CustomPaint size (#6790)

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

@@ -1783,17 +1783,27 @@ abstract class CustomPainter {
 /// tree as needing a new layout during the callback (the layout for this frame
 /// has already happened).
 ///
+/// Custom painters normally size themselves to their child. If they do not have
+/// a child, they attempt to size themselves to the [preferredSize], which
+/// defaults to [Size.zero].
+///
 /// See also:
 ///
-///  * [CustomPainter]
-///  * [Canvas]
+///  * [CustomPainter], the class that custom painter delegates should extend.
+///  * [Canvas], the API provided to custom painter delegates.
 class RenderCustomPaint extends RenderProxyBox {
   /// Creates a render object that delegates its painting.
   RenderCustomPaint({
     CustomPainter painter,
     CustomPainter foregroundPainter,
-    RenderBox child
-  }) : _painter = painter, _foregroundPainter = foregroundPainter, super(child);
+    Size preferredSize: Size.zero,
+    RenderBox child,
+  }) : _painter = painter,
+       _foregroundPainter = foregroundPainter,
+       _preferredSize = preferredSize,
+       super(child) {
+    assert(preferredSize != null);
+  }
 
   /// The background custom paint delegate.
   ///
@@ -1860,6 +1870,23 @@ class RenderCustomPaint extends RenderProxyBox {
     }
   }
 
+  /// The size that this [RenderCustomPaint] should aim for, given the layout
+  /// constraints, if there is no child.
+  ///
+  /// Defaults to [Size.zero].
+  ///
+  /// If there's a child, this is ignored, and the size of the child is used
+  /// instead.
+  Size get preferredSize => _preferredSize;
+  Size _preferredSize;
+  set preferredSize (Size value) {
+    assert(value != null);
+    if (preferredSize == value)
+      return;
+    _preferredSize = value;
+    markNeedsLayout();
+  }
+
   @override
   void attach(PipelineOwner owner) {
     super.attach(owner);
@@ -1886,6 +1913,11 @@ class RenderCustomPaint extends RenderProxyBox {
     return _painter != null && (_painter.hitTest(position) ?? true);
   }
 
+  @override
+  void performResize() {
+    size = constraints.constrain(preferredSize);
+  }
+
   void _paintWithPainter(Canvas canvas, Offset offset, CustomPainter painter) {
     int debugPreviousCanvasSaveCount;
     canvas.save();

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

@@ -197,9 +197,13 @@ class BackdropFilter extends SingleChildRenderObjectWidget {
 ///
 /// Painters are implemented by subclassing [CustomPainter].
 ///
-/// Because custom paint calls its painters during paint, you cannot mark the
-/// tree as needing a new layout during the callback (the layout for this frame
-/// has already happened).
+/// Because custom paint calls its painters during paint, you cannot call
+/// `setState` or `markNeedsLayout` during the callback (the layout for this
+/// frame has already happened).
+///
+/// Custom painters normally size themselves to their child. If they do not have
+/// a child, they attempt to size themselves to the [size], which defaults to
+/// [Size.zero].
 ///
 /// See also:
 ///
@@ -207,8 +211,10 @@ class BackdropFilter extends SingleChildRenderObjectWidget {
 ///  * [Canvas].
 class CustomPaint extends SingleChildRenderObjectWidget {
   /// Creates a widget that delegates its painting.
-  CustomPaint({ Key key, this.painter, this.foregroundPainter, Widget child })
-    : super(key: key, child: child);
+  CustomPaint({ Key key, this.painter, this.foregroundPainter, this.size: Size.zero, Widget child })
+    : super(key: key, child: child) {
+    assert(size != null);
+  }
 
   /// The painter that paints before the children.
   final CustomPainter painter;
@@ -216,17 +222,28 @@ class CustomPaint extends SingleChildRenderObjectWidget {
   /// The painter that paints after the children.
   final CustomPainter foregroundPainter;
 
+  /// The size that this [CustomPaint] should aim for, given the layout
+  /// constraints, if there is no child.
+  ///
+  /// Defaults to [Size.zero].
+  ///
+  /// If there's a child, this is ignored, and the size of the child is used
+  /// instead.
+  final Size size;
+
   @override
   RenderCustomPaint createRenderObject(BuildContext context) => new RenderCustomPaint(
     painter: painter,
-    foregroundPainter: foregroundPainter
+    foregroundPainter: foregroundPainter,
+    preferredSize: size,
   );
 
   @override
   void updateRenderObject(BuildContext context, RenderCustomPaint renderObject) {
     renderObject
       ..painter = painter
-      ..foregroundPainter = foregroundPainter;
+      ..foregroundPainter = foregroundPainter
+      ..preferredSize = size;
   }
 
   @override

packages/flutter/test/widgets/custom_paint_test.dart

@@ -42,4 +42,39 @@ void main() {
 
     expect(log, equals(<String>['background', 'child', 'foreground']));
   });
+
+  testWidgets('CustomPaint sizing', (WidgetTester tester) async {
+    GlobalKey target = new GlobalKey();
+
+    await tester.pumpWidget(new Center(
+      child: new CustomPaint(key: target)
+    ));
+    expect(target.currentContext.size, Size.zero);
+
+    await tester.pumpWidget(new Center(
+      child: new CustomPaint(key: target, child: new Container())
+    ));
+    expect(target.currentContext.size, const Size(800.0, 600.0));
+
+    await tester.pumpWidget(new Center(
+      child: new CustomPaint(key: target, size: const Size(20.0, 20.0))
+    ));
+    expect(target.currentContext.size, const Size(20.0, 20.0));
+
+    await tester.pumpWidget(new Center(
+      child: new CustomPaint(key: target, size: const Size(2000.0, 100.0))
+    ));
+    expect(target.currentContext.size, const Size(800.0, 100.0));
+
+    await tester.pumpWidget(new Center(
+      child: new CustomPaint(key: target, size: Size.zero, child: new Container())
+    ));
+    expect(target.currentContext.size, const Size(800.0, 600.0));
+
+    await tester.pumpWidget(new Center(
+      child: new CustomPaint(key: target, child: new Container(height: 0.0, width: 0.0))
+    ));
+    expect(target.currentContext.size, Size.zero);
+
+  });
 }