1
   2
   3
   4
   5
   6
   7
   8
   9
  10
  11
  12
  13
  14
  15
  16
  17
  18
  19
  20
  21
  22
  23
  24
  25
  26
  27
  28
  29
  30
  31
  32
  33
  34
  35
  36
  37
  38
  39
  40
  41
  42
  43
  44
  45
  46
  47
  48
  49
  50
  51
  52
  53
  54
  55
  56
  57
  58
  59
  60
  61
  62
  63
  64
  65
  66
  67
  68
  69
  70
  71
  72
  73
  74
  75
  76
  77
  78
  79
  80
  81
  82
  83
  84
  85
  86
  87
  88
  89
  90
  91
  92
  93
  94
  95
  96
  97
  98
  99
 100
 101
 102
 103
 104
 105
 106
 107
 108
 109
 110
 111
 112
 113
 114
 115
 116
 117
 118
 119
 120
 121
 122
 123
 124
 125
 126
 127
 128
 129
 130
 131
 132
 133
 134
 135
 136
 137
 138
 139
 140
 141
 142
 143
 144
 145
 146
 147
 148
 149
 150
 151
 152
 153
 154
 155
 156
 157
 158
 159
 160
 161
 162
 163
 164
 165
 166
 167
 168
 169
 170
 171
 172
 173
 174
 175
 176
 177
 178
 179
 180
 181
 182
 183
 184
 185
 186
 187
 188
 189
 190
 191
 192
 193
 194
 195
 196
 197
 198
 199
 200
 201
 202
 203
 204
 205
 206
 207
 208
 209
 210
 211
 212
 213
 214
 215
 216
 217
 218
 219
 220
 221
 222
 223
 224
 225
 226
 227
 228
 229
 230
 231
 232
 233
 234
 235
 236
 237
 238
 239
 240
 241
 242
 243
 244
 245
 246
 247
 248
 249
 250
 251
 252
 253
 254
 255
 256
 257
 258
 259
 260
 261
 262
 263
 264
 265
 266
 267
 268
 269
 270
 271
 272
 273
 274
 275
 276
 277
 278
 279
 280
 281
 282
 283
 284
 285
 286
 287
 288
 289
 290
 291
 292
 293
 294
 295
 296
 297
 298
 299
 300
 301
 302
 303
 304
 305
 306
 307
 308
 309
 310
 311
 312
 313
 314
 315
 316
 317
 318
 319
 320
 321
 322
 323
 324
 325
 326
 327
 328
 329
 330
 331
 332
 333
 334
 335
 336
 337
 338
 339
 340
 341
 342
 343
 344
 345
 346
 347
 348
 349
 350
 351
 352
 353
 354
 355
 356
 357
 358
 359
 360
 361
 362
 363
 364
 365
 366
 367
 368
 369
 370
 371
 372
 373
 374
 375
 376
 377
 378
 379
 380
 381
 382
 383
 384
 385
 386
 387
 388
 389
 390
 391
 392
 393
 394
 395
 396
 397
 398
 399
 400
 401
 402
 403
 404
 405
 406
 407
 408
 409
 410
 411
 412
 413
 414
 415
 416
 417
 418
 419
 420
 421
 422
 423
 424
 425
 426
 427
 428
 429
 430
 431
 432
 433
 434
 435
 436
 437
 438
 439
 440
 441
 442
 443
 444
 445
 446
 447
 448
 449
 450
 451
 452
 453
 454
 455
 456
 457
 458
 459
 460
 461
 462
 463
 464
 465
 466
 467
 468
 469
 470
 471
 472
 473
 474
 475
 476
 477
 478
 479
 480
 481
 482
 483
 484
 485
 486
 487
 488
 489
 490
 491
 492
 493
 494
 495
 496
 497
 498
 499
 500
 501
 502
 503
 504
 505
 506
 507
 508
 509
 510
 511
 512
 513
 514
 515
 516
 517
 518
 519
 520
 521
 522
 523
 524
 525
 526
 527
 528
 529
 530
 531
 532
 533
 534
 535
 536
 537
 538
 539
 540
 541
 542
 543
 544
 545
 546
 547
 548
 549
 550
 551
 552
 553
 554
 555
 556
 557
 558
 559
 560
 561
 562
 563
 564
 565
 566
 567
 568
 569
 570
 571
 572
 573
 574
 575
 576
 577
 578
 579
 580
 581
 582
 583
 584
 585
 586
 587
 588
 589
 590
 591
 592
 593
 594
 595
 596
 597
 598
 599
 600
 601
 602
 603
 604
 605
 606
 607
 608
 609
 610
 611
 612
 613
 614
 615
 616
 617
 618
 619
 620
 621
 622
 623
 624
 625
 626
 627
 628
 629
 630
 631
 632
 633
 634
 635
 636
 637
 638
 639
 640
 641
 642
 643
 644
 645
 646
 647
 648
 649
 650
 651
 652
 653
 654
 655
 656
 657
 658
 659
 660
 661
 662
 663
 664
 665
 666
 667
 668
 669
 670
 671
 672
 673
 674
 675
 676
 677
 678
 679
 680
 681
 682
 683
 684
 685
 686
 687
 688
 689
 690
 691
 692
 693
 694
 695
 696
 697
 698
 699
 700
 701
 702
 703
 704
 705
 706
 707
 708
 709
 710
 711
 712
 713
 714
 715
 716
 717
 718
 719
 720
 721
 722
 723
 724
 725
 726
 727
 728
 729
 730
 731
 732
 733
 734
 735
 736
 737
 738
 739
 740
 741
 742
 743
 744
 745
 746
 747
 748
 749
 750
 751
 752
 753
 754
 755
 756
 757
 758
 759
 760
 761
 762
 763
 764
 765
 766
 767
 768
 769
 770
 771
 772
 773
 774
 775
 776
 777
 778
 779
 780
 781
 782
 783
 784
 785
 786
 787
 788
 789
 790
 791
 792
 793
 794
 795
 796
 797
 798
 799
 800
 801
 802
 803
 804
 805
 806
 807
 808
 809
 810
 811
 812
 813
 814
 815
 816
 817
 818
 819
 820
 821
 822
 823
 824
 825
 826
 827
 828
 829
 830
 831
 832
 833
 834
 835
 836
 837
 838
 839
 840
 841
 842
 843
 844
 845
 846
 847
 848
 849
 850
 851
 852
 853
 854
 855
 856
 857
 858
 859
 860
 861
 862
 863
 864
 865
 866
 867
 868
 869
 870
 871
 872
 873
 874
 875
 876
 877
 878
 879
 880
 881
 882
 883
 884
 885
 886
 887
 888
 889
 890
 891
 892
 893
 894
 895
 896
 897
 898
 899
 900
 901
 902
 903
 904
 905
 906
 907
 908
 909
 910
 911
 912
 913
 914
 915
 916
 917
 918
 919
 920
 921
 922
 923
 924
 925
 926
 927
 928
 929
 930
 931
 932
 933
 934
 935
 936
 937
 938
 939
 940
 941
 942
 943
 944
 945
 946
 947
 948
 949
 950
 951
 952
 953
 954
 955
 956
 957
 958
 959
 960
 961
 962
 963
 964
 965
 966
 967
 968
 969
 970
 971
 972
 973
 974
 975
 976
 977
 978
 979
 980
 981
 982
 983
 984
 985
 986
 987
 988
 989
 990
 991
 992
 993
 994
 995
 996
 997
 998
 999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
//! Widgets are the core building blocks for every conrod user interface.
//!
//! This module contains items related to the implementation of the `Widget` trait. It also
//! re-exports all widgets (and their modules) that are provided by conrod.

