Add Container.align and Container.position (#5128)

These let you add Align widget to the inside or outside of a container.
Several customers have asked for these properties.

Fixes #4950

dev/manual_tests/card_collection.dart

@@ -284,11 +284,9 @@ class CardCollectionState extends State<CardCollection> {
       flexibleSpace: new Container(
         padding: const EdgeInsets.only(left: 72.0),
         height: 128.0,
-        child: new Align(
-          alignment: const FractionalOffset(0.0, 0.75),
+        align: const FractionalOffset(0.0, 0.75),
         child: new Text('Swipe Away: ${_cardModels.length}', style: Theme.of(context).primaryTextTheme.title)
       )
-      )
     );
   }
 

examples/flutter_gallery/lib/demo/full_screen_dialog_demo.dart

@@ -166,20 +166,16 @@ class FullScreenDialogDemoState extends State<FullScreenDialogDemo> {
             decoration: new BoxDecoration(
               border: new Border(bottom: new BorderSide(color: theme.dividerColor))
             ),
-            child: new Align(
-              alignment: FractionalOffset.bottomLeft,
+            align: FractionalOffset.bottomLeft,
             child: new Text('Event name', style: theme.textTheme.display2)
-            )
           ),
           new Container(
             padding: const EdgeInsets.symmetric(vertical: 8.0),
             decoration: new BoxDecoration(
               border: new Border(bottom: new BorderSide(color: theme.dividerColor))
             ),
-            child: new Align(
-              alignment: FractionalOffset.bottomLeft,
+            align: FractionalOffset.bottomLeft,
             child: new Text('Location', style: theme.textTheme.title.copyWith(color: Colors.black54))
-            )
           ),
           new Column(
             crossAxisAlignment: CrossAxisAlignment.start,

examples/flutter_gallery/lib/demo/pesto_demo.dart

@@ -331,7 +331,7 @@ class _RecipePageState extends State<_RecipePage> {
     const double fabHalfSize = 28.0;  // TODO(mpcomplete): needs to adapt to screen size
     return new Hero(
       tag: config.recipe.imagePath,
-      child: new DecoratedBox(
+      child: new Container(
         decoration: new BoxDecoration(
           backgroundColor: Theme.of(context).canvasColor,
           backgroundImage: new BackgroundImage(
@@ -340,8 +340,7 @@ class _RecipePageState extends State<_RecipePage> {
             fit: fullWidth ? ImageFit.fitWidth : ImageFit.cover
           )
         ),
-        child: new Align(
-          alignment: FractionalOffset.bottomCenter,
+        align: FractionalOffset.bottomCenter,
         child: new Block(
           children: <Widget>[
             new Padding(
@@ -368,7 +367,6 @@ class _RecipePageState extends State<_RecipePage> {
           ]
         )
       )
-      )
     );
   }
 

examples/flutter_gallery/lib/gallery/home.dart

@@ -71,11 +71,9 @@ class GalleryHomeState extends State<GalleryHome> {
           new Container(
             height: 48.0,
             padding: const EdgeInsets.only(left: 16.0),
-            child: new Align(
-              alignment: FractionalOffset.centerLeft,
+            align: FractionalOffset.centerLeft,
             child: new Text(galleryItem.category, style: headerStyle)
           )
-          )
         );
         category = galleryItem.category;
       }

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

@@ -406,8 +406,7 @@ class DataTable extends StatelessWidget {
     label = new Container(
       padding: padding,
       height: _kHeadingRowHeight,
-      child: new Align(
-        alignment: new FractionalOffset(numeric ? 1.0 : 0.0, 0.5), // TODO(ianh): RTL for non-numeric
+      align: new FractionalOffset(numeric ? 1.0 : 0.0, 0.5), // TODO(ianh): RTL for non-numeric
       child: new AnimatedDefaultTextStyle(
         style: new TextStyle(
           // TODO(ianh): font family should be Roboto; see https://github.com/flutter/flutter/issues/3116
@@ -421,7 +420,6 @@ class DataTable extends StatelessWidget {
         duration: _kSortArrowAnimationDuration,
         child: label
       )
-      )
     );
     if (tooltip != null) {
       label = new Tooltip(
@@ -458,8 +456,7 @@ class DataTable extends StatelessWidget {
     label = new Container(
       padding: padding,
       height: _kDataRowHeight,
-      child: new Align(
-        alignment: new FractionalOffset(numeric ? 1.0 : 0.0, 0.5), // TODO(ianh): RTL for non-numeric
+      align: new FractionalOffset(numeric ? 1.0 : 0.0, 0.5), // TODO(ianh): RTL for non-numeric
       child: new DefaultTextStyle(
         style: new TextStyle(
           // TODO(ianh): font family should be Roboto; see https://github.com/flutter/flutter/issues/3116
@@ -476,7 +473,6 @@ class DataTable extends StatelessWidget {
           child: new DropDownButtonHideUnderline(child: label)
         )
       )
-      )
     );
     if (onTap != null) {
       label = new InkWell(

