Add support for persistent footer buttons (#6345)

Fixes #616

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

@@ -10,6 +10,8 @@ import 'package:flutter/widgets.dart';
 
 import 'app_bar.dart';
 import 'bottom_sheet.dart';
+import 'button_bar.dart';
+import 'button.dart';
 import 'drawer.dart';
 import 'icon.dart';
 import 'icon_button.dart';
@@ -53,6 +55,7 @@ enum _ScaffoldSlot {
   appBar,
   bottomSheet,
   snackBar,
+  persistentFooter,
   bottomNavigationBar,
   floatingActionButton,
   drawer,
@@ -97,6 +100,12 @@ class _ScaffoldLayout extends MultiChildLayoutDelegate {
       positionChild(_ScaffoldSlot.bottomNavigationBar, new Offset(0.0, contentBottom));
     }
 
+    if (hasChild(_ScaffoldSlot.persistentFooter)) {
+      final double persistentFooterHeight = layoutChild(_ScaffoldSlot.persistentFooter, fullWidthConstraints.copyWith(maxHeight: contentBottom - contentTop)).height;
+      contentBottom -= persistentFooterHeight;
+      positionChild(_ScaffoldSlot.persistentFooter, new Offset(0.0, contentBottom));
+    }
+
     if (hasChild(_ScaffoldSlot.body)) {
       final double bodyHeight = contentBottom - contentTop;
       final BoxConstraints bodyConstraints = fullWidthConstraints.tighten(height: bodyHeight);
@@ -299,6 +308,7 @@ class Scaffold extends StatefulWidget {
     this.appBar,
     this.body,
     this.floatingActionButton,
+    this.persistentFooterButtons,
     this.drawer,
     this.bottomNavigationBar,
     this.scrollableKey,
@@ -321,6 +331,18 @@ class Scaffold extends StatefulWidget {
   /// Typically a [FloatingActionButton].
   final Widget floatingActionButton;
 
+  /// A set of buttons that are displayed at the bottom of the scaffold.
+  ///
+  /// Typically this is a list of [FlatButton] widgets. These buttons are
+  /// persistently visible, even of the [body] of the scaffold scrolls.
+  ///
+  /// These widgets will be wrapped in a [ButtonBar].
+  ///
+  /// See also:
+  ///
+  ///  * <https://material.google.com/components/buttons.html#buttons-persistent-footer-buttons>
+  final List<Widget> persistentFooterButtons;
+
   /// A panel displayed to the side of the body, often hidden on mobile devices.
   ///
   /// Typically a [Drawer].
@@ -761,6 +783,7 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
   @override
   Widget build(BuildContext context) {
     EdgeInsets padding = MediaQuery.of(context).padding;
+    ThemeData themeData = Theme.of(context);
     if (!config.resizeToAvoidBottomPadding)
       padding = new EdgeInsets.fromLTRB(padding.left, padding.top, padding.right, 0.0);
 
@@ -804,6 +827,26 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
     if (_snackBars.isNotEmpty)
       _addIfNonNull(children, _snackBars.first._widget, _ScaffoldSlot.snackBar);
 
+    if (config.persistentFooterButtons != null) {
+      children.add(new LayoutId(
+        id: _ScaffoldSlot.persistentFooter,
+        child: new Container(
+          decoration: new BoxDecoration(
+            border: new Border(
+              top: new BorderSide(
+                color: themeData.dividerColor
+              ),
+            ),
+          ),
+          child: new ButtonTheme.bar(
+            child: new ButtonBar(
+              children: config.persistentFooterButtons
+            ),
+          ),
+        ),
+      ));
+    }
+
     if (config.bottomNavigationBar != null) {
       children.add(new LayoutId(
         id: _ScaffoldSlot.bottomNavigationBar,
@@ -831,7 +874,7 @@ class ScaffoldState extends State<Scaffold> with TickerProviderStateMixin {
       )
     ));
 
-    if (Theme.of(context).platform == TargetPlatform.iOS) {
+    if (themeData.platform == TargetPlatform.iOS) {
       children.add(new LayoutId(
         id: _ScaffoldSlot.statusBar,
         child: new GestureDetector(

packages/flutter/test/material/scaffold_test.dart

@@ -239,6 +239,38 @@ void main() {
     expect(appBarBottomRight, equals(sheetTopRight));
   });
 
+  testWidgets('Persistent bottom buttons are persistent', (WidgetTester tester) async {
+    bool didPressButton = false;
+    await tester.pumpWidget(
+      new MaterialApp(
+        home: new Scaffold(
+          body: new ScrollableViewport(
+            child: new Container(
+              decoration: new BoxDecoration(
+                backgroundColor: Colors.amber[500],
+              ),
+              height: 5000.0,
+              child: new Text('body'),
+            ),
+          ),
+          persistentFooterButtons: <Widget>[
+            new FlatButton(
+              onPressed: () {
+                didPressButton = true;
+              },
+              child: new Text('X'),
+            )
+          ],
+        ),
+      ),
+    );
+
+    await tester.scroll(find.text('body'), const Offset(0.0, -1000.0));
+    expect(didPressButton, isFalse);
+    await tester.tap(find.text('X'));
+    expect(didPressButton, isTrue);
+  });
+
   group('back arrow', () {
     Future<Null> expectBackIcon(WidgetTester tester, TargetPlatform platform, IconData expectedIcon) async {
       GlobalKey rootKey = new GlobalKey();