adding added controller widget

druid/src/widget/added.rs

@@ -0,0 +1,62 @@
+// Copyright 2020 The Druid Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! A [`Controller`] widget that responds to [`LifeCycle::WidgetAdded`] event.
+//!
+//! [`Controller`]: crate::widget::Controller
+//! [`LifeCycle::WidgetAdded`]: crate::LifeCycle::WidgetAdded
+
+use crate::widget::Controller;
+use crate::{Data, Env, LifeCycleCtx, Widget};
+
+/// This [`Controller`] widget responds to [`LifeCycle::WidgetAdded`] event
+/// with the provided closure. Pass this and a child widget to [`ControllerHost`]
+/// to respond to the event when the child widget is added to the widget tree.
+/// This is also available, for convenience, as an `on_added` method
+/// via [`WidgetExt`].
+///
+/// [`Controller`]: crate::widget::Controller
+/// [`ControllerHost`]: crate::widget::ControllerHost
+/// [`WidgetExt`]: crate::widget::WidgetExt
+/// [`LifeCycle::WidgetAdded`]: crate::LifeCycle::WidgetAdded
+pub struct Added<T, W> {
+    /// A closure that will be invoked when the child widget is added
+    /// to the widget tree
+    action: Box<dyn Fn(&mut W, &mut LifeCycleCtx, &T, &Env)>,
+}
+
+impl<T: Data, W: Widget<T>> Added<T, W> {
+    /// Create a new [`Controller`] widget to respond to widget added to tree event.
+    pub fn new(action: impl Fn(&mut W, &mut LifeCycleCtx, &T, &Env) + 'static) -> Self {
+        Self {
+            action: Box::new(action),
+        }
+    }
+}
+
+impl<T: Data, W: Widget<T>> Controller<T, W> for Added<T, W> {
+    fn lifecycle(
+        &mut self,
+        child: &mut W,
+        ctx: &mut LifeCycleCtx,
+        event: &crate::LifeCycle,
+        data: &T,
+        env: &Env,
+    ) {
+        if let crate::LifeCycle::WidgetAdded = event {
+            (self.action)(child, ctx, data, env);
+        }
+        child.lifecycle(ctx, event, data, env)
+    }
+}

druid/src/widget/mod.rs

@@ -14,6 +14,7 @@
 
 //! Common widgets.
 
+mod added;
 mod align;
 mod button;
 mod checkbox;
@@ -55,6 +56,7 @@ mod widget;
 mod widget_ext;
 
 pub use self::image::Image;
+pub use added::Added;
 pub use align::Align;
 pub use button::Button;
 pub use checkbox::Checkbox;

druid/src/widget/widget_ext.rs

@@ -16,10 +16,12 @@
 
 use super::invalidation::DebugInvalidation;
 use super::{
-    Align, BackgroundBrush, Click, Container, Controller, ControllerHost, EnvScope,
+    Added, Align, BackgroundBrush, Click, Container, Controller, ControllerHost, EnvScope,
     IdentityWrapper, LensWrap, Padding, Parse, SizedBox, WidgetId,
 };
-use crate::{Color, Data, Env, EventCtx, Insets, KeyOrValue, Lens, UnitPoint, Widget};
+use crate::{
+    Color, Data, Env, EventCtx, Insets, KeyOrValue, Lens, LifeCycleCtx, UnitPoint, Widget,
+};
 
 /// A trait that provides extra methods for combining `Widget`s.
 pub trait WidgetExt<T: Data>: Widget<T> + Sized + 'static {
@@ -159,6 +161,21 @@ pub trait WidgetExt<T: Data>: Widget<T> + Sized + 'static {
         ControllerHost::new(self, controller)
     }
 
+    /// Provide a closure that will be called when this widget is added to the widget tree.
+    ///
+    /// You can use this to perform any initial setup.
+    ///
+    /// This is equivalent to handling the [`LifeCycle::WidgetAdded`] event in a
+    /// custom [`Controller`].
+    ///
+    /// [`LifeCycle::WidgetAdded`]: crate::LifeCycle::WidgetAdded
+    fn on_added<W: Widget<T>>(
+        self,
+        f: impl Fn(&mut W, &mut LifeCycleCtx, &T, &Env) + 'static,
+    ) -> ControllerHost<Self, Added<T, W>> {
+        ControllerHost::new(self, Added::new(f))
+    }
+
     /// Control the events of this widget with a [`Click`] widget. The closure
     /// provided will be called when the widget is clicked with the left mouse
     /// button.