Sprinkle some mixin magic incantations (#10442)

This prevents some of our mixins from being subclassed.

Also, move mixins to using 'extends' instead of 'implements' for
future compatibility with Dart changes.

Also, rename a class that had Mixin in the name but was not a mixin.

packages/flutter/lib/src/animation/animations.dart

@@ -115,6 +115,10 @@ class AlwaysStoppedAnimation<T> extends Animation<T> {
 /// given [parent] Animation. To implement an [Animation] that proxies to a
 /// parent, this class plus implementing "T get value" is all that is necessary.
 abstract class AnimationWithParentMixin<T> {
+  // This class is intended to be used as a mixin, and should not be
+  // extended directly.
+  factory AnimationWithParentMixin._() => null;
+
   /// The animation whose value this animation will proxy.
   ///
   /// This animation must remain the same for the lifetime of this object. If

packages/flutter/lib/src/animation/listener_helpers.dart

@@ -9,12 +9,20 @@ import 'package:flutter/foundation.dart';
 import 'animation.dart';
 
 abstract class _ListenerMixin {
+  // This class is intended to be used as a mixin, and should not be
+  // extended directly.
+  factory _ListenerMixin._() => null;
+
   void didRegisterListener();
   void didUnregisterListener();
 }
 
 /// A mixin that helps listen to another object only when this object has registered listeners.
-abstract class AnimationLazyListenerMixin implements _ListenerMixin {
+abstract class AnimationLazyListenerMixin extends _ListenerMixin {
+  // This class is intended to be used as a mixin, and should not be
+  // extended directly.
+  factory AnimationLazyListenerMixin._() => null;
+
   int _listenerCounter = 0;
 
   @override
@@ -47,7 +55,11 @@ abstract class AnimationLazyListenerMixin implements _ListenerMixin {
 
 /// A mixin that replaces the didRegisterListener/didUnregisterListener contract
 /// with a dispose contract.
-abstract class AnimationEagerListenerMixin implements _ListenerMixin {
+abstract class AnimationEagerListenerMixin extends _ListenerMixin {
+  // This class is intended to be used as a mixin, and should not be
+  // extended directly.
+  factory AnimationEagerListenerMixin._() => null;
+
   @override
   void didRegisterListener() { }
 
@@ -60,9 +72,13 @@ abstract class AnimationEagerListenerMixin implements _ListenerMixin {
   void dispose() { }
 }
 
-/// A mixin that implements the addListener/removeListener protocol and notifies
+/// A mixin that implements the [addListener]/[removeListener] protocol and notifies
-/// all the registered listeners when notifyListeners is called.
+/// all the registered listeners when [notifyListeners] is called.
 abstract class AnimationLocalListenersMixin extends _ListenerMixin {
+  // This class is intended to be used as a mixin, and should not be
+  // extended directly.
+  factory AnimationLocalListenersMixin._() => null;
+
   final ObserverList<VoidCallback> _listeners = new ObserverList<VoidCallback>();
 
   /// Calls the listener every time the value of the animation changes.
@@ -111,6 +127,10 @@ abstract class AnimationLocalListenersMixin extends _ListenerMixin {
 /// and notifies all the registered listeners when notifyStatusListeners is
 /// called.
 abstract class AnimationLocalStatusListenersMixin extends _ListenerMixin {
+  // This class is intended to be used as a mixin, and should not be
+  // extended directly.
+  factory AnimationLocalStatusListenersMixin._() => null;
+
   final ObserverList<AnimationStatusListener> _statusListeners = new ObserverList<AnimationStatusListener>();
 
   /// Calls listener every time the status of the animation changes.

packages/flutter/lib/src/foundation/tree_diagnostics_mixin.dart

@@ -6,6 +6,10 @@ import 'package:meta/meta.dart';
 
 /// A mixin that helps dump string representations of trees.
 abstract class TreeDiagnosticsMixin {
+  // This class is intended to be used as a mixin, and should not be
+  // extended directly.
+  factory TreeDiagnosticsMixin._() => null;
+
   @override
   String toString() => '$runtimeType#$hashCode';
 

packages/flutter/lib/src/gestures/binding.dart

@@ -15,7 +15,10 @@ import 'hit_test.dart';
 import 'pointer_router.dart';
 