use graph::{Container, UniqueWidgetState};
use position::{Align, Depth, Dimension, Dimensions, Padding, Position, Point,
               Positionable, Rect, Relative, Sizeable};
use std;
use text::font;
use theme::{self, Theme};
use ui::{self, Ui, UiCell};


pub use self::id::Id;

pub use self::primitive::line::{self, Line};
pub use self::primitive::image::{self, Image};
pub use self::primitive::point_path::{self, PointPath};
pub use self::primitive::shape::circle::{self, Circle};
pub use self::primitive::shape::oval::{self, Oval};
pub use self::primitive::shape::polygon::{self, Polygon};
pub use self::primitive::shape::rectangle::{self, Rectangle};
pub use self::primitive::shape::triangles::{self, Triangles};
pub use self::primitive::text::{self, Text};

pub use self::bordered_rectangle::BorderedRectangle;
pub use self::button::Button;
pub use self::canvas::Canvas;
pub use self::collapsible_area::CollapsibleArea;
pub use self::drop_down_list::DropDownList;
pub use self::envelope_editor::EnvelopeEditor;
pub use self::file_navigator::FileNavigator;
pub use self::grid::Grid;
pub use self::list::List;
pub use self::list_select::ListSelect;
pub use self::matrix::Matrix;
pub use self::graph::Graph;
pub use self::number_dialer::NumberDialer;
pub use self::plot_path::PlotPath;
pub use self::range_slider::RangeSlider;
pub use self::rounded_rectangle::RoundedRectangle;
pub use self::scrollbar::Scrollbar;
pub use self::slider::Slider;
pub use self::tabs::Tabs;
pub use self::text_box::TextBox;
pub use self::text_edit::TextEdit;
pub use self::title_bar::TitleBar;
pub use self::toggle::Toggle;
pub use self::xy_pad::XYPad;


// Macro providing modules.
#[macro_use] mod builder;

// Widget functionality modules.
#[macro_use] pub mod id;
pub mod scroll;

// Primitive widget modules.
pub mod primitive;

// Widget modules.

pub mod bordered_rectangle;
pub mod button;
pub mod canvas;
pub mod collapsible_area;
pub mod drop_down_list;
pub mod envelope_editor;
pub mod file_navigator;
pub mod grid;
pub mod list;
pub mod list_select;
pub mod matrix;
pub mod graph;
pub mod number_dialer;
pub mod plot_path;
pub mod range_slider;
pub mod rounded_rectangle;
pub mod scrollbar;
pub mod slider;
pub mod tabs;
pub mod text_box;
pub mod text_edit;
pub mod title_bar;
pub mod toggle;
pub mod xy_pad;


/// Arguments for the [**Widget::update**](./trait.Widget#method.update) method in a struct to
/// simplify the method signature.
pub struct UpdateArgs<'a, 'b: 'a, 'c, 'd: 'c, W>
    where W: Widget,
{
    /// The **Widget**'s unique index.
    pub id: Id,
    /// The **Widget**'s parent unique index, if there is one.
    pub maybe_parent_id: Option<Id>,
    /// The **Widget**'s previous state. Specifically, the state that is common between all widgets,
    /// such as positioning, floatability, draggability, etc.
    pub prev: &'a CommonState,
    /// A wrapper around the **Widget**'s unique state, providing methods for both immutably viewing
    /// and mutably updating the state.
    ///
    /// We wrap mutation in a method so that we can keep track of whether or not the unique state
    /// has been updated.
    ///
    /// If **State::update** is called, we assume that there has been some mutation and in turn
    /// will require re-drawing the **Widget**. Thus, it is recommended that you *only* call
    /// **State::update** if you need to update the unique state in some way.
    pub state: &'a mut State<'b, W::State>,
    /// The rectangle describing the **Widget**'s area.
    pub rect: Rect,
    /// The **Widget**'s current **Widget::Style**.
    pub style: &'a W::Style,
    /// Restricted access to the `Ui`.
    ///
    /// Provides methods for immutably accessing the `Ui`'s `Theme` and `GlyphCache`.  Also allows
    /// calling `Widget::set` within the `Widget::update` method.
    pub ui: &'c mut UiCell<'d>,
}

/// Arguments to the [**Widget::kid_area**](./trait.Widget#method.kid_area) method in a struct to
/// simplify the method signature.
pub struct KidAreaArgs<'a, W>
    where W: Widget,
{
    /// The **Rect** describing the **Widget**'s position and dimensions.
    pub rect: Rect,
    /// Current **Widget::Style** of the **Widget**.
    pub style: &'a W::Style,
    /// The active **Theme** within the **Ui**.
    pub theme: &'a Theme,
    /// The **Font** (for determining text width).
    pub fonts: &'a font::Map,
}

/// The area upon which a **Widget**'s child widgets will be placed.
#[derive(Copy, Clone, Debug, PartialEq)]
pub struct KidArea {
    /// The **Rect** bounds describing the position and area.
    pub rect: Rect,
    /// The distance between the edge of the area and where the widgets will be placed.
    pub pad: Padding,
}

/// The builder argument for the **Widget**'s parent.
#[derive(Copy, Clone, Debug, PartialEq)]
pub enum MaybeParent {
    /// The user specified the widget should not have any parents, so the Root will be used.
    None,
    /// The user gave a specific parent widget.
    Some(Id),
    /// No parent widget was specified, so we will assume they want the last parent.
    Unspecified,
}

impl MaybeParent {
    /// Convert the **MaybeParent** into an **Option<Id>**.
    ///
    /// If `Unspecified`, check the positioning to retrieve the **Id** from there.
    ///
    /// If `None`, the `Ui`'s `window` widget will be used.
    ///
    /// **Note:** This method does not check whether or not using the `window` widget as the parent
    /// would cause a cycle. If it is important that the inferred parent should not cause a cycle,
    /// use `get` instead.
    pub fn get_unchecked(&self, ui: &Ui, x_pos: Position, y_pos: Position) -> Id {
        match *self {
            MaybeParent::Some(id) => id,
            MaybeParent::None => ui.window.into(),
            MaybeParent::Unspecified => ui::infer_parent_unchecked(ui, x_pos, y_pos),
        }
    }

    /// The same as `get_unchecked`, but checks whether or not the widget that we're inferring the
    /// parent for is the `Ui`'s window (which cannot have a parent, without creating a cycle).
    pub fn get(&self, id: Id, ui: &Ui, x_pos: Position, y_pos: Position) -> Option<Id> {
        if id == ui.window {
            None
        } else {
            Some(self.get_unchecked(ui, x_pos, y_pos))
        }
    }
}

/// State necessary for "floating" (pop-up style) widgets.
#[derive(Copy, Clone, Debug, PartialEq)]
pub struct Floating {
    /// The time the **Widget** was last clicked (used for depth sorting in the widget **Graph**).
    pub time_last_clicked: std::time::Instant,
}

