Trait Widget

pub trait Widget: Layout {
    type Data;

    // Provided methods
    fn as_node<'a>(&'a mut self, data: &'a Self::Data) -> Node<'a> { ... }
    fn for_child_node(
        &mut self,
        data: &Self::Data,
        index: usize,
        closure: Box<dyn FnOnce(Node<'_>) + '_>,
    ) { ... }
}

The Widget trait

The primary widget trait covers event handling over super trait Layout which governs layout, drawing, child enumeration and identification. Most methods of Widget are hidden and only for use within the Kas library.

Widget is dyn-safe given a type parameter, e.g. dyn Widget<Data = ()>. Layout is dyn-safe without a type parameter. Node is a dyn-safe abstraction over a &dyn Widget<Data = T> plus a &T data parameter.

Widget lifecycle

1. The widget is configured (Events::configure) and immediately updated (Events::update).
2. The widget has its size-requirements checked by calling Layout::size_rules for each axis.
3. Layout::set_rect is called to position elements. This may use data cached by size_rules.
4. The widget is updated again after any data change (see ConfigCx::update).
5. The widget is ready for event-handling and drawing (Events::handle_event, Layout::find_id, Layout::draw).

Widgets are responsible for ensuring that their children may observe this lifecycle. Usually this simply involves inclusion of the child in layout operations. Steps of the lifecycle may be postponed until a widget becomes visible.

Implementing Widget

To implement a widget, use the #widget macro within an impl_scope. This is the only supported method of implementing Widget.

Explicit (partial) implementations of Widget, Layout and Events are optional. The #widget macro completes implementations.

Synopsis:

impl_scope! {
    #[widget {
        // macro properties (all optional)
        Data = T;
        layout = self.foo;
    }]
    struct MyWidget {
        core: widget_core!(),
        #[widget] foo: impl Widget<Data = T> = make_foo(),
        // ...
    }

    // Optional implementations:
    impl Layout for Self { /* ... */ }
    impl Events for Self { /* ... */ }
    impl Self { /* ... */ }
}

Details may be categorised as follows:

• Data: the type Widget::Data must be specified exactly once, but this type may be given in any of three locations: as a property of the #widget macro or as Widget::Data.
• Core methods of Layout are always implemented via the #widget macro, whether or not an impl Layout { ... } item is present.
• Introspection methods Layout::num_children, Layout::get_child and Widget::for_child_node are implemented by the #widget macro in most cases: child widgets embedded within a layout descriptor or included as fields marked with #[widget] are enumerated.
• Introspection methods Layout::find_child_index and Events::make_child_id have default implementations which usually suffice.
• Layout is specified either via layout syntax or via implementation of at least Layout::size_rules and Layout::draw (optionally also set_rect, nav_next, translation and find_id).
• Event handling is optional, implemented through Events.

For examples, check the source code of widgets in the widgets library or examples apps. (Check that the code uses the same Kas version since the widget traits are not yet stable.)

Required Associated Types

type Data

Input data type

Widget expects data of this type to be provided by reference when calling any event-handling operation on this widget.

This type may be specified using a #widget macro property in case this trait is not explicitly implemented.

Provided Methods

fn as_node<'a>(&'a mut self, data: &'a Self::Data) -> Node<'a>

Erase type

This method is implemented by the #[widget] macro.

fn for_child_node( &mut self, data: &Self::Data, index: usize, closure: Box<dyn FnOnce(Node<'_>) + '_>, )

Call closure on child with given index, if index < self.num_children().

Widgets with no children or using the #[widget] attribute on fields do not need to implement this. Widgets with an explicit implementation of Layout::num_children also need to implement this.

It is recommended to use the methods on Node instead of calling this method.

Implementations on Foreign Types

impl<T: Widget + ?Sized> Widget for &mut T

type Data = <T as Widget>::Data

fn as_node<'a>(&'a mut self, data: &'a Self::Data) -> Node<'a>

fn for_child_node( &mut self, data: &Self::Data, index: usize, closure: Box<dyn FnOnce(Node<'_>) + '_>, )

impl<T: Widget + ?Sized> Widget for Box<T>

type Data = <T as Widget>::Data

fn as_node<'a>(&'a mut self, data: &'a Self::Data) -> Node<'a>

fn for_child_node( &mut self, data: &Self::Data, index: usize, closure: Box<dyn FnOnce(Node<'_>) + '_>, )

Implementors

impl Widget for TitleBar

type Data = ()

impl Widget for TitleBarButtons

type Data = ()

impl<Data: 'static> Widget for Window<Data>

type Data = Data

impl<W: Widget> Widget for Popup<W>

type Data = <W as Widget>::Data