 /// A binding for the gesture subsystem.
-abstract class GestureBinding extends BindingBase implements HitTestable, HitTestDispatcher, HitTestTarget {
+abstract class GestureBinding extends BindingBase with HitTestable, HitTestDispatcher, HitTestTarget {
+  // This class is intended to be used as a mixin, and should not be
+  // extended directly.
+  factory GestureBinding._() => null;
 
   @override
   void initInstances() {

packages/flutter/lib/src/gestures/hit_test.dart

@@ -6,6 +6,10 @@ import 'events.dart';
 
 /// An object that can hit-test pointers.
 abstract class HitTestable { // ignore: one_member_abstracts
+  // This class is intended to be used as an interface with the implements
+  // keyword, and should not be extended directly.
+  factory HitTestable._() => null;
+
   /// Check whether the given position hits this object.
   ///
   /// If this given position hits this object, consider adding a [HitTestEntry]
@@ -15,12 +19,20 @@ abstract class HitTestable { // ignore: one_member_abstracts
 
 /// An object that can dispatch events.
 abstract class HitTestDispatcher { // ignore: one_member_abstracts
+  // This class is intended to be used as an interface with the implements
+  // keyword, and should not be extended directly.
+  factory HitTestDispatcher._() => null;
+
   /// Override this method to dispatch events.
   void dispatchEvent(PointerEvent event, HitTestResult result);
 }
 
 /// An object that can handle events.
 abstract class HitTestTarget { // ignore: one_member_abstracts
+  // This class is intended to be used as an interface with the implements
+  // keyword, and should not be extended directly.
+  factory HitTestTarget._() => null;
+
   /// Override this method to receive events.
   void handleEvent(PointerEvent event, HitTestEntry entry);
 }

packages/flutter/lib/src/rendering/binding.dart

@@ -20,7 +20,11 @@ import 'view.dart';
 export 'package:flutter/gestures.dart' show HitTestResult;
 
 /// The glue between the render tree and the Flutter engine.
-abstract class RendererBinding extends BindingBase implements SchedulerBinding, ServicesBinding, HitTestable {
+abstract class RendererBinding extends BindingBase with SchedulerBinding, ServicesBinding, HitTestable {
+  // This class is intended to be used as a mixin, and should not be
+  // extended directly.
+  factory RendererBinding._() => null;
+
   @override
   void initInstances() {
     super.initInstances();

packages/flutter/lib/src/rendering/block.dart

@@ -8,7 +8,7 @@ import 'box.dart';
 import 'object.dart';
 
 /// Parent data for use with [RenderListBody].
-class ListBodyParentData extends ContainerBoxParentDataMixin<RenderBox> { }
+class ListBodyParentData extends ContainerBoxParentData<RenderBox> { }
 
 typedef double _ChildSizingFunction(RenderBox child);
 

packages/flutter/lib/src/rendering/box.dart

@@ -530,7 +530,10 @@ class BoxParentData extends ParentData {
 
 /// Abstract ParentData subclass for RenderBox subclasses that want the
 /// ContainerRenderObjectMixin.
-abstract class ContainerBoxParentDataMixin<ChildType extends RenderObject> extends BoxParentData with ContainerParentDataMixin<ChildType> { }
+///
+/// This is a convenience class that mixes in the relevant classes with
+/// the relevant type arguments.
+abstract class ContainerBoxParentData<ChildType extends RenderObject> extends BoxParentData with ContainerParentDataMixin<ChildType> { }
 