/// A struct containing builder data common to all **Widget** types.
///
/// This type also allows us to do a blanket impl of **Positionable** and **Sizeable** for `T: Widget`.
///
/// When Rust gets some sort of field inheritance feature, this will most likely be refactored to
/// take advantage of that.
#[derive(Clone, Copy, Debug)]
pub struct CommonBuilder {
    /// Styling and positioning data that is common between all widget types.
    pub style: CommonStyle,
    /// The parent widget of the Widget.
    pub maybe_parent_id: MaybeParent,
    /// Whether or not the Widget is a "floating" Widget.
    pub is_floating: bool,
    /// Whether or not the children of this **Widget** should be cropped to its `kid_area`.
    ///
    /// By default, the kid_area is the size of the entire widget, though it may be specified
    /// otherwise via the `Widget::kid_area` method.
    pub crop_kids: bool,
    /// Arguments to the scrolling of the widget's *x* axis.
    pub maybe_x_scroll: Option<scroll::Scroll>,
    /// Arguments to the scrolling of the widget's *y* axis.
    pub maybe_y_scroll: Option<scroll::Scroll>,
    /// Whether or not the **Widget** should be placed on the kid_area.
    ///
    /// If `true`, the **Widget** will be placed on the `kid_area` of the parent **Widget** if the
    /// **Widget** is given a **Place** variant for its **Position**.
    ///
    /// If `false`, the **Widget** will be placed on the parent **Widget**'s *total* area.
    pub place_on_kid_area: bool,
    /// Describes whether or not the **Widget** is instantiated as a graphical element for some
    /// other **Widget**.
    ///
    /// When adding an edge *a -> b* where *b* is considered to be a graphical element of *a*,
    /// several things are implied about *b*:
    ///
    /// - If *b* is picked within either **Graph::pick_widget** or
    /// **Graph::pick_top_scrollable_widget**, it will instead return the index for *a*.
    /// - When determining the **Graph::scroll_offset** for *b*, *a*'s scrolling (if it is
    /// scrollable, that is) will be skipped.
    /// - *b* will always be placed upon *a*'s total area, rather than its kid_area which is the
    /// default.
    /// - Any **Graphic** child of *b* will be considered as a **Graphic** child of *a*.
    pub maybe_graphics_for: Option<Id>,
}

/// Styling and positioning data that is common between all widget types.
#[derive(Clone, Copy, Debug, Default, PartialEq)]
pub struct CommonStyle {
    /// The width of a Widget.
    pub maybe_x_dimension: Option<Dimension>,
    /// The height of a Widget.
    pub maybe_y_dimension: Option<Dimension>,
    /// The position of a Widget along the *x* axis.
    pub maybe_x_position: Option<Position>,
    /// The position of a Widget along the *y* axis.
    pub maybe_y_position: Option<Position>,
    /// The rendering Depth of the Widget.
    pub maybe_depth: Option<Depth>,
}

/// A wrapper around a **Widget**'s unique **Widget::State**.
///
/// This type is used to provide limited access to the **Widget::State** within the
/// [**Widget::update**](./trait.Widget#method.update) method (to which it is passed via the
/// [**UpdateArgs**](./struct.UpdateArgs)).
///
/// The type provides only two methods. One for viewing the state, the other for mutating it.
///
/// We do this so that we can keep track of whether or not the **Widget::State** has been mutated
/// (using an internal `has_updated` flag). This allows us to know whether or not we need to
/// re-draw the **Widget**, without having to compare the previous and new **Widget::State**s.
pub struct State<'a, T: 'a> {
    state: &'a mut T,
    /// A flag indicating whether or not the widget's State has been updated.
    has_updated: bool,
}

/// A wrapper around state that is common to all **Widget** types.
#[derive(Copy, Clone, Debug, PartialEq)]
pub struct CommonState {
    /// The rectangle describing the `Widget`'s area.
    pub rect: Rect,
    /// The rendering depth for the Widget (the default is 0.0).
    pub depth: Depth,
    /// Floating state for the widget if it is floating.
    pub maybe_floating: Option<Floating>,
    /// The area of the widget upon which kid widgets are placed.
    pub kid_area: KidArea,
    /// If the widget is scrollable across the *x* axis.
    pub maybe_x_scroll_state: Option<scroll::StateX>,
    /// If the widget is scrollable across the *y* axis.
    pub maybe_y_scroll_state: Option<scroll::StateY>,
}

// **Widget** data to be cached prior to the **Widget::update** call in the **widget::set_widget**
// function.
//
// We do this so that if this **Widget** were to internally `set` some other **Widget**s, this
// **Widget**'s positioning and dimension data already exists within the widget **Graph** for
// reference.
#[allow(missing_docs)]
#[allow(missing_copy_implementations)]
pub struct PreUpdateCache {
    /// The **Widget**'s unique type identifier.
    pub type_id: std::any::TypeId,
    /// The **Widget**'s unique Id.
    pub id: Id,
    /// The **Widget**'s parent's unique index (if it has a parent).
    pub maybe_parent_id: Option<Id>,
    /// If this **Widget** is relatively positioned to another **Widget**, this will be the index
    /// of the **Widget** to which this **Widget** is relatively positioned along the *x* axis.
    pub maybe_x_positioned_relatively_id: Option<Id>,
    /// If this **Widget** is relatively positioned to another **Widget**, this will be the index
    /// of the **Widget** to which this **Widget** is relatively positioned along the *y* axis.
    pub maybe_y_positioned_relatively_id: Option<Id>,
    /// The **Rect** describing the **Widget**'s position and dimensions.
    pub rect: Rect,
    /// The z-axis depth - affects the render order of sibling widgets.
    pub depth: Depth,
    /// The area upon which the **Widget**'s children widgets will be placed.
    pub kid_area: KidArea,
    /// Floating data for the **Widget** if there is some.
    pub maybe_floating: Option<Floating>,
    /// Whether or not the children of the **Widget** should be cropped to its `kid_area`.
    pub crop_kids: bool,
    /// Scrolling data for the **Widget**'s *x* axis if there is some.
    pub maybe_x_scroll_state: Option<scroll::StateX>,
    /// Scrolling data for the **Widget**'s *y* axis if there is some.
    pub maybe_y_scroll_state: Option<scroll::StateY>,
    /// Whether or not the **Widget** has been instantiated as a graphical element for some other
    /// widget.
    pub maybe_graphics_for: Option<Id>,
    /// A function describing whether or not a given point is over the widget.
    pub is_over: IsOverFn
}

// **Widget** data to be cached after the **Widget::update** call in the **widget::set_widget**
// function.
//
// We do this so that if this **Widget** were to internally **Widget::set** some other
// **Widget**s, this **Widget**'s positioning and dimension data will already exist within the
// widget **Graph** for reference.
#[allow(missing_docs)]
pub struct PostUpdateCache<W>
    where W: Widget,
{
    /// The **Widget**'s unique **Id**.
    pub id: Id,
    /// The **Widget**'s parent's unique **Id** (if it has a parent).
    pub maybe_parent_id: Option<Id>,
    /// The newly produced unique **Widget::State** associated with the **Widget**.
    pub state: W::State,
    /// The newly produced unique **Widget::Style** associated with the **Widget**.
    pub style: W::Style,
}

/// Returned by the `Widget::is_over` method.
#[derive(Copy, Clone)]
pub enum IsOver {
    /// Whether or not the point was over the widget.
    Bool(bool),
    /// Check whether or not the point is over the widget at the given `Id` and if so, assume it is
    /// over this widget.
    Widget(Id),
}

impl From<bool> for IsOver {
    fn from(b: bool) -> Self {
        IsOver::Bool(b)
    }
}

impl From<Id> for IsOver {
    fn from(id: Id) -> Self {
        IsOver::Widget(id)
    }
}

