Clean up some RenderObject layer stuff (#6883)

More idiomatic use of constraints in performResize.

Trivial fixes to comments.

Make ProxyBox not use BoxParentData since it ignores the field.

Make applyPaintTransform more helpful if you use a different ParentData
subclass than RenderBox expects.

Make debugAssertIsValid actually fulfill its contract in RenderObject as
documented.

Add a childBefore for symmetry (we already had childAfter).

Fix the way we dump the child list when there's no children in a
multichild render object.

More asserts in the rendering test library.

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

@@ -1631,7 +1631,7 @@ abstract class RenderBox extends RenderObject {
   @override
   void performResize() {
     // default behavior for subclasses that have sizedByParent = true
-    size = constraints.constrain(Size.zero);
+    size = constraints.smallest;
     assert(!size.isInfinite);
   }
 
@@ -1657,7 +1657,7 @@ abstract class RenderBox extends RenderObject {
   /// given hit test result.
   ///
   /// The caller is responsible for transforming [position] into the local
-  /// coordinate space of the callee.  The callee is responsible for checking
+  /// coordinate space of the callee. The callee is responsible for checking
   /// whether the given position is within its bounds.
   ///
   /// Hit testing requires layout to be up-to-date but does not require painting
@@ -1712,7 +1712,7 @@ abstract class RenderBox extends RenderObject {
   /// Override this method to check whether any children are located at the
   /// given position.
   ///
-  /// Typically children should be hit tested in reverse paint order so that
+  /// Typically children should be hit-tested in reverse paint order so that
   /// hit tests at locations where children overlap hit the child that is
   /// visually "on top" (i.e., paints later).
   ///
@@ -1733,9 +1733,28 @@ abstract class RenderBox extends RenderObject {
   /// child's [parentData] in the [BoxParentData.offset] field.
   @override
   void applyPaintTransform(RenderObject child, Matrix4 transform) {
+    assert(child != null);
     assert(child.parent == this);
-    BoxParentData childParentData = child.parentData;
-    Offset offset = childParentData.offset;
+    assert(() {
+      if (child.parentData is! BoxParentData) {
+        throw new FlutterError(
+          '$runtimeType does not implement applyPaintTransform.\n'
+          'The following $runtimeType object:\n'
+          '  ${this.toStringShallow()}\n'
+          '...did not use a BoxParentData class for the parentData field of the following child:\n'
+          '  ${child.toStringShallow()}\n'
+          'The $runtimeType class inherits from RenderBox. '
+          'The default applyPaintTransform implementation provided by RenderBox assumes that the '
+          'children all use BoxParentData objects for their parentData field. '
+          'Since $runtimeType does not in fact use that ParentData class for its children, it must '
+          'provide an implementation of applyPaintTransform that supports the specific ParentData '
+          'subclass used by its children (which apparently is ${child.parentData.runtimeType}).'
+        );
+      }
+      return true;
+    });
+    final BoxParentData childParentData = child.parentData;
+    final Offset offset = childParentData.offset;
     transform.translate(offset.dx, offset.dy);
   }
 

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

@@ -515,7 +515,10 @@ abstract class Constraints {
   bool debugAssertIsValid({
     bool isAppliedConstraint: false,
     InformationCollector informationCollector
-  });
+  }) {
+    assert(isNormalized);
+    return isNormalized;
+  }
 }
 
 /// Signature for a function that is called for each [RenderObject].
@@ -1296,7 +1299,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
 
   /// Calls visitor for each immediate child of this render object.
   ///
-  /// Override in subclasses with children and call the visitor for each child
+  /// Override in subclasses with children and call the visitor for each child.
   void visitChildren(RenderObjectVisitor visitor) { }
 
   /// The object responsible for creating this render object.
@@ -2606,6 +2609,9 @@ abstract class ContainerRenderObjectMixin<ChildType extends RenderObject, Parent
     }
   }
   /// Insert child into this render object's child list after the given child.
+  ///
+  /// If `after` is null, then this inserts the child at the start of the list,
+  /// and the child becomes the new [firstChild].
   void insert(ChildType child, { ChildType after }) {
     assert(child != this);
     assert(after != this);
@@ -2744,30 +2750,41 @@ abstract class ContainerRenderObjectMixin<ChildType extends RenderObject, Parent
   /// The last child in the child list.
   ChildType get lastChild => _lastChild;
 
+  /// The previous child before the given child in the child list.
+  ChildType childBefore(ChildType child) {
+    assert(child != null);
+    assert(child.parent == this);
+    final ParentDataType childParentData = child.parentData;
+    return childParentData.previousSibling;
+  }
+
   /// The next child after the given child in the child list.
   ChildType childAfter(ChildType child) {
+    assert(child != null);
+    assert(child.parent == this);
     final ParentDataType childParentData = child.parentData;
     return childParentData.nextSibling;
   }
 
   @override
   String debugDescribeChildren(String prefix) {
-    String result = '$prefix \u2502\n';
-    if (_firstChild != null) {
-      ChildType child = _firstChild;
+    if (firstChild != null) {
+      String result = '$prefix \u2502\n';
+      ChildType child = firstChild;
       int count = 1;
-      while (child != _lastChild) {
+      while (child != lastChild) {
         result += '${child.toStringDeep("$prefix \u251C\u2500child $count: ", "$prefix \u2502")}';
         count += 1;
         final ParentDataType childParentData = child.parentData;
         child = childParentData.nextSibling;
       }
       if (child != null) {
-        assert(child == _lastChild);
+        assert(child == lastChild);
         result += '${child.toStringDeep("$prefix \u2514\u2500child $count: ", "$prefix  ")}';
       }
+      return result;
     }
-    return result;
+    return '';
   }
 }
 