 enum _IntrinsicDimension { minWidth, maxWidth, minHeight, maxHeight }
 
@@ -714,13 +717,13 @@ class _IntrinsicDimensionsCacheEntry {
 /// [parentData]. The class used for [parentData] must itself have the
 /// [ContainerParentDataMixin] class mixed into it; this is where
 /// [ContainerRenderObjectMixin] stores the linked list. A [ParentData] class
-/// can extend [ContainerBoxParentDataMixin]; this is essentially
+/// can extend [ContainerBoxParentData]; this is essentially
 /// [BoxParentData] mixed with [ContainerParentDataMixin]. For example, if a
 /// `RenderFoo` class wanted to have a linked list of [RenderBox] children, one
 /// might create a `FooParentData` class as follows:
 ///
 /// ```dart
-/// class FooParentData extends ContainerBoxParentDataMixin<RenderBox> {
+/// class FooParentData extends ContainerBoxParentData<RenderBox> {
 ///   // (any fields you might need for these children)
 /// }
 /// ```
@@ -2024,7 +2027,10 @@ abstract class RenderBox extends RenderObject {
 /// By convention, this class doesn't override any members of the superclass.
 /// Instead, it provides helpful functions that subclasses can call as
 /// appropriate.
-abstract class RenderBoxContainerDefaultsMixin<ChildType extends RenderBox, ParentDataType extends ContainerBoxParentDataMixin<ChildType>> implements ContainerRenderObjectMixin<ChildType, ParentDataType> {
+abstract class RenderBoxContainerDefaultsMixin<ChildType extends RenderBox, ParentDataType extends ContainerBoxParentData<ChildType>> implements ContainerRenderObjectMixin<ChildType, ParentDataType> {
+  // This class is intended to be used as a mixin, and should not be
+  // extended directly.
+  factory RenderBoxContainerDefaultsMixin._() => null;
 
   /// Returns the baseline of the first child with a baseline.
   ///

packages/flutter/lib/src/rendering/custom_layout.dart

@@ -10,7 +10,7 @@ import 'object.dart';
 // For SingleChildLayoutDelegate and RenderCustomSingleChildLayoutBox, see shifted_box.dart
 
 /// [ParentData] used by [RenderCustomMultiChildLayoutBox].
-class MultiChildLayoutParentData extends ContainerBoxParentDataMixin<RenderBox> {
+class MultiChildLayoutParentData extends ContainerBoxParentData<RenderBox> {
   /// An object representing the identity of this child.
   Object id;
 

packages/flutter/lib/src/rendering/flex.dart

@@ -29,7 +29,7 @@ enum FlexFit {
 }
 
 /// Parent data for use with [RenderFlex].
-class FlexParentData extends ContainerBoxParentDataMixin<RenderBox> {
+class FlexParentData extends ContainerBoxParentData<RenderBox> {
   /// The flex factor to use for this child
   ///
   /// If null or zero, the child is inflexible and determines its own size. If
@@ -48,7 +48,7 @@ class FlexParentData extends ContainerBoxParentDataMixin<RenderBox> {
   FlexFit fit;
 
   @override
-  String toString() => '${super.toString()}; flex=$flex';
+  String toString() => '${super.toString()}; flex=$flex; fit=$fit';
 }
 
 /// How much space should be occupied in the main axis.

packages/flutter/lib/src/rendering/flow.dart

@@ -143,7 +143,7 @@ int _getAlphaFromOpacity(double opacity) => (opacity * 255).round();
 /// transformation matrix, which is private to the [RenderFlow]. To set the
 /// matrix, use the [FlowPaintingContext.paintChild] function from an override
 /// of the [FlowDelegate.paintChildren] function.
-class FlowParentData extends ContainerBoxParentDataMixin<RenderBox> {
+class FlowParentData extends ContainerBoxParentData<RenderBox> {
   Matrix4 _transform;
 }
 

packages/flutter/lib/src/rendering/object.dart

@@ -2739,7 +2739,11 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
 /// Generic mixin for render objects with one child.
 ///
 /// Provides a child model for a render object subclass that has a unique child.
-abstract class RenderObjectWithChildMixin<ChildType extends RenderObject> implements RenderObject {
+abstract class RenderObjectWithChildMixin<ChildType extends RenderObject> extends RenderObject {
+  // This class is intended to be used as a mixin, and should not be
+  // extended directly.
+  factory RenderObjectWithChildMixin._() => null;
+
   /// Checks whether the given render object has the correct [runtimeType] to be
   /// a child of this render object.
   ///
@@ -2816,7 +2820,11 @@ abstract class RenderObjectWithChildMixin<ChildType extends RenderObject> implem
 }
 