/// A function type used to determine whether or not a given point is over a widget.
pub type IsOverFn = fn(&Container, Point, &Theme) -> IsOver;

/// The default `IsOverFn` used if the `Widget::is_over` method is not overridden.
pub fn is_over_rect(container: &Container, point: Point, _: &Theme) -> IsOver {
    container.rect.is_over(point).into()
}

/// The necessary bounds for a **Widget**'s associated **Style** type.
pub trait Style: std::any::Any + std::fmt::Debug + PartialEq + Sized {}

/// Auto-implement the **Style** trait for all applicable types.
impl<T> Style for T where T: std::any::Any + std::fmt::Debug + PartialEq + Sized {}


/// Determines the default **Dimension** for a **Widget**.
///
/// This function checks for a default dimension in the following order.
/// 1. Check for a default value within the **Ui**'s **Theme**.
/// 2. Otherwise attempts to copy the dimension of the previously set widget if there is one.
/// 3. Otherwise attempts to copy the dimension of our parent widget.
/// 4. If no parent widget can be inferred, the window dimensions are used.
fn default_dimension<W, F>(widget: &W, ui: &Ui, f: F) -> Dimension
    where W: Widget,
          F: FnOnce(theme::UniqueDefault<W::Style>) -> Option<Dimension>,
{
    ui.theme.widget_style::<W::Style>()
        .and_then(f)
        .or_else(|| ui.maybe_prev_widget().map(|id| Dimension::Of(id, None)))
        .unwrap_or_else(|| {
            let x_pos = widget.get_x_position(ui);
            let y_pos = widget.get_y_position(ui);
            let parent_id = widget.common().maybe_parent_id.get_unchecked(ui, x_pos, y_pos);
            Dimension::Of(parent_id, None)
        })
}

/// Determines the default **Dimension** for a **Widget**.
///
/// This function checks for a default dimension in the following order.
/// 1. Check for a default value within the **Ui**'s **Theme**.
/// 2. Otherwise attempts to copy the dimension of the previously set widget if there is one.
/// 3. Otherwise attempts to copy the dimension of our parent widget.
/// 4. If no parent widget can be inferred, the window dimensions are used.
///
/// This is called by the default implementations of **Widget::default_x_dimension**.
///
/// If you wish to override **Widget::default_x_dimension**, feel free to call this function
/// internally if you partly require the bahaviour of the default implementations.
pub fn default_x_dimension<W>(widget: &W, ui: &Ui) -> Dimension
    where W: Widget,
{
    default_dimension(widget, ui, |default| default.common.maybe_x_dimension)
}

/// Determines the default **Dimension** for a **Widget**.
///
/// This function checks for a default dimension in the following order.
/// 1. Check for a default value within the **Ui**'s **Theme**.
/// 2. Otherwise attempts to copy the dimension of the previously set widget if there is one.
/// 3. Otherwise attempts to copy the dimension of our parent widget.
/// 4. If no parent widget can be inferred, the window dimensions are used.
///
/// This is called by the default implementations of **Widget::default_y_dimension**.
///
/// If you wish to override **Widget::default_y_dimension**, feel free to call this function
/// internally if you partly require the bahaviour of the default implementations.
pub fn default_y_dimension<W>(widget: &W, ui: &Ui) -> Dimension
    where W: Widget,
{
    default_dimension(widget, ui, |default| default.common.maybe_y_dimension)
}


/// A trait implemented by all **Widget** types.
///
/// This trait provides access to a field of type **CommonBuilder** on the implementor. This allows
/// the `Widget` trait to automatically provide a large number of methods including those from the
/// **Positionable** and **Sizeable** traits.
///
/// The **Common** trait can be automatically derived for widgets like so:
///
/// ```ignore
/// extern crate conrod;
/// #[macro_use] extern crate conrod_derive;
///
/// #[derive(WidgetCommon)]
/// struct MyWidget {
///     #[conrod(common_builder)]
///     common: conrod::widget::CommonBuilder,
///     // etc
/// }
/// ```
pub trait Common {
    /// Borrows the `CommonBuilder` field.
    fn common(&self) -> &CommonBuilder;
    /// Mutably borrows the `CommonBuilder` field.
    fn common_mut(&mut self) -> &mut CommonBuilder;
}


/// A trait to be implemented by all **Widget** types.
///
/// A type that implements **Widget** can be thought of as a collection of arguments to the
/// **Widget**'s **Widget::update** method. They type itself is not stored between updates, but
/// rather is used to update an instance of the **Widget**'s **Widget::State**, which *is* stored.
///
/// Methods that *must* be overridden:
///
/// - init_state
/// - style
/// - update
///
/// Methods that can be optionally overridden:
///
/// - default_x_position
/// - default_y_position
/// - default_width
/// - default_height
/// - drag_area
/// - kid_area
///
/// Methods that should not be overridden:
///
/// - floating
/// - scroll_kids
/// - scroll_kids_vertically
/// - scroll_kids_horizontally
/// - place_widget_on_kid_area
/// - parent
/// - no_parent
/// - set
pub trait Widget: Common + Sized {
    /// State to be stored within the `Ui`s widget cache.
    ///
    /// Take advantage of this type for any large allocations that you would like to avoid
    /// repeating between updates, or any calculations that you'd like to avoid repeating between
    /// calls to `update`.
    ///
    /// Conrod will never clone the state, it will only ever be moved.
    type State: std::any::Any + Send;
    /// Every widget is required to have its own associated `Style` type. This type is intended to
    /// contain high-level styling information for the widget that can be *optionally specified* by
    /// a user of the widget.
    ///
    /// All `Style` structs are typically `Copy` and contain simple, descriptive fields like
    /// `color`, `font_size`, `line_spacing`, `border_width`, etc. These types are also required to
    /// be `PartialEq`. This is so that the `Ui` may automatically compare the previous style to
    /// the new style each time `.set` is called, allowing conrod to automatically determine
    /// whether or not something has changed and if a re-draw is required.
    ///
    /// Each field in a `Style` struct is typically an `Option<T>`. This is so that each field may
    /// be *optionally specified*, indicating to fall back to defaults if the fields are `None`
    /// upon style retrieval.
    ///
    /// The reason this data is required to be in its own `Style` type (rather than in the widget
    /// type itself) is so that conrod can distinguish between default style data that may be
    /// stored within the `Theme`'s `widget_styling` field, and other data that is necessary for
    /// the widget's behaviour logic. Having `Style` be an associated type makes it trivial to
    /// retrieve unique, widget-specific styling data for each widget from a single method (see
    /// [`Theme::widget_style`](./theme/struct.Theme.html#method.widget_style)).
    ///
    /// ## `#[derive(WidgetStyle)]`
    ///
    /// These `Style` types are often quite similar and their implementations can involve a lot of
    /// boilerplate when written by hand. To get around this, conrod provides
    /// `#[derive(WidgetStyle)]`.
    ///
    /// This procedural macro generates a "getter"-style method for each struct field that is
    /// decorated with a `#[conrod(default = "expr")]` attribute. The generated methods have the
    /// same name as their respective fields and behave as follows:
    ///
    /// 1. First, the method will attempt to return the value directly if the field is `Some`.
    /// 2. If the field is `None`, the method will fall back to the `Widget::Style` stored within
    ///    the `Theme`'s `widget_styling` map.
    /// 3. If there are no style defaults for the widget in the `Theme`, or if there is but the
    ///    default field is also `None`, the method will fall back to the expression specified
    ///    within the field's `#[conrod(default = "expr")]` attribute.
    ///
    /// The given "expr" can be a string containing any expression that returns the type specified
    /// within the field's `Option` type parameter. The expression may also utilise the `theme` and
    /// `self` bindings, where `theme` is a binding to the borrowed `Theme` and `self` is a binding
    /// to the borrowed instance of this `Style` type.
    ///
    /// # Examples
    ///
    /// ```
    /// # extern crate conrod;
    /// # #[macro_use] extern crate conrod_derive;
    /// # use conrod::{Color, FontSize, Scalar};
    /// # fn main() {}
    /// /// Unique styling for a Button widget.
    /// #[derive(Copy, Clone, Debug, Default, PartialEq, WidgetStyle)]
    /// pub struct Style {
    ///     /// Color of the Button's pressable area.
    ///     #[conrod(default = "theme.shape_color")]
    ///     pub color: Option<Color>,
    ///     /// Width of the border surrounding the button.
    ///     #[conrod(default = "1.0")]
    ///     pub border: Option<Scalar>,
    ///     /// The color of the Button's rectangular border.
    ///     #[conrod(default = "conrod::color::BLACK")]
    ///     pub border_color: Option<Color>,
    ///     /// The color of the Button's label.
    ///     #[conrod(default = "theme.label_color")]
    ///     pub label_color: Option<Color>,
    ///     /// The font size for the Button's label.
    ///     #[conrod(default = "12")]
    ///     pub label_font_size: Option<FontSize>,
    /// }
    /// ```
    type Style: Style + Send;
    /// The type of event yielded by the widget, returned via the `Widget::set` function.
    ///
    /// For a `Toggle` widget, this might be a `bool`.
    ///
    /// For a non-interactive, purely graphical widget, this might be `()`.
    type Event;

