Factor WillPopScope out of Form (#8311)

This functionality is independently useful.

Fixes #8293

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

@@ -6,6 +6,7 @@ import 'package:flutter/foundation.dart';
 
 import 'framework.dart';
 import 'routes.dart';
+import 'will_pop_scope.dart';
 
 /// An optional container for grouping together multiple form field widgets
 /// (e.g. [Input] widgets).
@@ -64,35 +65,6 @@ class Form extends StatefulWidget {
 class FormState extends State<Form> {
   int _generation = 0;
   Set<FormFieldState<dynamic>> _fields = new Set<FormFieldState<dynamic>>();
-  ModalRoute<dynamic> _route;
-
-  @override
-  void dependenciesChanged() {
-    super.dependenciesChanged();
-    if (_route != null && config.onWillPop != null)
-      _route.removeScopedWillPopCallback(config.onWillPop);
-    _route = ModalRoute.of(context);
-    if (_route != null && config.onWillPop != null)
-      _route.addScopedWillPopCallback(config.onWillPop);
-  }
-
-  @override
-  void didUpdateConfig(Form oldConfig) {
-    assert(_route == ModalRoute.of(context));
-    if (config.onWillPop != oldConfig.onWillPop && _route != null) {
-      if (oldConfig.onWillPop != null)
-        _route.removeScopedWillPopCallback(oldConfig.onWillPop);
-      if (config.onWillPop != null)
-        _route.addScopedWillPopCallback(config.onWillPop);
-    }
-  }
-
-  @override
-  void dispose() {
-    if (_route != null && config.onWillPop != null)
-      _route.removeScopedWillPopCallback(config.onWillPop);
-    super.dispose();
-  }
 
   // Called when a form field has changed. This will cause all form fields
   // to rebuild, useful if form fields have interdependencies.
@@ -114,10 +86,13 @@ class FormState extends State<Form> {
   Widget build(BuildContext context) {
     if (config.autovalidate)
       _validate();
-    return new _FormScope(
-      formState: this,
-      generation: _generation,
-      child: config.child
+    return new WillPopScope(
+      onWillPop: config.onWillPop,
+      child: new _FormScope(
+        formState: this,
+        generation: _generation,
+        child: config.child,
+      ),
     );
   }
 

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

@@ -688,10 +688,29 @@ abstract class ModalRoute<T> extends TransitionRoute<T> with LocalHistoryRoute<T
 
   /// Enables this route to veto attempts by the user to dismiss it.
   ///
-  /// This callback is typically added by a stateful descendant of the modal route.
-  /// A stateful widget shown in a modal route, like the child passed to
-  /// [showDialog], can look up its modal route and then add a callback in its
-  /// `dependenciesChanged` method:
+  /// This callback is typically added using a [WillPopScope] widget. That
+  /// widget finds the enclosing [ModalRoute] and uses this function to register
+  /// this callback:
+  ///
+  /// ```dart
+  /// Widget build(BuildContext context) {
+  ///   return new WillPopScope(
+  ///     onWillPop: askTheUserIfTheyAreSure,
+  ///     child: ...,
+  ///   );
+  /// }
+  /// ```
+  ///
+  /// This callback runs asynchronously and it's possible that it will be called
+  /// after its route has been disposed. The callback should check [mounted]
+  /// before doing anything.
+  ///
+  /// A typical application of this callback would be to warn the user about
+  /// unsaved [Form] data if the user attempts to back out of the form. In that
+  /// case, use the [Form.onWillPop] property to register the callback.
+  ///
+  /// To register a callback manually, look up the enclosing [ModalRoute] in a
+  /// [State.dependenciesChanged] callback:
   ///
   /// ```dart
   /// ModalRoute<dynamic> _route;
@@ -705,32 +724,28 @@ abstract class ModalRoute<T> extends TransitionRoute<T> with LocalHistoryRoute<T
   /// }
   /// ```
   ///
-  /// A typical application of this callback would be to warn the user about
-  /// unsaved [Form] data if the user attempts to back out of the form.
-  ///
-  /// This callback runs asynchronously and it's possible that it will be called
-  /// after its route has been disposed. The callback should check [mounted] before
-  /// doing anything.
-  ///
-  /// A widget that adds a scopedWillPopCallback must ensure that the callback
-  /// is removed with [removeScopedWillPopCallback] by the time the widget has
-  /// been disposed. A stateful widget can do this in its dispose method
-  /// (continuing the previous example):
+  /// If you register a callback manually, be sure to remove the callback with
+  /// [removeScopedWillPopCallback] by the time the widget has been disposed. A
+  /// stateful widget can do this in its dispose method (continuing the previous
+  /// example):
   ///
   /// ```dart
   /// @override
   /// void dispose() {
   ///   _route?.removeScopedWillPopCallback(askTheUserIfTheyAreSure);
+  ///   _route = null;
   ///   super.dispose();
   /// }
   /// ```
   ///
   /// See also:
   ///
-  /// * [Form], which provides an `onWillPop` callback that uses this mechanism.
-  /// * [willPop], which runs the callbacks added with this method.
-  /// * [removeScopedWillPopCallback], which removes a callback from the list
-  ///   that [willPop] checks.
+  ///  * [WillPopScope], which manages the registration and unregistration
+  ///    process automatically.
+  ///  * [Form], which provides an `onWillPop` callback that uses this mechanism.
+  ///  * [willPop], which runs the callbacks added with this method.
+  ///  * [removeScopedWillPopCallback], which removes a callback from the list
+  ///    that [willPop] checks.
   void addScopedWillPopCallback(WillPopCallback callback) {
     assert(_scopeKey.currentState != null);
     _scopeKey.currentState.addWillPopCallback(callback);

