Struct druid::widget::LensWrap

pub struct LensWrap<T, U, L, W> { /* private fields */ }

A wrapper for its widget subtree to have access to a part of its parent’s data.

Every widget in Druid is instantiated with access to data of some type; the root widget has access to the entire application data. Often, a part of the widget hierarchy is only concerned with a part of that data. The LensWrap widget is a way to “focus” the data reference down, for the subtree. One advantage is performance; data changes that don’t intersect the scope of the lens aren’t propagated.

Another advantage is generality and reuse. If a widget (or tree of widgets) is designed to work with some chunk of data, then with a lens that same code can easily be reused across all occurrences of that chunk within the application state.

This wrapper takes a Lens as an argument, which is a specification of a struct field, or some other way of narrowing the scope.

Implementations

impl<T, U, L, W> LensWrap<T, U, L, W>

pub fn new(child: W, lens: L) -> LensWrap<T, U, L, W>

Wrap a widget with a lens.

When the lens has type Lens<T, U>, the child widget has data of type U, and the wrapped widget has data of type T.

Examples found in repository

examples/switches.rs (line 37)

fn build_widget() -> impl Widget<DemoState> {
    let mut col = Flex::column();
    let mut row = Flex::row();
    let switch = LensWrap::new(Switch::new(), DemoState::value);
    let check_box = LensWrap::new(Checkbox::new(""), DemoState::value);
    let switch_label = Label::new("Setting label");

    row.add_child(Padding::new(5.0, switch_label));
    row.add_child(Padding::new(5.0, switch));
    row.add_child(Padding::new(5.0, check_box));

    let stepper = LensWrap::new(
        Stepper::new()
            .with_range(0.0, 10.0)
            .with_step(0.5)
            .with_wraparound(false),
        DemoState::stepper_value,
    );

    let mut textbox_row = Flex::row();
    // TODO: Replace Parse usage with TextBox::with_formatter
    #[allow(deprecated)]
    let textbox = LensWrap::new(
        Parse::new(TextBox::new()),
        DemoState::stepper_value.map(|x| Some(*x), |x, y| *x = y.unwrap_or(0.0)),
    );
    textbox_row.add_child(Padding::new(5.0, textbox));
    textbox_row.add_child(Padding::new(5.0, stepper.center()));

    let mut label_row = Flex::row();

    let label = Label::new(|data: &DemoState, _env: &_| {
        format!("Stepper value: {0:.2}", data.stepper_value)
    });

    label_row.add_child(Padding::new(5.0, label));

    col.set_main_axis_alignment(MainAxisAlignment::Center);
    col.add_child(Padding::new(5.0, row));
    col.add_child(Padding::new(5.0, textbox_row));
    col.add_child(Padding::new(5.0, label_row));
    col.center()
}

examples/styled_text.rs (lines 133-136)

fn ui_builder() -> impl Widget<AppData> {
    let my_painter = Painter::new(|ctx, _, _| {
        let bounds = ctx.size().to_rect();
        if ctx.is_hot() {
            ctx.fill(bounds, &Color::rgba8(0, 0, 0, 128));
        }

        if ctx.is_active() {
            ctx.stroke(bounds, &Color::WHITE, 2.0);
        }
    });

    // This is Druid's default text style.
    // It's set by theme::LABEL_COLOR and theme::UI_FONT
    let label =
        Label::new(|data: &String, _env: &_| format!("Default: {data}")).lens(AppData::text);

