Appbar should update when you add a drawer. (#9755)

Also, I had a question about flutter_test matchers and our style guide
says that when I have a question I should update the docs so I did
that and then got a bit carried away.

packages/flutter/lib/src/material/app_bar.dart

@@ -319,20 +319,6 @@ class AppBar extends StatefulWidget implements PreferredSizeWidget {
 }
 
 class _AppBarState extends State<AppBar> {
-  bool _hasDrawer = false;
-  bool _canPop = false;
-  bool _useCloseButton = false;
-
-  @override
-  void didChangeDependencies() {
-    super.didChangeDependencies();
-    final ScaffoldState scaffold = Scaffold.of(context, nullOk: true);
-    _hasDrawer = scaffold?.hasDrawer ?? false;
-    final ModalRoute<dynamic> parentRoute = ModalRoute.of(context);
-    _canPop = parentRoute?.canPop ?? false;
-    _useCloseButton = parentRoute is MaterialPageRoute<dynamic> && parentRoute.fullscreenDialog;
-  }
-
   void _handleDrawerButton() {
     Scaffold.of(context).openDrawer();
   }
@@ -341,6 +327,12 @@ class _AppBarState extends State<AppBar> {
   Widget build(BuildContext context) {
     assert(!widget.primary || debugCheckHasMediaQuery(context));
     final ThemeData themeData = Theme.of(context);
+    final ScaffoldState scaffold = Scaffold.of(context, nullOk: true);
+    final ModalRoute<dynamic> parentRoute = ModalRoute.of(context);
+
+    final bool hasDrawer = scaffold?.hasDrawer ?? false;
+    final bool canPop = parentRoute?.canPop ?? false;
+    final bool useCloseButton = parentRoute is MaterialPageRoute<dynamic> && parentRoute.fullscreenDialog;
 
     IconThemeData appBarIconTheme = widget.iconTheme ?? themeData.primaryIconTheme;
     TextStyle centerStyle = widget.textTheme?.title ?? themeData.primaryTextTheme.title;
@@ -365,15 +357,15 @@ class _AppBarState extends State<AppBar> {
     final List<Widget> toolbarChildren = <Widget>[];
     Widget leading = widget.leading;
     if (leading == null) {
-      if (_hasDrawer) {
+      if (hasDrawer) {
         leading = new IconButton(
           icon: const Icon(Icons.menu),
           onPressed: _handleDrawerButton,
           tooltip: 'Open navigation menu' // TODO(ianh): Figure out how to localize this string
         );
       } else {
-        if (_canPop)
-          leading = _useCloseButton ? const CloseButton() : const BackButton();
+        if (canPop)
+          leading = useCloseButton ? const CloseButton() : const BackButton();
       }
     }
     if (leading != null) {

packages/flutter/test/material/app_bar_test.dart

@@ -308,7 +308,6 @@ void main() {
     expect(find.text('A2'), findsOneWidget);
   });
 
-
   testWidgets('AppBar render at zero size', (WidgetTester tester) async {
     await tester.pumpWidget(
       new MaterialApp(
@@ -760,4 +759,24 @@ void main() {
     expect(appBarTop(tester), 0.0);
     expect(tester.getTopLeft(find.text('title')).dy, lessThan(100.0));
   });
+
+  testWidgets('AppBar updates when you add a drawer', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new MaterialApp(
+        home: new Scaffold(
+          appBar: new AppBar(),
+        ),
+      ),
+    );
+    expect(find.byIcon(Icons.menu), findsNothing);
+    await tester.pumpWidget(
+      new MaterialApp(
+        home: new Scaffold(
+          drawer: const Drawer(),
+          appBar: new AppBar(),
+        ),
+      ),
+    );
+    expect(find.byIcon(Icons.menu), findsOneWidget);
+  });
 }

packages/flutter_test/lib/src/matchers.dart

@@ -9,30 +9,62 @@ import 'finders.dart';
 
 /// Asserts that the [Finder] matches no widgets in the widget tree.
 ///
-/// Example:
+/// ## Sample code
 ///
+/// ```dart
 /// expect(find.text('Save'), findsNothing);
+/// ```
+///
+/// See also:
+///
+///  * [findsWidgets], when you want the finder to find one or more widgets.
+///  * [findsOneWidgets], when you want the finder to find exactly one widget.
+///  * [findsNWidgets], when you want the finder to find a specific number of widgets.
 const Matcher findsNothing = const _FindsWidgetMatcher(null, 0);
 
 /// Asserts that the [Finder] locates at least one widget in the widget tree.
 ///
-/// Example:
+/// ## Sample code
 ///
+/// ```dart
 /// expect(find.text('Save'), findsWidgets);
+/// ```
+///
+/// See also:
+///
+///  * [findsNothing], when you want the finder to not find anything.
+///  * [findsOneWidgets], when you want the finder to find exactly one widget.
+///  * [findsNWidgets], when you want the finder to find a specific number of widgets.
 const Matcher findsWidgets = const _FindsWidgetMatcher(1, null);
 