    /// Return the initial **State** of the Widget.
    ///
    /// The `Ui` will only call this once, immediately prior to the first time that
    /// **Widget::update** is first called.
    fn init_state(&self, id::Generator) -> Self::State;

    /// Return the styling of the widget.
    ///
    /// The `Ui` will call this once prior to each `update`. It does this so that it can check for
    /// differences in `Style` in case we need to re-draw the widget.
    fn style(&self) -> Self::Style;

    /// Update our **Widget**'s unique **Widget::State** via the **State** wrapper type (the
    /// `state` field within the [**UpdateArgs**](./struct.UpdateArgs)).
    ///
    /// Whenever [**State::update**](./struct.State.html#method.update) is called, a `has_updated`
    /// flag is set within the **State**, indicating that there has been some change to the unique
    /// **Widget::State** and that we require re-drawing the **Widget**. As a result, widget
    /// designers should only call **State::update** when necessary, checking whether or not the
    /// state has changed before invoking the method. See the custom_widget.rs example for a
    /// demonstration of this.
    ///
    /// # Arguments
    /// * id - The `Widget`'s unique index (whether `Public` or `Internal`).
    /// * prev - The previous common state of the Widget. If this is the first time **update** is
    /// called, `Widget::init_state` will be used to produce some initial state instead.
    /// * state - A wrapper around the `Widget::State`. See the [**State** docs](./struct.State)
    /// for more details.
    /// * rect - The position (centered) and dimensions of the widget.
    /// * style - The style produced by the `Widget::style` method.
    /// * ui - A wrapper around the `Ui`, offering restricted access to its functionality. See the
    /// docs for `UiCell` for more details.
    fn update(self, args: UpdateArgs<Self>) -> Self::Event;

    /// The default **Position** for the widget along the *x* axis.
    ///
    /// This is used when no **Position** is explicitly given when instantiating the Widget.
    fn default_x_position(&self, ui: &Ui) -> Position {
        ui.theme.widget_style::<Self::Style>()
            .and_then(|style| style.common.maybe_x_position)
            .unwrap_or(ui.theme.x_position)
    }

    /// The default **Position** for the widget along the *y* axis.
    ///
    /// This is used when no **Position** is explicitly given when instantiating the Widget.
    fn default_y_position(&self, ui: &Ui) -> Position {
        ui.theme.widget_style::<Self::Style>()
            .and_then(|style| style.common.maybe_y_position)
            .unwrap_or(ui.theme.y_position)
    }

    /// The default width for the **Widget**.
    ///
    /// This method is only used if no height is explicitly given.
    ///
    /// By default, this simply calls [**default_dimension**](./fn.default_dimension) with a
    /// fallback absolute dimension of 0.0.
    fn default_x_dimension(&self, ui: &Ui) -> Dimension {
        default_x_dimension(self, ui)
    }

    /// The default height of the widget.
    ///
    /// By default, this simply calls [**default_dimension**](./fn.default_dimension) with a
    /// fallback absolute dimension of 0.0.
    fn default_y_dimension(&self, ui: &Ui) -> Dimension {
        default_y_dimension(self, ui)
    }

    /// If the widget is draggable, implement this method and return the position and dimensions of
    /// the draggable space. The position should be relative to the center of the widget.
    fn drag_area(&self,
                 _dim: Dimensions,
                 _style: &Self::Style,
                 _theme: &Theme) -> Option<Rect>
    {
        None
    }

    /// The area on which child widgets will be placed when using the `Place` `Position` methods.
    fn kid_area(&self, args: KidAreaArgs<Self>) -> KidArea {
        KidArea {
            rect: args.rect,
            pad: Padding::none(),
        }
    }

    /// Returns either of the following:
    ///
    /// - A `Function` that can be used to describe whether or not a given point is over the
    /// widget.
    /// - The `Id` of another `Widget` that can be used to determine if the point is over this
    /// widget.
    ///
    /// By default, this is a function returns `true` if the given `Point` is over the bounding
    /// `Rect` and returns `false` otherwise.
    ///
    /// *NOTE: It could be worth removing this in favour of adding a `widget::State` trait, adding
    /// an `is_over` method to it and then refactoring the `Container` to store a
    /// `Box<widget::State>` and `Box<widget::Style>` rather than `Box<Any>`. This would however
    /// involve some significant breakage (which could perhaps be mitigated by adding a
    /// `derive(WidgetState)` macro - a fair chunk of work) so this might be the easiest temporary
    /// way forward for now.*
    fn is_over(&self) -> IsOverFn {
        is_over_rect
    }


    // None of the following methods should require overriding. Perhaps they should be split off
    // into a separate trait which is impl'ed for W: Widget to make this clearer?
    // Most of them would benefit by some sort of field inheritance as they are mainly just used to
    // set common data.