 /// Parent data to support a doubly-linked list of children.
-abstract class ContainerParentDataMixin<ChildType extends RenderObject> implements ParentData {
+abstract class ContainerParentDataMixin<ChildType extends RenderObject> extends ParentData {
+  // This class is intended to be used as a mixin, and should not be
+  // extended directly.
+  factory ContainerParentDataMixin._() => null;
+
   /// The previous sibling in the parent's child list.
   ChildType previousSibling;
   /// The next sibling in the parent's child list.
@@ -2847,7 +2855,10 @@ abstract class ContainerParentDataMixin<ChildType extends RenderObject> implemen
 ///
 /// Provides a child model for a render object subclass that has a doubly-linked
 /// list of children.
-abstract class ContainerRenderObjectMixin<ChildType extends RenderObject, ParentDataType extends ContainerParentDataMixin<ChildType>> implements RenderObject {
+abstract class ContainerRenderObjectMixin<ChildType extends RenderObject, ParentDataType extends ContainerParentDataMixin<ChildType>> extends RenderObject {
+  // This class is intended to be used as a mixin, and should not be
+  // extended directly.
+  factory ContainerRenderObjectMixin._() => null;
 
   bool _debugUltimatePreviousSiblingOf(ChildType child, { ChildType equals }) {
     ParentDataType childParentData = child.parentData;

packages/flutter/lib/src/rendering/proxy_box.dart

@@ -50,7 +50,11 @@ class RenderProxyBox extends RenderBox with RenderObjectWithChildMixin<RenderBox
 /// of [RenderProxyBox] is desired but inheriting from [RenderProxyBox] is
 /// impractical (e.g. because you want to mix in other classes as well).
 // TODO(ianh): Remove this class once https://github.com/dart-lang/sdk/issues/15101 is fixed
-abstract class RenderProxyBoxMixin implements RenderBox, RenderObjectWithChildMixin<RenderBox> {
+abstract class RenderProxyBoxMixin extends RenderBox with RenderObjectWithChildMixin<RenderBox> {
+  // This class is intended to be used as a mixin, and should not be
+  // extended directly.
+  factory RenderProxyBoxMixin._() => null;
+
   @override
   void setupParentData(RenderObject child) {
     // We don't actually use the offset argument in BoxParentData, so let's

packages/flutter/lib/src/rendering/stack.dart

@@ -138,7 +138,7 @@ class RelativeRect {
 }
 
 /// Parent data for use with [RenderStack].
-class StackParentData extends ContainerBoxParentDataMixin<RenderBox> {
+class StackParentData extends ContainerBoxParentData<RenderBox> {
   /// The distance by which the child's top edge is inset from the top of the stack.
   double top;
 

packages/flutter/lib/src/rendering/viewport.dart

@@ -20,7 +20,11 @@ import 'viewport_offset.dart';
 /// content, which can be controlled by a [ViewportOffset]. This interface lets
 /// the framework recognize such render objects and interact with them without
 /// having specific knowledge of all the various types of viewports.
-abstract class RenderAbstractViewport implements RenderObject {
+abstract class RenderAbstractViewport extends RenderObject {
+  // This class is intended to be used as an interface with the implements
+  // keyword, and should not be extended directly.
+  factory RenderAbstractViewport._() => null;
+
   /// Returns the [RenderAbstractViewport] that most tightly encloses the given
   /// render object.
   ///

packages/flutter/lib/src/rendering/wrap.dart

@@ -59,7 +59,7 @@ class _RunMetrics {
 }
 
 /// Parent data for use with [RenderWrap].
-class WrapParentData extends ContainerBoxParentDataMixin<RenderBox> {
+class WrapParentData extends ContainerBoxParentData<RenderBox> {
   int _runIndex = 0;
 }
 

packages/flutter/lib/src/scheduler/binding.dart

@@ -159,6 +159,9 @@ enum SchedulerPhase {
 ///   priority and are executed in priority order according to a
 ///   [schedulingStrategy].
 abstract class SchedulerBinding extends BindingBase {
+  // This class is intended to be used as a mixin, and should not be
+  // extended directly.
+  factory SchedulerBinding._() => null;
 