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

@@ -337,10 +337,8 @@ class DropDownMenuItem<T> extends StatelessWidget {
   Widget build(BuildContext context) {
     return new Container(
       height: _kMenuItemHeight,
-      child: new Align(
-        alignment: FractionalOffset.centerLeft,
+      align: FractionalOffset.centerLeft,
       child: child
-      )
     );
   }
 }

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

@@ -146,10 +146,9 @@ class _FlexibleSpaceBarState extends State<FlexibleSpaceBar> {
         );
         final bool effectiveCenterTitle = _getEffectiveCenterTitle(theme);
         final FractionalOffset titleAlignment = effectiveCenterTitle ? FractionalOffset.bottomCenter : FractionalOffset.bottomLeft;
-        children.add(new Padding(
+        children.add(new Container(
           padding: new EdgeInsets.only(left: effectiveCenterTitle ? 0.0 : 72.0, bottom: 14.0),
-          child: new Align(
-            alignment: new Tween<FractionalOffset>(
+          align: new Tween<FractionalOffset>(
             begin: new FractionalOffset(0.0, yAlignStart),
             end: new FractionalOffset(0.0, yAlignEnd)
           ).evaluate(scaleAndAlignCurve),
@@ -161,7 +160,6 @@ class _FlexibleSpaceBarState extends State<FlexibleSpaceBar> {
               child: new DefaultTextStyle(style: titleStyle, child: config.title)
             )
           )
-          )
         ));
       }
     }

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

@@ -154,10 +154,8 @@ class ListItem extends StatelessWidget {
       children.add(new Container(
         margin: const EdgeInsets.only(right: 16.0),
         width: 40.0,
-        child: new Align(
-          alignment: FractionalOffset.centerLeft,
+        align: FractionalOffset.centerLeft,
         child: leading
-        )
       ));
     }
 
@@ -188,10 +186,8 @@ class ListItem extends StatelessWidget {
     if (trailing != null) {
       children.add(new Container(
         margin: const EdgeInsets.only(left: 16.0),
-        child: new Align(
-          alignment: FractionalOffset.centerRight,
+        align: FractionalOffset.centerRight,
         child: trailing
-        )
       ));
     }
 

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

@@ -370,8 +370,7 @@ class RefreshIndicatorState extends State<RefreshIndicator> {
                 padding: _isIndicatorAtTop
                   ? new EdgeInsets.only(top: config.displacement)
                   : new EdgeInsets.only(bottom: config.displacement),
-                child: new Align(
-                  alignment: _isIndicatorAtTop
+                align: _isIndicatorAtTop
                   ? FractionalOffset.bottomCenter
                   : FractionalOffset.topCenter,
                 child: new ScaleTransition(
@@ -390,7 +389,6 @@ class RefreshIndicatorState extends State<RefreshIndicator> {
               )
             )
           )
-          )
         ]
       )
     );

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

@@ -74,6 +74,7 @@ class Container extends StatelessWidget {
   /// Creates a widget that combines common painting, positioning, and sizing widgets.
   Container({
     Key key,
+    this.align,
     this.padding,
     this.decoration,
     this.foregroundDecoration,
@@ -99,6 +100,12 @@ class Container extends StatelessWidget {
   /// If null, the container will expand to fill all available space in its parent.
   final Widget child;
 
+  /// Align the child within the container.
+  ///
+  /// If non-null, the container will expand to fill its parent and position its
+  /// child within itself according to the given value.
+  final FractionalOffset align;
+
   /// Empty space to inscribe inside the decoration.
   final EdgeInsets padding;
 
@@ -133,6 +140,9 @@ class Container extends StatelessWidget {
     if (child == null && (constraints == null || !constraints.isTight))
       current = new ConstrainedBox(constraints: const BoxConstraints.expand());
 
+    if (align != null)
+      current = new Align(alignment: align, child: current);
+
     EdgeInsets effectivePadding = _paddingIncludingDecoration;
     if (effectivePadding != null)
       current = new Padding(padding: effectivePadding, child: current);