    /// Set the parent widget for this Widget by passing the WidgetId of the parent.
    ///
    /// This will attach this Widget to the parent widget.
    fn parent(mut self, parent_id: Id) -> Self {
        self.common_mut().maybe_parent_id = MaybeParent::Some(parent_id);
        self
    }

    /// Specify that this widget has no parent widgets.
    fn no_parent(mut self) -> Self {
        self.common_mut().maybe_parent_id = MaybeParent::None;
        self
    }

    /// Set whether or not the **Widget** should be placed on the kid_area.
    ///
    /// If `true`, the **Widget** will be placed on the `kid_area` of the parent **Widget** if the
    /// **Widget** is given a **Place** variant for its **Position**.
    ///
    /// If `false`, the **Widget** will be placed on the parent **Widget**'s *total* area.
    ///
    /// By default, conrod will automatically determine this for you by checking whether or not the
    /// **Widget** that our **Widget** is being placed upon returns `Some` from its
    /// **Widget::kid_area** method.
    fn place_on_kid_area(mut self, b: bool) -> Self {
        self.common_mut().place_on_kid_area = b;
        self
    }

    /// Indicates that the **Widget** is used as a non-interactive graphical element for some other
    /// widget.
    ///
    /// This is useful for **Widget**s that are used to compose some other **Widget**.
    ///
    /// When adding an edge *a -> b* where *b* is considered to be a graphical element of *a*,
    /// several things are implied about *b*:
    ///
    /// - If *b* is picked within either **Graph::pick_widget** or
    /// **Graph::pick_top_scrollable_widget**, it will instead return the index for *a*.
    /// - When determining the **Graph::scroll_offset** for *b*, *a*'s scrolling (if it is
    /// scrollable, that is) will be skipped.
    /// - *b* will always be placed upon *a*'s total area, rather than its kid_area which is the
    /// default.
    /// - Any **Graphic** child of *b* will be considered as a **Graphic** child of *a*.
    fn graphics_for(mut self, id: Id) -> Self {
        self.common_mut().maybe_graphics_for = Some(id);
        self
    }

    /// Set whether or not the widget is floating (the default is `false`).
    /// A typical example of a floating widget would be a pop-up or alert window.
    ///
    /// A "floating" widget will always be rendered *after* its parent tree and all widgets
    /// connected to its parent tree. If two sibling widgets are both floating, then the one that
    /// was last clicked will be rendered last. If neither are clicked, they will be rendered in
    /// the order in which they were cached into the `Ui`.
    fn floating(mut self, is_floating: bool) -> Self {
        self.common_mut().is_floating = is_floating;
        self
    }

    /// Indicates that all widgets who are children of this widget should be cropped to the
    /// `kid_area` of this widget.
    fn crop_kids(mut self) -> Self {
        self.common_mut().crop_kids = true;
        self
    }

    /// Makes the widget's `KidArea` scrollable.
    ///
    /// If a widget is scrollable and it has children widgets that fall outside of its `KidArea`,
    /// the `KidArea` will become scrollable.
    ///
    /// This method calls `Widget::crop_kids` internally.
    fn scroll_kids(self) -> Self {
        self.scroll_kids_vertically().scroll_kids_horizontally().crop_kids()
    }

    /// Makes the widget's `KidArea` scrollable.
    ///
    /// If a widget is scrollable and it has children widgets that fall outside of its `KidArea`,
    /// the `KidArea` will become scrollable.
    ///
    /// This method calls `Widget::crop_kids` internally.
    fn scroll_kids_vertically(mut self) -> Self {
        self.common_mut().maybe_y_scroll = Some(scroll::Scroll::new());
        self.crop_kids()
    }

    /// Set whether or not the widget's `KidArea` is scrollable (the default is false).
    ///
    /// If a widget is scrollable and it has children widgets that fall outside of its `KidArea`,
    /// the `KidArea` will become scrollable.
    ///
    /// This method calls `Widget::crop_kids` internally.
    fn scroll_kids_horizontally(mut self) -> Self {
        self.common_mut().maybe_x_scroll = Some(scroll::Scroll::new());
        self.crop_kids()
    }

    /// A builder method that "lifts" the **Widget** through the given `build` function.
    ///
    /// This method is solely for providing slight ergonomic improvement by helping to maintain
    /// the symmetry of the `builder` pattern in some cases.
    #[inline]
    fn and<F>(self, build: F) -> Self
        where F: FnOnce(Self) -> Self,
    {
        build(self)
    }

    /// A builder method that mutates the **Widget** with the given `mutate` function.
    ///
    /// This method is solely for providing slight ergonomic improvement by helping to maintain
    /// the symmetry of the `builder` pattern in some cases.
    #[inline]
    fn and_mut<F>(mut self, mutate: F) -> Self
        where F: FnOnce(&mut Self),
    {
        mutate(&mut self);
        self
    }

    /// A method that conditionally builds the **Widget** with the given `build` function.
    ///
    /// If `cond` is `true`, `build(self)` is evaluated and returned.
    ///
    /// If `false`, `self` is returned.
    #[inline]
    fn and_if<F>(self, cond: bool, build: F) -> Self
        where F: FnOnce(Self) -> Self,
    {
        if cond { build(self) } else { self }
    }

    /// A method that optionally builds the **Widget** with the given `build` function.
    ///
    /// If `maybe` is `Some(t)`, `build(self, t)` is evaluated and returned.
    ///
    /// If `None`, `self` is returned.
    #[inline]
    fn and_then<T, F>(self, maybe: Option<T>, build: F) -> Self
        where F: FnOnce(Self, T) -> Self,
    {
        if let Some(t) = maybe { build(self, t) } else { self }
    }

    /// Note: There should be no need to override this method.
    ///
    /// After building the widget, you call this method to set its current state into the given
    /// `Ui`. More precisely, the following will occur when calling this method:
    /// - The widget's previous state and style will be retrieved.
    /// - The widget's current `Style` will be retrieved (from the `Widget::style` method).
    /// - The widget's state will be updated (using the `Widget::udpate` method).
    /// - If the widget's state or style has changed, the **Ui** will be notified that the widget
    /// needs to be re-drawn.
    /// - The new State and Style will be cached within the `Ui`.
    fn set<'a, 'b>(self, id: Id, ui_cell: &'a mut UiCell<'b>) -> Self::Event {
        set_widget(self, id, ui_cell)
    }

}



/// Updates the given widget and caches it within the given `Ui`'s `widget_graph`.
///
/// If it is the first time a widget has been set, it will be cached into the `Ui`'s widget_graph.
/// For all following occasions, the pre-existing cached state will be compared and updated.
///
/// Note that this is a very imperative, mutation oriented segment of code. We try to move as much
/// imperativeness and mutation out of the users hands and into this function as possible, so that
/// users have a clear, concise, purely functional `Widget` API. As a result, we try to keep this
/// as verbosely annotated as possible. If anything is unclear, feel free to post an issue or PR
/// with concerns/improvements to the github repo.
fn set_widget<'a, 'b, W>(widget: W, id: Id, ui: &'a mut UiCell<'b>) -> W::Event
    where W: Widget,
{
    let type_id = std::any::TypeId::of::<W::State>();

    // Take the previous state of the widget from the cache if there is some to collect.
    let (maybe_prev_unique_state, maybe_prev_common, maybe_prev_style) =
        ui::widget_graph_mut(ui::ref_mut_from_ui_cell(ui))
            .widget_mut(id)
            .and_then(|container| {

                // If the cache is already initialised for a different widget type, warn the user.
                if container.type_id != type_id {
                    use std::io::Write;
                    writeln!(std::io::stderr(),
                             "A widget of a different type already exists at the given WidgetId \
                             ({:?}). You tried to insert a {:?}, however the existing widget is a \
                             {:?}. Check your widgets' `WidgetId`s for errors.",
                              id, type_id, container.type_id).unwrap();
                    return None;
                }