   @override
   void initInstances() {

packages/flutter/lib/src/services/binding.dart

@@ -17,6 +17,10 @@ import 'platform_messages.dart';
 /// the licenses found in the `LICENSE` file stored at the root of the asset
 /// bundle.
 abstract class ServicesBinding extends BindingBase {
+  // This class is intended to be used as a mixin, and should not be
+  // extended directly.
+  factory ServicesBinding._() => null;
+
   @override
   void initInstances() {
     super.initInstances();

packages/flutter/lib/src/widgets/binding.dart

@@ -22,6 +22,11 @@ export 'dart:ui' show AppLifecycleState, Locale;
 /// Interface for classes that register with the Widgets layer binding.
 ///
 /// See [WidgetsBinding.addObserver] and [WidgetsBinding.removeObserver].
+///
+/// This class can be extended directly, to get default behaviors for all of the
+/// handlers, or can used with the `implements` keyword, in which case all the
+/// handlers must be implemented (and the analyzer will list those that have
+/// been omitted).
 abstract class WidgetsBindingObserver {
   /// Called when the system tells the app to pop the current route.
   /// For example, on Android, this is called when the user presses
@@ -63,7 +68,11 @@ abstract class WidgetsBindingObserver {
 }
 
 /// The glue between the widgets layer and the Flutter engine.
-abstract class WidgetsBinding extends BindingBase implements GestureBinding, RendererBinding {
+abstract class WidgetsBinding extends BindingBase with GestureBinding, RendererBinding {
+  // This class is intended to be used as a mixin, and should not be
+  // extended directly.
+  factory WidgetsBinding._() => null;
+
   @override
   void initInstances() {
     super.initInstances();

packages/flutter/lib/src/widgets/scroll_notification.dart

@@ -15,6 +15,10 @@ import 'scroll_metrics.dart';
 ///
 /// This is used by [ScrollNotification] and [OverscrollIndicatorNotification].
 abstract class ViewportNotificationMixin extends Notification {
+  // This class is intended to be used as a mixin, and should not be
+  // extended directly.
+  factory ViewportNotificationMixin._() => null;
+
   /// The number of viewports that this notification has bubbled through.
   ///
   /// Typically listeners only respond to notifications with a [depth] of zero.

packages/flutter/lib/src/widgets/sliver_persistent_header.dart

@@ -166,7 +166,11 @@ abstract class _SliverPersistentHeaderRenderObjectWidget extends RenderObjectWid
   }
 }
 
-abstract class _RenderSliverPersistentHeaderForWidgetsMixin implements RenderSliverPersistentHeader {
+abstract class _RenderSliverPersistentHeaderForWidgetsMixin extends RenderSliverPersistentHeader {
+  // This class is intended to be used as a mixin, and should not be
+  // extended directly.
+  factory _RenderSliverPersistentHeaderForWidgetsMixin._() => null;
+
   _SliverPersistentHeaderElement _element;
 
   @override

packages/flutter/lib/src/widgets/ticker_provider.dart

@@ -73,7 +73,10 @@ class TickerMode extends InheritedWidget {
 /// This mixin only supports vending a single ticker. If you might have multiple
 /// [AnimationController] objects over the lifetime of the [State], use a full
 /// [TickerProviderStateMixin] instead.
-abstract class SingleTickerProviderStateMixin implements State<dynamic>, TickerProvider { // ignore: TYPE_ARGUMENT_NOT_MATCHING_BOUNDS, https://github.com/dart-lang/sdk/issues/25232
+abstract class SingleTickerProviderStateMixin extends State<dynamic> implements TickerProvider { // ignore: TYPE_ARGUMENT_NOT_MATCHING_BOUNDS, https://github.com/dart-lang/sdk/issues/25232
+  // This class is intended to be used as a mixin, and should not be
+  // extended directly.
+  factory SingleTickerProviderStateMixin._() => null;
 
   Ticker _ticker;
 
@@ -150,7 +153,10 @@ abstract class SingleTickerProviderStateMixin implements State<dynamic>, TickerP
 /// If you only have a single [Ticker] (for example only a single
 /// [AnimationController]) for the lifetime of your [State], then using a
 /// [SingleTickerProviderStateMixin] is more efficient. This is the common case.
-abstract class TickerProviderStateMixin implements State<dynamic>, TickerProvider { // ignore: TYPE_ARGUMENT_NOT_MATCHING_BOUNDS, https://github.com/dart-lang/sdk/issues/25232
+abstract class TickerProviderStateMixin extends State<dynamic> implements TickerProvider { // ignore: TYPE_ARGUMENT_NOT_MATCHING_BOUNDS, https://github.com/dart-lang/sdk/issues/25232
+  // This class is intended to be used as a mixin, and should not be
+  // extended directly.
+  factory TickerProviderStateMixin._() => null;
 
   Set<Ticker> _tickers;