Allow null DropdownButton values (flutter#6971)

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

@@ -52,8 +52,9 @@ class _DropdownMenuPainter extends CustomPainter {
 
   @override
   void paint(Canvas canvas, Size size) {
+    final double selectedItemOffset = selectedIndex * _kMenuItemHeight + _kMenuVerticalPadding.top;
     final Tween<double> top = new Tween<double>(
-      begin: (selectedIndex * _kMenuItemHeight + _kMenuVerticalPadding.top).clamp(0.0, size.height - _kMenuItemHeight),
+      begin: selectedItemOffset.clamp(0.0, size.height - _kMenuItemHeight),
       end: 0.0
     );
 
@@ -411,28 +412,31 @@ class DropdownButtonHideUnderline extends InheritedWidget {
 class DropdownButton<T> extends StatefulWidget {
   /// Creates a dropdown button.
   ///
-  /// The [items] must have distinct values and [value] must be among them.
+  /// The [items] must have distinct values and if [value] isn't null it must be among them.
   ///
   /// The [elevation] and [iconSize] arguments must not be null (they both have
   /// defaults, so do not need to be specified).
   DropdownButton({
     Key key,
     @required this.items,
-    @required this.value,
+    this.value,
     @required this.onChanged,
     this.elevation: 8,
     this.style,
     this.iconSize: 24.0,
     this.isDense: false,
   }) : super(key: key) {
     assert(items != null);
-    assert(items.where((DropdownMenuItem<T> item) => item.value == value).length == 1);
+    assert(value == null ||
+      items.where((DropdownMenuItem<T> item) => item.value == value).length == 1);
   }
 
   /// The list of possible items to select among.
   final List<DropdownMenuItem<T>> items;
 
-  /// The currently selected item.
+  /// The currently selected item, or null if no item has been selected. If
+  /// value is null then the menu is popped up as if the first item was
+  /// selected.
   final T value;
 
   /// Called when the user selects an item.
@@ -470,22 +474,23 @@ class DropdownButton<T> extends StatefulWidget {
 }
 
 class _DropdownButtonState<T> extends State<DropdownButton<T>> {
+  int _selectedIndex;
+
   @override
   void initState() {
     super.initState();
     _updateSelectedIndex();
-    assert(_selectedIndex != null);
   }
 
  @override
   void didUpdateConfig(DropdownButton<T> oldConfig) {
-    if (config.items[_selectedIndex].value != config.value)
-      _updateSelectedIndex();
+    _updateSelectedIndex();
   }
 
-  int _selectedIndex;
-
   void _updateSelectedIndex() {
+    assert(config.value == null ||
+      config.items.where((DropdownMenuItem<T> item) => item.value == config.value).length == 1);
+    _selectedIndex = null;
     for (int itemIndex = 0; itemIndex < config.items.length; itemIndex++) {
       if (config.items[itemIndex].value == config.value) {
         _selectedIndex = itemIndex;
@@ -502,7 +507,7 @@ class _DropdownButtonState<T> extends State<DropdownButton<T>> {
     Navigator.push(context, new _DropdownRoute<T>(
       items: config.items,
       buttonRect: _kMenuHorizontalPadding.inflateRect(itemRect),
-      selectedIndex: _selectedIndex,
+      selectedIndex: _selectedIndex ?? 0,
       elevation: config.elevation,
       theme: Theme.of(context, shadowThemeOnly: true),
       style: _textStyle,
@@ -533,10 +538,12 @@ class _DropdownButtonState<T> extends State<DropdownButton<T>> {
           mainAxisAlignment: MainAxisAlignment.spaceBetween,
           mainAxisSize: MainAxisSize.min,
           children: <Widget>[
+            // The button's size is defined by its largest menu item. If value is
+            // null then an item does not appear.
             new IndexedStack(
               index: _selectedIndex,
               alignment: FractionalOffset.centerLeft,
-              children: config.items
+              children: config.items,
             ),
             new Icon(Icons.arrow_drop_down,
               size: config.iconSize,

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

@@ -454,30 +454,28 @@ class RenderStack extends RenderBox
 class RenderIndexedStack extends RenderStack {
   /// Creates a stack render object that paints a single child.
   ///
-  /// The [index] argument must not be null.
+  /// If the [index] parameter is null, nothing is displayed.
   RenderIndexedStack({
     List<RenderBox> children,
     FractionalOffset alignment: FractionalOffset.topLeft,
     int index: 0
   }) : _index = index, super(
    children: children,
    alignment: alignment
-  ) {
-    assert(index != null);
-  }
+  );
 
-  /// The index of the child to show.
+  /// The index of the child to show, null if nothing is to be displayed.
   int get index => _index;
   int _index;
   set index (int value) {
-    assert(value != null);
     if (_index != value) {
       _index = value;
       markNeedsLayout();
     }
   }
 
   RenderBox _childAtIndex() {
+    assert(index != null);
     RenderBox child = firstChild;
     int i = 0;
     while (child != null && i < index) {
@@ -492,7 +490,7 @@ class RenderIndexedStack extends RenderStack {
 
   @override
   bool hitTestChildren(HitTestResult result, { Point position }) {
-    if (firstChild == null)
+    if (firstChild == null || index == null)
       return false;
     assert(position != null);
     RenderBox child = _childAtIndex();
@@ -504,7 +502,7 @@ class RenderIndexedStack extends RenderStack {
 