                // Destructure the cached state.
                let Container {
                    ref mut maybe_state,
                    rect,
                    depth,
                    kid_area,
                    maybe_floating,
                    maybe_x_scroll_state,
                    maybe_y_scroll_state,
                    ..
                } = *container;

                let (state, style) = match maybe_state.take().and_then(|a| a.downcast().ok()) {
                    Some(boxed) => {
                        let unique: UniqueWidgetState<W::State, W::Style> = *boxed;
                        let UniqueWidgetState { state, style } = unique;
                        (state, style)
                    },
                    None => return None,
                };

                // Use the cached state to construct the prev_state (to be fed to Widget::update).
                let prev_common = CommonState {
                    rect: rect,
                    depth: depth,
                    maybe_floating: maybe_floating,
                    kid_area: kid_area,
                    maybe_x_scroll_state: maybe_x_scroll_state,
                    maybe_y_scroll_state: maybe_y_scroll_state,
                };

                Some((Some(state), Some(prev_common), Some(style)))
            })
            .unwrap_or_else(|| (None, None, None));

    // We need to hold onto the current "previously set widget", as this may change during our
    // `Widget`'s update method (i.e. if it sets any of its own widgets, they will become the last
    // previous widget).
    let maybe_prev_widget_id = ui.maybe_prev_widget();

    let new_style = widget.style();
    let depth = widget.get_depth();
    let dim = widget.get_wh(&ui).unwrap_or([0.0, 0.0]);
    let x_pos = widget.get_x_position(ui);
    let y_pos = widget.get_y_position(ui);
    let place_on_kid_area = widget.common().place_on_kid_area;

    // Determine the id of the canvas that the widget is attached to. If not given explicitly,
    // check the positioning to retrieve the Id from there.
    let maybe_parent_id = widget.common().maybe_parent_id.get(id, ui, x_pos, y_pos);

    // Calculate the `xy` location of the widget, considering drag.
    let xy = maybe_prev_common
        .as_ref()
        .and_then(|prev| {
            let maybe_drag_area = widget.drag_area(dim, &new_style, &ui.theme);
            maybe_drag_area.map(|drag_area| {
                let mut left_mouse_drags = ui.widget_input(id).drags().left();
                let maybe_first_drag = left_mouse_drags.next();
                let prev_xy = prev.rect.xy();
                maybe_first_drag
                    .and_then(|first_drag| {
                        if drag_area.is_over(first_drag.from) {
                            let total_drag_xy = left_mouse_drags
                                .fold(first_drag.delta_xy, |total, drag| {
                                    [total[0] + drag.delta_xy[0], total[1] + drag.delta_xy[1]]
                                });
                            Some([prev_xy[0] + total_drag_xy[0], prev_xy[1] + total_drag_xy[1]])
                        } else {
                            None
                        }
                    })
                    .unwrap_or(prev_xy)
            })
        })
        // If there is no previous state to compare for dragging, return an initial state.
        //
        // A function for generating the xy coords from the given alignment and Position.
        .unwrap_or_else(|| ui.calc_xy(Some(id), x_pos, y_pos, dim, place_on_kid_area));

    // Construct the rectangle describing our Widget's area.
    let rect = Rect::from_xy_dim(xy, dim);

    // Check whether or not the widget is a "floating" (hovering / pop-up style) widget.
    let maybe_floating = if widget.common().is_floating {

        fn new_floating() -> Floating {
            Floating { time_last_clicked: std::time::Instant::now() }
        }

        // If it is floating, check to see if we need to update the last time it was clicked.
        match maybe_prev_common.as_ref() {
            Some(prev) => {
                let maybe_mouse = ui.widget_input(id).mouse();
                match (prev.maybe_floating, maybe_mouse) {
                    (Some(prev_floating), Some(mouse)) => {
                        if mouse.buttons.left().is_down() {
                            Some(new_floating())
                        } else {
                            Some(prev_floating)
                        }
                    },
                    (Some(prev_floating), None) => Some(prev_floating),
                    _ => Some(new_floating()),
                }
            },
            None => Some(new_floating()),
        }
    } else {
        None
    };

    // Retrieve the area upon which kid widgets will be placed.
    let kid_area = {
        let args: KidAreaArgs<W> = KidAreaArgs {
            rect: rect,
            style: &new_style,
            theme: &ui.theme,
            fonts: &ui.fonts,
        };
        widget.kid_area(args)
    };

    // If either axis is scrollable, retrieve the up-to-date `scroll::State` for that axis.
    //
    // We must step the scrolling using the previous `kid_area` state so that the bounding box
    // around our kid widgets is in sync with the position of the `kid_area`.
    let prev_kid_area = maybe_prev_common.map(|common| common.kid_area)
        .unwrap_or_else(|| kid_area);

    // If the widget is scrollable, check for given `Scroll` events.
    //
    // TODO: On the first time the widget is set (i.e. if `maybe_prev_*_scroll_state` is `None` and
    // `maybe_*_scroll` is `Some`) we should consider and handle the `scroll_args`'
    // `maybe_initial_alignment` field.
    let mut maybe_x_scroll_state = widget.common().maybe_x_scroll.map(|_scroll_args| {
        let maybe_prev = maybe_prev_common.as_ref().and_then(|p| p.maybe_x_scroll_state);
        scroll::State::update(ui, id, &prev_kid_area, maybe_prev, 0.0)
    });
    let mut maybe_y_scroll_state = widget.common().maybe_y_scroll.map(|_scroll_args| {
        let maybe_prev = maybe_prev_common.as_ref().and_then(|p| p.maybe_y_scroll_state);
        scroll::State::update(ui, id, &prev_kid_area, maybe_prev, 0.0)
    });

    for scroll in ui.widget_input(id).scrolls() {

        if widget.common().maybe_x_scroll.is_some() {
            maybe_x_scroll_state =
                Some(scroll::State::update(ui, id, &prev_kid_area, maybe_x_scroll_state, scroll.x))
        }

        if widget.common().maybe_y_scroll.is_some() {
            maybe_y_scroll_state =
                Some(scroll::State::update(ui, id, &prev_kid_area, maybe_y_scroll_state, scroll.y))
        }
    }

    // Determine whether or not this is the first time set has been called.
    // We'll use this to determine whether or not we need to draw for the first time.
    let is_first_set = maybe_prev_common.is_none();

    // Update all positioning and dimension related data prior to calling `Widget::update`.
    // We do this so that if this widget were to internally `set` some other `Widget`s, this
    // `Widget`s positioning and dimension data already exists within the `Graph`.
    {
        // Some widget to which this widget is relatively positioned (if there is one).
        let maybe_positioned_relatively_id = |pos: Position| match pos {
            Position::Relative(_, maybe_id) => maybe_id.or(maybe_prev_widget_id),
            Position::Absolute(_) => None,
        };

        let maybe_x_positioned_relatively_id = maybe_positioned_relatively_id(x_pos);
        let maybe_y_positioned_relatively_id = maybe_positioned_relatively_id(y_pos);

        // Retrieve whether or not the widget's children should be cropped to it.
        let crop_kids = widget.common().crop_kids;

        // This will cache the given data into the `ui`'s `widget_graph`.
        let ui: &mut Ui = ui::ref_mut_from_ui_cell(ui);
        ui::pre_update_cache(ui, PreUpdateCache {
            type_id: type_id,
            id: id,
            maybe_parent_id: maybe_parent_id,
            maybe_x_positioned_relatively_id: maybe_x_positioned_relatively_id,
            maybe_y_positioned_relatively_id: maybe_y_positioned_relatively_id,
            rect: rect,
            depth: depth,
            kid_area: kid_area,
            maybe_floating: maybe_floating,
            crop_kids: crop_kids,
            maybe_y_scroll_state: maybe_y_scroll_state,
            maybe_x_scroll_state: maybe_x_scroll_state,
            maybe_graphics_for: widget.common().maybe_graphics_for,
            is_over: widget.is_over(),
        });
    }