 /// Asserts that the [Finder] locates at exactly one widget in the widget tree.
 ///
-/// Example:
+/// ## Sample code
 ///
+/// ```dart
 /// expect(find.text('Save'), findsOneWidget);
+/// ```
+///
+/// See also:
+///
+///  * [findsNothing], when you want the finder to not find anything.
+///  * [findsWidgets], when you want the finder to find one or more widgets.
+///  * [findsNWidgets], when you want the finder to find a specific number of widgets.
 const Matcher findsOneWidget = const _FindsWidgetMatcher(1, 1);
 
 /// Asserts that the [Finder] locates the specified number of widgets in the widget tree.
 ///
-/// Example:
+/// ## Sample code
 ///
+/// ```dart
 /// expect(find.text('Save'), findsNWidgets(2));
+/// ```
+///
+/// See also:
+///
+///  * [findsNothing], when you want the finder to not find anything.
+///  * [findsWidgets], when you want the finder to find one or more widgets.
+///  * [findsOneWidgets], when you want the finder to find exactly one widget.
 Matcher findsNWidgets(int n) => new _FindsWidgetMatcher(n, n);
 
 /// Asserts that the [Finder] locates the a single widget that has at
@@ -41,21 +73,41 @@ Matcher findsNWidgets(int n) => new _FindsWidgetMatcher(n, n);
 /// It's important to use a full finder, since by default finders exclude
 /// offstage widgets.
 ///
-/// Example:
+/// ## Sample code
 ///
+/// ```dart
 /// expect(find.text('Save', skipOffstage: false), isOffstage);
+/// ```
+///
+/// See also:
+///
+///  * [isOnStage], the opposite.
 const Matcher isOffstage = const _IsOffstage();
 
 /// Asserts that the [Finder] locates the a single widget that has no
 /// [Offstage] widget ancestors.
+///
+/// See also:
+///
+///  * [isOffStage], the opposite.
 const Matcher isOnstage = const _IsOnstage();
 
 /// Asserts that the [Finder] locates the a single widget that has at
 /// least one [Card] widget ancestor.
+///
+/// See also:
+///
+///  * [isNotInCard], the opposite.
 const Matcher isInCard = const _IsInCard();
 
 /// Asserts that the [Finder] locates the a single widget that has no
 /// [Card] widget ancestors.
+///
+/// This is equivalent to `isNot(isInCard)`.
+///
+/// See also:
+///
+///  * [isInCard], the opposite.
 const Matcher isNotInCard = const _IsNotInCard();
 
 /// Asserts that an object's toString() is a plausible one-line description.
@@ -66,15 +118,47 @@ const Matcher isNotInCard = const _IsNotInCard();
 const Matcher hasOneLineDescription = const _HasOneLineDescription();
 
 /// A matcher for functions that throw [FlutterError].
+///
+/// This is equivalent to `throwsA(const isInstanceOf<FlutterError>())`.
+///
+/// See also:
+///
+///  * [throwsAssertionError], to test if a function throws any [AssertionError].
+///  * [isFlutterError], to test if any object is a [FlutterError].
+///  * [isAssertionError], to test if any object is any kind of [AssertionError].
 Matcher throwsFlutterError = throwsA(isFlutterError);
 
 /// A matcher for functions that throw [AssertionError].
+///
+/// This is equivalent to `throwsA(const isInstanceOf<AssertionError>())`.
+///
+/// See also:
+///
+///  * [throwsFlutterError], to test if a function throws a [FlutterError].
+///  * [isFlutterError], to test if any object is a [FlutterError].
+///  * [isAssertionError], to test if any object is any kind of [AssertionError].
 Matcher throwsAssertionError = throwsA(isAssertionError);
 
 /// A matcher for [FlutterError].
+///
+/// This is equivalent to `const isInstanceOf<FlutterError>()`.
+///
+/// See also:
+///
+///  * [throwsFlutterError], to test if a function throws a [FlutterError].
+///  * [throwsAssertionError], to test if a function throws any [AssertionError].
+///  * [isAssertionError], to test if any object is any kind of [AssertionError].
 const Matcher isFlutterError = const isInstanceOf<FlutterError>();
 
 /// A matcher for [AssertionError].
+///
+/// This is equivalent to `const isInstanceOf<AssertionError>()`.
+///
+/// See also:
+///
+///  * [throwsFlutterError], to test if a function throws a [FlutterError].
+///  * [throwsAssertionError], to test if a function throws any [AssertionError].
+///  * [isFlutterError], to test if any object is a [FlutterError].
 const Matcher isAssertionError = const isInstanceOf<AssertionError>();
 
 /// Asserts that two [double]s are equal, within some tolerated error.
@@ -84,6 +168,13 @@ const Matcher isAssertionError = const isInstanceOf<AssertionError>();
 /// using the `epsilon` argument. This matcher is intended to compare floating
 /// point numbers that are the result of different sequences of operations, such
 /// that they may have accumulated slightly different errors.
+///
+/// See also:
+///
+///  * [closeTo], which is identical except that the epsilon argument is
+///    required and not named.
+///  * [isInclusiveRange], which matches if the argument is in a specified
+///    range.
 Matcher moreOrLessEquals(double value, { double epsilon: 1e-10 }) {
   return new _MoreOrLessEquals(value, epsilon);
 }