Add the LifeCycle::Size event.

CHANGELOG.md

@@ -46,6 +46,7 @@ This means that druid no longer requires cairo on macOS and uses Core Graphics i
 - `WidgetExt::debug_widget_id`, for displaying widget ids on hover. ([#876] by [@cmyr])
 - `im` feature, with `Data` support for the [`im` crate](https://docs.rs/im/) collections. ([#924] by [@cmyr])
 - `im::Vector` support for the `List` widget. ([#940] by [@xStrom])
+- `LifeCycle::Size` event to inform widgets that their size changed. ([#953] by [@xStrom])
 
 ### Changed
 
@@ -194,6 +195,7 @@ This means that druid no longer requires cairo on macOS and uses Core Graphics i
 [#942]: https://github.com/xi-editor/druid/pull/942
 [#943]: https://github.com/xi-editor/druid/pull/943
 [#951]: https://github.com/xi-editor/druid/pull/951
+[#953]: https://github.com/xi-editor/druid/pull/953
 
 ## [0.5.0] - 2020-04-01
 

druid/src/core.rs

@@ -188,13 +188,37 @@ impl<T, W: Widget<T>> WidgetPod<T, W> {
         self.state.id
     }
 
-    /// Set layout rectangle.
+    /// Set the layout [`Rect`].
     ///
-    /// Intended to be called on child widget in container's `layout`
-    /// implementation.
+    /// A container widget should call the [`Widget::layout`] method on its children in
+    /// its own [`Widget::layout`] implementation, then possibly modify the returned [`Size`], and
+    /// finally call this `set_layout_rect` method on the child to set the final layout [`Rect`].
+    ///
+    /// The child will receive the [`LifeCycle::Size`] event informing them of the final [`Size`].
+    ///
+    /// [`Widget::layout`]: trait.Widget.html#tymethod.layout
+    /// [`Rect`]: struct.Rect.html
+    /// [`Size`]: struct.Size.html
+    /// [`LifeCycle::Size`]: enum.LifeCycle.html#variant.Size
     pub fn set_layout_rect(&mut self, ctx: &mut LayoutCtx, data: &T, env: &Env, layout_rect: Rect) {
+        let mut needs_merge = false;
+
+        let old_size = self.state.layout_rect.map(|r| r.size());
+        let new_size = layout_rect.size();
+
         self.state.layout_rect = Some(layout_rect);
 
+        if old_size.is_none() || old_size.unwrap() != new_size {
+            let mut child_ctx = LifeCycleCtx {
+                command_queue: ctx.command_queue,
+                base_state: &mut self.state,
+                window_id: ctx.window_id,
+            };
+            let size_event = LifeCycle::Size(new_size);
+            self.inner.lifecycle(&mut child_ctx, &size_event, data, env);
+            needs_merge = true;
+        }
+
         if WidgetPod::set_hot_state(
             &mut self.inner,
             ctx.command_queue,
@@ -205,6 +229,10 @@ impl<T, W: Widget<T>> WidgetPod<T, W> {
             data,
             env,
         ) {
+            needs_merge = true;
+        }
+
+        if needs_merge {
             ctx.base_state.merge_up(&self.state);
         }
     }
@@ -215,9 +243,12 @@ impl<T, W: Widget<T>> WidgetPod<T, W> {
         self.layout_rect()
     }
 
-    /// The layout rectangle.
+    /// Returns the layout [`Rect`].
+    ///
+    /// This will be the same [`Rect`] that was set by [`set_layout_rect`].
     ///
-    /// This will be same value as set by `set_layout_rect`.
+    /// [`Rect`]: struct.Rect.html
+    /// [`set_layout_rect`]: #method.set_layout_rect
     pub fn layout_rect(&self) -> Rect {
         self.state.layout_rect.unwrap_or_default()
     }
@@ -790,6 +821,11 @@ impl<T: Data, W: Widget<T>> WidgetPod<T, W> {
 
                 true
             }
+            LifeCycle::Size(_) => {
+                // We are a descendant of a widget that received the Size event.
+                // This event was meant only for our parent, so don't recurse.
+                false
+            }
             //NOTE: this is not sent here, but from the special set_hot_state method
             LifeCycle::HotChanged(_) => false,
             LifeCycle::FocusChanged(_) => {

druid/src/event.rs

@@ -178,6 +178,14 @@ pub enum LifeCycle {
     /// [`WidgetPod`]: struct.WidgetPod.html
     /// [`LifeCycleCtx::register_for_focus`]: struct.LifeCycleCtx.html#method.register_for_focus
     WidgetAdded,
+    /// Called when the size of the widget changes.
+    ///
+    /// The [`Size`] is derived from the [`Rect`] that was set with [`WidgetPod::set_layout_rect`].
+    ///
+    /// [`Size`]: struct.Size.html
+    /// [`Rect`]: struct.Rect.html
+    /// [`WidgetPod::set_layout_rect`]: struct.WidgetPod.html#method.set_layout_rect
+    Size(Size),
     /// Called at the beginning of a new animation frame.
     ///
     /// On the first frame when transitioning from idle to animating, `interval`