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

@@ -51,6 +51,14 @@ class RenderProxyBox extends RenderBox with RenderObjectWithChildMixin<RenderBox
 /// impractical (e.g. because you want to mix in other classes as well).
 // TODO(ianh): Remove this class once https://github.com/dart-lang/sdk/issues/15101 is fixed
 abstract class RenderProxyBoxMixin implements RenderBox, RenderObjectWithChildMixin<RenderBox> {
+  @override
+  void setupParentData(RenderObject child) {
+    // We don't actually use the offset argument in BoxParentData, so let's
+    // avoid allocating it at all.
+    if (child.parentData is! ParentData)
+      child.parentData = new ParentData();
+  }
+
   @override
   double computeMinIntrinsicWidth(double height) {
     if (child != null)
@@ -101,6 +109,9 @@ abstract class RenderProxyBoxMixin implements RenderBox, RenderObjectWithChildMi
     return child?.hitTest(result, position: position) ?? false;
   }
 
+  @override
+  void applyPaintTransform(RenderObject child, Matrix4 transform) { }
+
   @override
   void paint(PaintingContext context, Offset offset) {
     if (child != null)

packages/flutter/test/rendering/box_test.dart

@@ -15,8 +15,10 @@ void main() {
         backgroundColor: const Color(0xFF00FF00),
         gradient: new RadialGradient(
           center: FractionalOffset.topLeft, radius: 1.8,
-          colors: <Color>[Colors.yellow[500], Colors.blue[500]]),
-        boxShadow: kElevationToShadow[3])
+          colors: <Color>[Colors.yellow[500], Colors.blue[500]],
+        ),
+        boxShadow: kElevationToShadow[3],
+      ),
     );
     layout(root);
     expect(root.size.width, equals(800.0));
@@ -25,28 +27,28 @@ void main() {
 
   test('Flex and padding', () {
     RenderBox size = new RenderConstrainedBox(
-      additionalConstraints: new BoxConstraints().tighten(height: 100.0)
+      additionalConstraints: new BoxConstraints().tighten(height: 100.0),
     );
     RenderBox inner = new RenderDecoratedBox(
       decoration: new BoxDecoration(
-        backgroundColor: const Color(0xFF00FF00)
+        backgroundColor: const Color(0xFF00FF00),
       ),
-      child: size
+      child: size,
     );
     RenderBox padding = new RenderPadding(
       padding: new EdgeInsets.all(50.0),
-      child: inner
+      child: inner,
     );
     RenderBox flex = new RenderFlex(
       children: <RenderBox>[padding],
       direction: Axis.vertical,
-      crossAxisAlignment: CrossAxisAlignment.stretch
+      crossAxisAlignment: CrossAxisAlignment.stretch,
     );
     RenderBox outer = new RenderDecoratedBox(
       decoration: new BoxDecoration(
         backgroundColor: const Color(0xFF0000FF)
       ),
-      child: flex
+      child: flex,
     );
 
     layout(outer);
@@ -65,13 +67,15 @@ void main() {
 
   test("should not have a 0 sized colored Box", () {
     RenderBox coloredBox = new RenderDecoratedBox(
-      decoration: new BoxDecoration()
+      decoration: new BoxDecoration(),
+    );
+    RenderBox paddingBox = new RenderPadding(
+      padding: const EdgeInsets.all(10.0),
+      child: coloredBox,
     );
-    RenderBox paddingBox = new RenderPadding(padding: const EdgeInsets.all(10.0),
-        child: coloredBox);
     RenderBox root = new RenderDecoratedBox(
       decoration: new BoxDecoration(),
-      child: paddingBox
+      child: paddingBox,
     );
     layout(root);
     expect(coloredBox.size.width, equals(780.0));
@@ -80,22 +84,23 @@ void main() {
 
   test("reparenting should clear position", () {
     RenderDecoratedBox coloredBox = new RenderDecoratedBox(
-      decoration: new BoxDecoration());
+      decoration: new BoxDecoration(),
+    );
+
     RenderPadding paddedBox = new RenderPadding(
-      child: coloredBox, padding: const EdgeInsets.all(10.0));
-
+      child: coloredBox,
+      padding: const EdgeInsets.all(10.0),
+    );
     layout(paddedBox);
-
     BoxParentData parentData = coloredBox.parentData;
     expect(parentData.offset.dx, isNot(equals(0.0)));
-
     paddedBox.child = null;
+
     RenderConstrainedBox constraintedBox = new RenderConstrainedBox(
-      child: coloredBox, additionalConstraints: const BoxConstraints());
-
+      child: coloredBox,
+      additionalConstraints: const BoxConstraints(),
+    );
     layout(constraintedBox);
-
-    parentData = coloredBox.parentData;
-    expect(parentData.offset.dx, equals(0.0));
+    expect(coloredBox.parentData?.runtimeType, ParentData);
   });
 }

packages/flutter/test/rendering/rendering_tester.dart

@@ -70,6 +70,8 @@ void layout(RenderBox box, {
 
 void pumpFrame({ EnginePhase phase: EnginePhase.layout }) {
   assert(renderer != null);
+  assert(renderer.renderView != null);
+  assert(renderer.renderView.child != null); // call layout() first!
   renderer.phase = phase;
   renderer.beginFrame();
 }