Dismissable doesn't resize from its full size (#3194)

Previously we'd snap to the size of the background widget before resizing to

zero. Now we force the background widget to match the size of the dimissable

widget at the time it was dismissed.



Fixes #3092

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

diff --git a/packages/flutter/lib/src/widgets/dismissable.dart b/packages/flutter/lib/src/widgets/dismissable.dart
index 1b32cb9..619658b 100644
--- a/packages/flutter/lib/src/widgets/dismissable.dart
+++ b/packages/flutter/lib/src/widgets/dismissable.dart
@@ -76,7 +76,7 @@
   /// A widget that is stacked behind the child. If secondaryBackground is also
   /// specified then this widget only appears when the child has been dragged
   /// down or to the right.
-  final  Widget background;
+  final Widget background;
 
   /// A widget that is stacked behind the child and is exposed when the child
   /// has been dragged up or to the left. It may only be specified when background
@@ -119,6 +119,7 @@
 
   double _dragExtent = 0.0;
   bool _dragUnderway = false;
+  Size _sizePriorToCollapse;
 
   @override
   void dispose() {
@@ -261,6 +262,7 @@
     assert(_moveController != null);
     assert(_moveController.isCompleted);
     assert(_resizeController == null);
+    assert(_sizePriorToCollapse == null);
     if (config.resizeDuration == null) {
       if (config.onDismissed != null)
         config.onDismissed(_dismissDirection);
@@ -269,6 +271,8 @@
         ..addListener(_handleResizeProgressChanged);
       _resizeController.forward();
       setState(() {
+        RenderBox box = context.findRenderObject();
+        _sizePriorToCollapse = box.size;
         _resizeAnimation = new Tween<double>(
           begin: 1.0,
           end: 0.0
@@ -315,8 +319,12 @@
 
       return new SizeTransition(
         sizeFactor: _resizeAnimation,
-        axis: _directionIsXAxis ? Axis.horizontal : Axis.vertical,
-        child: background
+        axis: _directionIsXAxis ? Axis.vertical : Axis.horizontal,
+        child: new SizedBox(
+          width: _sizePriorToCollapse.width,
+          height: _sizePriorToCollapse.height,
+          child: background
+        )
       );
     }
 

packages/flutter/test/widget/dismissable_test.dart

diff --git a/packages/flutter/test/widget/dismissable_test.dart b/packages/flutter/test/widget/dismissable_test.dart
index 97a1e07..5188775 100644
--- a/packages/flutter/test/widget/dismissable_test.dart
+++ b/packages/flutter/test/widget/dismissable_test.dart
@@ -12,6 +12,7 @@
 DismissDirection dismissDirection = DismissDirection.horizontal;
 DismissDirection reportedDismissDirection;
 List<int> dismissedItems = <int>[];
+Widget background;
 
 void handleOnResize(int item) {
   expect(dismissedItems.contains(item), isFalse);
@@ -29,6 +30,7 @@
     direction: dismissDirection,
     onDismissed: (DismissDirection direction) { handleOnDismissed(direction, item); },
     onResize: () { handleOnResize(item); },
+    background: background,
     child: new Container(
       width: itemExtent,
       height: itemExtent,
@@ -123,11 +125,15 @@
 }
 
 void main() {
+  setUp(() {
+    dismissedItems = <int>[];
+    background = null;
+  });
+
   test('Horizontal drag triggers dismiss scrollDirection=vertical', () {
     testWidgets((WidgetTester tester) {
       scrollDirection = Axis.vertical;
       dismissDirection = DismissDirection.horizontal;
-      dismissedItems = <int>[];
 
       tester.pumpWidget(widgetBuilder());
       expect(dismissedItems, isEmpty);
@@ -148,7 +154,6 @@
     testWidgets((WidgetTester tester) {
       scrollDirection = Axis.horizontal;
       dismissDirection = DismissDirection.vertical;
-      dismissedItems = <int>[];
 
       tester.pumpWidget(widgetBuilder());
       expect(dismissedItems, isEmpty);
@@ -169,7 +174,6 @@
     testWidgets((WidgetTester tester) {
       scrollDirection = Axis.vertical;
       dismissDirection = DismissDirection.endToStart;
-      dismissedItems = <int>[];
 
       tester.pumpWidget(widgetBuilder());
       expect(dismissedItems, isEmpty);
@@ -190,7 +194,6 @@
     testWidgets((WidgetTester tester) {
       scrollDirection = Axis.vertical;
       dismissDirection = DismissDirection.startToEnd;
-      dismissedItems = <int>[];
 
       tester.pumpWidget(widgetBuilder());
       expect(dismissedItems, isEmpty);
@@ -209,7 +212,6 @@
     testWidgets((WidgetTester tester) {
       scrollDirection = Axis.horizontal;
       dismissDirection = DismissDirection.up;
-      dismissedItems = <int>[];
 
       tester.pumpWidget(widgetBuilder());
       expect(dismissedItems, isEmpty);
@@ -228,7 +230,6 @@
     testWidgets((WidgetTester tester) {
       scrollDirection = Axis.horizontal;
       dismissDirection = DismissDirection.down;
-      dismissedItems = <int>[];
 
       tester.pumpWidget(widgetBuilder());
       expect(dismissedItems, isEmpty);
@@ -253,7 +254,6 @@
     testWidgets((WidgetTester tester) {
       scrollDirection = Axis.horizontal;
       dismissDirection = DismissDirection.down;
-      dismissedItems = <int>[];
 
       tester.pumpWidget(widgetBuilder());
       Element itemElement = tester.findText('0');
@@ -307,4 +307,24 @@
       expect(tester.findText('2'), isNull);
     });
   });
+
+  test('Dismissable starts from the full size when collapsing', () {
+    testWidgets((WidgetTester tester) {
+      scrollDirection = Axis.vertical;
+      dismissDirection = DismissDirection.horizontal;
+      background = new Text('background');
+
+      tester.pumpWidget(widgetBuilder());
+      expect(dismissedItems, isEmpty);
+
+      Element itemElement = tester.findText(0.toString());
+      expect(itemElement, isNotNull);
+      dismissElement(tester, itemElement, gestureDirection: DismissDirection.startToEnd);
+      tester.pump();
+
+      Element backgroundElement = tester.findText('background');
+      RenderBox backgroundBox = backgroundElement.findRenderObject();
+      expect(backgroundBox.size.height, equals(100.0));
+    });
+  });
 }