Updating example for MultiChildLayoutDelegate (#15597)
diff --git a/packages/flutter/lib/src/rendering/custom_layout.dart b/packages/flutter/lib/src/rendering/custom_layout.dart
index 7ed7543..46e9939 100644
--- a/packages/flutter/lib/src/rendering/custom_layout.dart
+++ b/packages/flutter/lib/src/rendering/custom_layout.dart
@@ -41,26 +41,43 @@
/// Used with [CustomMultiChildLayout], the widget for the
/// [RenderCustomMultiChildLayoutBox] render object.
///
-/// ## Example
+/// Each child must be wrapped in a [LayoutId] widget to assign the id that
+/// identifies it to the delegate. The [LayoutId.id] needs to be unique among
+/// the children that the [CustomMultiChildLayout] manages.
+///
+/// ## Sample code
///
/// Below is an example implementation of [performLayout] that causes one widget
-/// to be the same size as another:
+/// (the follower) to be the same size as another (the leader):
///
/// ```dart
-/// @override
-/// void performLayout(Size size) {
-/// Size followerSize = Size.zero;
-///
-/// if (hasChild(_Slots.leader) {
-/// followerSize = layoutChild(_Slots.leader, new BoxConstraints.loose(size));
-/// positionChild(_Slots.leader, Offset.zero);
+/// // Define your own slot numbers, depending upon the id assigned by LayoutId.
+/// // Typical usage is to define an enum like the one below, and use those
+/// // values as the ids.
+/// enum _Slot {
+/// leader,
+/// follower,
+/// }
+///
+/// class FollowTheLeader extends MultiChildLayoutDelegate {
+/// @override
+/// void performLayout(Size size) {
+/// Size leaderSize = Size.zero;
+///
+/// if (hasChild(_Slot.leader)) {
+/// leaderSize = layoutChild(_Slot.leader, new BoxConstraints.loose(size));
+/// positionChild(_Slot.leader, Offset.zero);
+/// }
+///
+/// if (hasChild(_Slot.follower)) {
+/// layoutChild(_Slot.follower, new BoxConstraints.tight(leaderSize));
+/// positionChild(_Slot.follower, new Offset(size.width - leaderSize.width,
+/// size.height - leaderSize.height));
+/// }
/// }
-///
-/// if (hasChild(_Slots.follower)) {
-/// layoutChild(_Slots.follower, new BoxConstraints.tight(followerSize));
-/// positionChild(_Slots.follower, new Offset(size.width - followerSize.width,
-/// size.height - followerSize.height));
-/// }
+///
+/// @override
+/// bool shouldRelayout(MultiChildLayoutDelegate oldDelegate) => false;
/// }
/// ```
///
diff --git a/packages/flutter/lib/src/widgets/basic.dart b/packages/flutter/lib/src/widgets/basic.dart
index e431039..0a1cbad 100644
--- a/packages/flutter/lib/src/widgets/basic.dart
+++ b/packages/flutter/lib/src/widgets/basic.dart
@@ -1448,7 +1448,7 @@
}
}
-/// Meta data for identifying children in a [CustomMultiChildLayout].
+/// Metadata for identifying children in a [CustomMultiChildLayout].
///
/// The [MultiChildLayoutDelegate.hasChild],
/// [MultiChildLayoutDelegate.layoutChild], and
@@ -1466,6 +1466,9 @@
super(key: key ?? new ValueKey<Object>(id), child: child);
/// An object representing the identity of this child.
+ ///
+ /// The [id] needs to be unique among the children that the
+ /// [CustomMultiChildLayout] manages.
final Object id;
@override
diff --git a/packages/flutter/lib/src/widgets/framework.dart b/packages/flutter/lib/src/widgets/framework.dart
index 5474783..00a2cd6 100644
--- a/packages/flutter/lib/src/widgets/framework.dart
+++ b/packages/flutter/lib/src/widgets/framework.dart
@@ -4462,13 +4462,17 @@
@override
void deactivate() {
super.deactivate();
- assert(!renderObject.attached);
+ assert(!renderObject.attached,
+ 'A RenderObject was still attached when attempting to deactivate its '
+ 'RenderObjectElement: $renderObject');
}
@override
void unmount() {
super.unmount();
- assert(!renderObject.attached);
+ assert(!renderObject.attached,
+ 'A RenderObject was still attached when attempting to unmount its '
+ 'RenderObjectElement: $renderObject');
widget.didUnmountRenderObject(renderObject);
}