    // The text_color, text_size, and font builder methods can override the
    // defaults provided by the theme by passing in a Key or a concrete value.
    //
    // In this example, text_color receives a Key from the theme, text_size
    // gets a custom key which we set with the env_scope wrapper, and the
    // default font key (theme::FONT_NAME) is overridden in the env_scope
    // wrapper. (Like text_color and text_size, the font can be set using the
    // with_font builder method, but overriding here makes it easy to fall back
    // to the default font)
    let styled_label = Label::new(|data: &AppData, _env: &_| format!("{data}"))
        .with_text_color(theme::PRIMARY_LIGHT)
        .with_font(MY_CUSTOM_FONT)
        .background(my_painter)
        .on_click(|_, data, _| {
            data.size *= 1.1;
        })
        .env_scope(|env: &mut druid::Env, data: &AppData| {
            let new_font = if data.mono {
                FontDescriptor::new(FontFamily::MONOSPACE)
            } else {
                FontDescriptor::new(FontFamily::SYSTEM_UI)
            }
            .with_size(data.size);
            env.set(MY_CUSTOM_FONT, new_font);
        });

    let labels = Scroll::new(
        Flex::column()
            .cross_axis_alignment(CrossAxisAlignment::Start)
            .with_child(label)
            .with_default_spacer()
            .with_child(styled_label),
    )
    .expand_height()
    .fix_width(COLUMN_WIDTH);

    let stepper = Stepper::new()
        .with_range(0.0, 100.0)
        .with_step(1.0)
        .with_wraparound(false)
        .lens(AppData::size);

    // TODO: Replace Parse usage with TextBox::with_formatter
    #[allow(deprecated)]
    let stepper_textbox = LensWrap::new(
        Parse::new(TextBox::new()),
        AppData::size.map(|x| Some(*x), |x, y| *x = y.unwrap_or(24.0)),
    );

    let mono_checkbox = Checkbox::new("Monospace").lens(AppData::mono);
    let stepper_row = Flex::row()
        .with_child(stepper_textbox)
        .with_child(stepper)
        .with_default_spacer()
        .with_child(mono_checkbox);

    let input = TextBox::multiline()
        .with_placeholder("Your sample text here :)")
        .fix_width(COLUMN_WIDTH)
        .fix_height(140.0)
        .lens(AppData::text);

    Flex::column()
        .main_axis_alignment(MainAxisAlignment::Center)
        .with_default_spacer()
        .with_flex_child(labels, 1.0)
        .with_default_spacer()
        .with_child(input)
        .with_default_spacer()
        .with_child(stepper_row)
        .with_default_spacer()
}

pub fn lens(&self) -> &L

Get a reference to the lens.

pub fn lens_mut(&mut self) -> &mut L

Get a mutable reference to the lens.

Trait Implementations

impl<T, U, L, W> Widget<T> for LensWrap<T, U, L, W>where T: Data, U: Data, L: Lens<T, U>, W: Widget<U>,

fn event( &mut self, ctx: &mut EventCtx<'_, '_>, event: &Event, data: &mut T, env: &Env )

Handle an event.

fn lifecycle( &mut self, ctx: &mut LifeCycleCtx<'_, '_>, event: &LifeCycle, data: &T, env: &Env )

Handle a life cycle notification.

fn update(&mut self, ctx: &mut UpdateCtx<'_, '_>, old_data: &T, data: &T, env: &Env)

Update the widget’s appearance in response to a change in the app’s Data or Env.

fn layout( &mut self, ctx: &mut LayoutCtx<'_, '_>, bc: &BoxConstraints, data: &T, env: &Env ) -> Size

Compute layout.

fn paint(&mut self, ctx: &mut PaintCtx<'_, '_, '_>, data: &T, env: &Env)

Paint the widget appearance.

fn compute_max_intrinsic( &mut self, axis: Axis, ctx: &mut LayoutCtx<'_, '_>, bc: &BoxConstraints, data: &T, env: &Env ) -> f64

Computes max intrinsic/preferred dimension of a widget on the provided axis.

impl<T, U, L, W> WidgetWrapper for LensWrap<T, U, L, W>

type Wrapped = W

The type of the wrapped widget. Maybe we would like to constrain this to Widget<impl Data> (if existential bounds were supported). Any other scheme leads to T being unconstrained in unification at some point

fn wrapped(&self) -> &Self::Wrapped

Get immutable access to the wrapped child

fn wrapped_mut(&mut self) -> &mut Self::Wrapped

Get mutable access to the wrapped child