    // Unwrap the widget's previous common state. If there is no previous common state, we'll
    // use the new state in it's place.
    let prev_common = maybe_prev_common.unwrap_or_else(|| CommonState {
        rect: rect,
        depth: depth,
        maybe_floating: maybe_floating,
        kid_area: kid_area,
        maybe_x_scroll_state: maybe_x_scroll_state,
        maybe_y_scroll_state: maybe_y_scroll_state,
    });

    // Retrieve the widget's unique state and update it via `Widget::update`.
    let (unique_state, has_state_updated, event) = {

        // Unwrap our unique widget state. If there is no previous state to unwrap, call the
        // `init_state` method to construct some initial state.
        let mut unique_state = maybe_prev_unique_state.unwrap_or_else(|| {
            widget.init_state(ui.widget_id_generator())
        });
        let (has_updated, event) = {

            // A wrapper around the widget's unique state in order to keep track of whether or not it
            // has been updated during the `Widget::update` method.
            let mut state = State {
                state: &mut unique_state,
                has_updated: false,
            };

            let event = widget.update(UpdateArgs {
                id: id,
                maybe_parent_id: maybe_parent_id,
                state: &mut state,
                prev: &prev_common,
                rect: rect,
                style: &new_style,
                ui: ui,
            });

            (state.has_updated, event)
        };

        (unique_state, has_updated, event)
    };

    // Determine whether or not the `State` has changed.
    let state_has_changed = has_state_updated
        || rect != prev_common.rect
        || depth != prev_common.depth
        || is_first_set;

    // Determine whether or not the widget's `Style` has changed.
    let style_has_changed = maybe_prev_style.map(|style| style != new_style).unwrap_or(false);

    // We need to know if the scroll state has changed to see if we need to redraw.
    let scroll_has_changed =
        maybe_x_scroll_state != maybe_prev_common.as_ref().and_then(|p| p.maybe_x_scroll_state)
        || maybe_y_scroll_state != maybe_prev_common.as_ref().and_then(|p| p.maybe_y_scroll_state);

    // We only need to redraw if some visible part of our widget has changed.
    let requires_redraw = style_has_changed || state_has_changed || scroll_has_changed;

    let ui: &mut Ui = ui::ref_mut_from_ui_cell(ui);

    // If we require a redraw, we should notify the `Ui`.
    if requires_redraw {
        ui.needs_redraw();
    }

    // Finally, cache the `Widget`'s newly updated `State` and `Style` within the `ui`'s
    // `widget_graph`.
    ui::post_update_cache::<W>(ui, PostUpdateCache {
        id: id,
        maybe_parent_id: maybe_parent_id,
        state: unique_state,
        style: new_style,
    });

    event
}


impl<'a, T> State<'a, T> {

    /// Mutate the internal widget state and set a flag notifying us that there has been a mutation.
    ///
    /// If this method is *not* called, we assume that there has been no mutation and in turn we do
    /// not need to re-draw the Widget.
    ///
    /// If this method *is* called, we assume that there has been some mutation and in turn will
    /// need to re-draw the Widget. Thus, it is recommended that you *only* call this method if you
    /// need to update the unique state in some way.
    pub fn update<F>(&mut self, f: F) where F: FnOnce(&mut T) {
        self.has_updated = true;
        f(self.state);
    }

}

impl<'a, T> std::ops::Deref for State<'a, T> {
    type Target = T;
    fn deref(&self) -> &T {
        &self.state
    }
}

impl Default for CommonBuilder {
    fn default() -> Self {
        CommonBuilder {
            style: CommonStyle::default(),
            maybe_parent_id: MaybeParent::Unspecified,
            place_on_kid_area: true,
            maybe_graphics_for: None,
            is_floating: false,
            maybe_x_scroll: None,
            maybe_y_scroll: None,
            crop_kids: false,
        }
    }
}

impl<W> Positionable for W
    where W: Widget,
{
    fn x_position(mut self, x: Position) -> Self {
        self.common_mut().style.maybe_x_position = Some(x);
        self
    }
    fn y_position(mut self, y: Position) -> Self {
        self.common_mut().style.maybe_y_position = Some(y);
        self
    }
    fn get_x_position(&self, ui: &Ui) -> Position {
        let from_y_position = || self.common().style.maybe_y_position
            .and_then(|y_pos| infer_position_from_other_position(y_pos, Align::Start));
        self.common().style.maybe_x_position
            .or_else(from_y_position)
            .unwrap_or(self.default_x_position(ui))
    }
    fn get_y_position(&self, ui: &Ui) -> Position {
        let from_x_position = || self.common().style.maybe_x_position
            .and_then(|x_pos| infer_position_from_other_position(x_pos, Align::End));
        self.common().style.maybe_y_position
            .or_else(from_x_position)
            .unwrap_or(self.default_y_position(ui))
    }
    fn depth(mut self, depth: Depth) -> Self {
        self.common_mut().style.maybe_depth = Some(depth);
        self
    }
    fn get_depth(&self) -> Depth {
        const DEFAULT_DEPTH: Depth = 0.0;
        self.common().style.maybe_depth.unwrap_or(DEFAULT_DEPTH)
    }
}


/// In the case that a position hasn't been given for one of the axes, we must first check to see
/// if we can infer the missing axis position from the other axis.
///
/// This is used within the impl of **Positionable** for **Widget**.
fn infer_position_from_other_position(other_pos: Position, dir_align: Align) -> Option<Position> {
    match other_pos {
        Position::Relative(Relative::Direction(_, _), maybe_id) =>
            Some(Position::Relative(Relative::Align(dir_align), maybe_id)),
        Position::Relative(Relative::Place(_), maybe_id) =>
            Some(Position::Relative(Relative::Align(Align::Middle), maybe_id)),
        Position::Relative(Relative::Scalar(_), maybe_id) =>
            Some(Position::Relative(Relative::Scalar(0.0), maybe_id)),
        Position::Relative(Relative::Align(_), _) |
        Position::Absolute(_) => None,
    }
}


impl<W> Sizeable for W
    where W: Widget,
{
    fn x_dimension(mut self, w: Dimension) -> Self {
        self.common_mut().style.maybe_x_dimension = Some(w);
        self
    }
    fn y_dimension(mut self, h: Dimension) -> Self {
        self.common_mut().style.maybe_y_dimension = Some(h);
        self
    }
    /// We attempt to retrieve the `x` **Dimension** for the widget via the following:
    /// - Check for specified value at `maybe_x_dimension`
    /// - Otherwise, use the default returned by **Widget::default_x_dimension**.
    fn get_x_dimension(&self, ui: &Ui) -> Dimension {
        self.common().style.maybe_x_dimension.unwrap_or_else(|| self.default_x_dimension(ui))
    }
    /// We attempt to retrieve the `y` **Dimension** for the widget via the following:
    /// - Check for specified value at `maybe_y_dimension`
    /// - Otherwise, use the default returned by **Widget::default_y_dimension**.
    fn get_y_dimension(&self, ui: &Ui) -> Dimension {
        self.common().style.maybe_y_dimension.unwrap_or_else(|| self.default_y_dimension(ui))
    }
}