   @override
   void paintStack(PaintingContext context, Offset offset) {
-    if (firstChild == null)
+    if (firstChild == null || index == null)
       return;
     RenderBox child = _childAtIndex();
     final StackParentData childParentData = child.parentData;

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

@@ -1534,7 +1534,10 @@ class Stack extends MultiChildRenderObjectWidget {
 
 /// A [Stack] that shows a single child from a list of children.
 ///
-/// The displayed child is the one with the given [index].
+/// The displayed child is the one with the given [index]. The stack is
+/// always as big as the largest child.
+///
+/// If value is null, then nothing is displayed.
 ///
 /// For more details, see [Stack].
 class IndexedStack extends Stack {
@@ -1546,9 +1549,7 @@ class IndexedStack extends Stack {
     FractionalOffset alignment: FractionalOffset.topLeft,
     this.index: 0,
     List<Widget> children: const <Widget>[],
-  }) : super(key: key, alignment: alignment, children: children) {
-    assert(index != null);
-  }
+  }) : super(key: key, alignment: alignment, children: children);
 
   /// The index of the child to show.
   final int index;

packages/flutter/test/material/drop_down_test.dart

@@ -7,8 +7,9 @@ import 'dart:math' as math;
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/material.dart';
 
+final List<String> menuItems = <String>['one', 'two', 'three', 'four'];
+
 Widget buildFrame({ Key buttonKey, String value: 'two',  ValueChanged<String> onChanged, bool isDense: false }) {
-  final List<String> items = <String>['one', 'two', 'three', 'four'];
   return new MaterialApp(
     home: new Material(
       child: new Center(
@@ -17,7 +18,7 @@ Widget buildFrame({ Key buttonKey, String value: 'two',  ValueChanged<String> on
           value: value,
           onChanged: onChanged,
           isDense: isDense,
-          items: items.map((String item) {
+          items: menuItems.map((String item) {
             return new DropdownMenuItem<String>(
               key: new ValueKey<String>(item),
               value: item,
@@ -265,4 +266,55 @@ void main() {
     // should have the same size and location.
     checkSelectedItemTextGeometry(tester, 'two');
   });
+
+  testWidgets('Size of DropdownButton with null value', (WidgetTester tester) async {
+    Key buttonKey = new UniqueKey();
+    String value;
+
+    Widget build() => buildFrame(buttonKey: buttonKey, value: value);
+
+    await tester.pumpWidget(build());
+    RenderBox buttonBoxNullValue = tester.renderObject(find.byKey(buttonKey));
+    assert(buttonBoxNullValue.attached);
+
+
+    value = 'three';
+    await tester.pumpWidget(build());
+    RenderBox buttonBox = tester.renderObject(find.byKey(buttonKey));
+    assert(buttonBox.attached);
+
+    // A DropDown button with a null value should be the same size as a
+    // one with a non-null value.
+    expect(buttonBox.localToGlobal(Point.origin), equals(buttonBoxNullValue.localToGlobal(Point.origin)));
+    expect(buttonBox.size, equals(buttonBoxNullValue.size));
+  });
+
+  testWidgets('Layout of a DropdownButton with null value', (WidgetTester tester) async {
+    Key buttonKey = new UniqueKey();
+    String value;
+
+    void onChanged(String newValue) {
+      value = newValue;
+    }
+
+    Widget build() => buildFrame(buttonKey: buttonKey, value: value, onChanged: onChanged);
+
+    await tester.pumpWidget(build());
+    RenderBox buttonBox = tester.renderObject(find.byKey(buttonKey));
+    assert(buttonBox.attached);
+
+    // Show the menu.
+    await tester.tap(find.byKey(buttonKey));
+    await tester.pump();
+    await tester.pump(const Duration(seconds: 1)); // finish the menu animation
+
+    // Tap on item 'one', which must appear over the button.
+    await tester.tap(find.byKey(buttonKey));
+    await tester.pump();
+    await tester.pump(const Duration(seconds: 1)); // finish the menu animation
+
+    await tester.pumpWidget(build());
+    expect(value, equals('one'));
+  });
+
 }

packages/flutter/test/widgets/stack_test.dart

@@ -262,6 +262,33 @@ void main() {
     expect(renderBox.size.height, equals(12.0));
   });
 
+  testWidgets('IndexedStack with null index', (WidgetTester tester) async {
+    bool tapped;
+
+    await tester.pumpWidget(
+      new Center(
+        child: new IndexedStack(
+          index: null,
+          children: <Widget>[
+            new GestureDetector(
+              behavior: HitTestBehavior.opaque,
+              onTap: () { print("HELLO"); tapped = true; },
+              child: new SizedBox(
+                width: 200.0,
+                height: 200.0,
+              ),
+            ),
+          ],
+        ),
+      ),
+    );
+
+    await tester.tap(find.byType(IndexedStack));
+    RenderBox box = tester.renderObject(find.byType(IndexedStack));
+    expect(box.size, equals(const Size(200.0, 200.0)));
+    expect(tapped, isNull);
+  });
+
   testWidgets('Stack clip test', (WidgetTester tester) async {
     await tester.pumpWidget(
       new Center(