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

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/foundation.dart';
+
+import 'framework.dart';
+import 'routes.dart';
+
+/// Registers a callback to veto attempts by the user to dismiss the enclosing
+/// [ModalRoute].
+///
+/// See also:
+///
+///  * [ModalRoute.addScopedWillPopCallback] and [ModalScope.removeScopedWillPopCallback],
+///    which this widget uses to register and unregister [onWillPop].
+class WillPopScope extends StatefulWidget {
+  /// Creates a widget that registers a callback to veto attempts by the user to
+  /// dismiss the enclosing [ModalRoute].
+  ///
+  /// The [child] argument must not be null.
+  WillPopScope({
+    Key key,
+    @required this.child,
+    @required this.onWillPop,
+  }) : super(key: key) {
+    assert(child != null);
+  }
+
+  /// The widget below this widget in the tree.
+  final Widget child;
+
+  /// Called to veto attempts by the user to dismiss the enclosing [ModalRoute].
+  ///
+  /// If the callback returns a Future that resolves to false, the enclosing
+  /// route will not be popped.
+  WillPopCallback onWillPop;
+
+  @override
+  _WillPopScopeState createState() => new _WillPopScopeState();
+}
+
+class _WillPopScopeState extends State<WillPopScope> {
+  ModalRoute<dynamic> _route;
+
+  @override
+  void dependenciesChanged() {
+    super.dependenciesChanged();
+    if (config.onWillPop != null)
+      _route?.removeScopedWillPopCallback(config.onWillPop);
+    _route = ModalRoute.of(context);
+    if (config.onWillPop != null)
+      _route?.addScopedWillPopCallback(config.onWillPop);
+  }
+
+  @override
+  void didUpdateConfig(WillPopScope oldConfig) {
+    assert(_route == ModalRoute.of(context));
+    if (config.onWillPop != oldConfig.onWillPop && _route != null) {
+      if (oldConfig.onWillPop != null)
+        _route.removeScopedWillPopCallback(oldConfig.onWillPop);
+      if (config.onWillPop != null)
+        _route.addScopedWillPopCallback(config.onWillPop);
+    }
+  }
+
+  @override
+  void dispose() {
+    if (config.onWillPop != null)
+      _route?.removeScopedWillPopCallback(config.onWillPop);
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) => config.child;
+}

packages/flutter/lib/widgets.dart

@@ -66,5 +66,6 @@ export 'src/widgets/title.dart';
 export 'src/widgets/transitions.dart';
 export 'src/widgets/unique_widget.dart';
 export 'src/widgets/viewport.dart';
+export 'src/widgets/will_pop_scope.dart';
 
 export 'package:vector_math/vector_math_64.dart' show Matrix4;