Add offstage attribute to OffStage (#5059)

Also, add a few more debugging flags.

packages/flutter/lib/rendering.dart

@@ -39,7 +39,6 @@ export 'src/rendering/layer.dart';
 export 'src/rendering/list.dart';
 export 'src/rendering/node.dart';
 export 'src/rendering/object.dart';
-export 'src/rendering/overflow.dart';
 export 'src/rendering/paragraph.dart';
 export 'src/rendering/performance_overlay.dart';
 export 'src/rendering/proxy_box.dart';

packages/flutter/lib/scheduler.dart

@@ -3,7 +3,7 @@
 // found in the LICENSE file.
 
 /// The Flutter Scheduler library.
-/// 
+///
 /// To use, import `package:flutter/scheduler.dart`.
 ///
 /// This library is responsible for scheduler frame callbacks, and tasks at
@@ -14,5 +14,6 @@
 library scheduler;
 
 export 'src/scheduler/binding.dart';
+export 'src/scheduler/debug.dart';
 export 'src/scheduler/priority.dart';
 export 'src/scheduler/ticker.dart';

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

-// Copyright 2015 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-import 'box.dart';
-import 'object.dart';
-
-/// Lays the child out as if it was in the tree, but without painting anything,
-/// without making the child available for hit testing, and without taking any
-/// room in the parent.
-class RenderOffStage extends RenderBox with RenderObjectWithChildMixin<RenderBox> {
-  /// Creates an off-stage render object.
-  RenderOffStage({ RenderBox child }) {
-    this.child = child;
-  }
-
-  @override
-  bool get sizedByParent => true;
-
-  @override
-  void performResize() {
-    size = constraints.smallest;
-  }
-
-  @override
-  void performLayout() {
-    if (child != null)
-      child.layout(constraints);
-  }
-
-  @override
-  bool hitTest(HitTestResult result, { Point position }) => false;
-
-  @override
-  void paint(PaintingContext context, Offset offset) { }
-
-  @override
-  void visitChildrenForSemantics(RenderObjectVisitor visitor) { }
-}

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

@@ -1980,6 +1980,108 @@ class RenderIgnorePointer extends RenderProxyBox {
   }
 }
 
+/// Lays the child out as if it was in the tree, but without painting anything,
+/// without making the child available for hit testing, and without taking any
+/// room in the parent.
+class RenderOffStage extends RenderProxyBox {
+  /// Creates an off-stage render object.
+  RenderOffStage({
+    bool offstage: true,
+    RenderBox child
+  }) : _offstage = offstage, super(child) {
+    assert(offstage != null);
+  }
+
+  /// Whether the child is hidden from the rest of the tree.
+  ///
+  /// If true, the child is laid out as if it was in the tree, but without
+  /// painting anything, without making the child available for hit testing, and
+  /// without taking any room in the parent.
+  ///
+  /// If false, the child is included in the tree as normal.
+  bool get offstage => _offstage;
+  bool _offstage;
+  set offstage(bool value) {
+    assert(value != null);
+    if (value == _offstage)
+      return;
+    _offstage = value;
+    markNeedsLayout();
+  }
+
+  @override
+  double computeMinIntrinsicWidth(double height) {
+    if (offstage)
+      return 0.0;
+    return super.computeMinIntrinsicWidth(height);
+  }
+
+  @override
+  double computeMaxIntrinsicWidth(double height) {
+    if (offstage)
+      return 0.0;
+    return super.computeMaxIntrinsicWidth(height);
+  }
+
+  @override
+  double computeMinIntrinsicHeight(double width) {
+    if (offstage)
+      return 0.0;
+    return super.computeMinIntrinsicHeight(width);
+  }
+
+  @override
+  double computeMaxIntrinsicHeight(double width) {
+    if (offstage)
+      return 0.0;
+    return super.computeMaxIntrinsicHeight(width);
+  }
+
+  @override
+  double computeDistanceToActualBaseline(TextBaseline baseline) {
+    if (offstage)
+      return null;
+    return super.computeDistanceToActualBaseline(baseline);
+  }
+
+  @override
+  bool get sizedByParent => offstage;
+
+  @override
+  void performResize() {
+    assert(offstage);
+    size = constraints.smallest;
+  }
+
+  @override
+  void performLayout() {
+    if (offstage) {
+      child?.layout(constraints);
+    } else {
+      super.performLayout();
+    }
+  }
+
+  @override
+  bool hitTest(HitTestResult result, { Point position }) {
+    return !offstage && super.hitTest(result, position: position);
+  }
+
+  @override
+  void paint(PaintingContext context, Offset offset) {
+    if (offstage)
+      return;
+    super.paint(context, offset);
+  }
+
+  @override
+  void visitChildrenForSemantics(RenderObjectVisitor visitor) {
+    if (offstage)
+      return;
+    super.visitChildrenForSemantics(visitor);
+  }
+}
+
 /// A render object that absorbs pointers during hit testing.
 ///
 /// When [absorbing] is `true`, this render object prevents its subtree from

packages/flutter/lib/src/scheduler/binding.dart

@@ -11,6 +11,7 @@ import 'dart:ui' show VoidCallback;
 import 'package:collection/collection.dart';
 import 'package:flutter/foundation.dart';
 
+import 'debug.dart';
 import 'priority.dart';
 
 export 'dart:ui' show VoidCallback;
@@ -426,6 +427,11 @@ abstract class SchedulerBinding extends BindingBase {
     Timeline.startSync('Frame');
     _firstRawTimeStampInEpoch ??= rawTimeStamp;
     Duration timeStamp = _adjustForEpoch(rawTimeStamp);
+    assert(() {
+      if (debugPrintBeginFrameBanner)
+        print('━‬━‬━‬━‬━‬━‬━┫ Begin Frame ($timeStamp) ┣━‬━‬━‬━‬━‬━‬━');
+      return true;
+    });
     _lastRawTimeStamp = rawTimeStamp;
     assert(!_isProducingFrame);
     _isProducingFrame = true;

packages/flutter/lib/src/scheduler/debug.dart

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+/// Print a banner at the beginning of each frame.
+bool debugPrintBeginFrameBanner = false;

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

@@ -1087,11 +1087,33 @@ class SizedOverflowBox extends SingleChildRenderObjectWidget {
 /// room in the parent.
 class OffStage extends SingleChildRenderObjectWidget {
   /// Creates a widget that visually hides its child.
-  OffStage({ Key key, Widget child })
-    : super(key: key, child: child);
+  OffStage({ Key key, this.offstage: true, Widget child })
+    : super(key: key, child: child) {
+    assert(offstage != null);
+  }
+
+  /// Whether the child is hidden from the rest of the tree.
+  ///
+  /// If true, the child is laid out as if it was in the tree, but without
+  /// painting anything, without making the child available for hit testing, and
+  /// without taking any room in the parent.
+  ///
+  /// If false, the child is included in the tree as normal.
+  final bool offstage;
+
+  @override
+  RenderOffStage createRenderObject(BuildContext context) => new RenderOffStage(offstage: offstage);
 
   @override
-  RenderOffStage createRenderObject(BuildContext context) => new RenderOffStage();
+  void updateRenderObject(BuildContext context, RenderOffStage renderObject) {
+    renderObject.offstage = offstage;
+  }
+
+  @override
+  void debugFillDescription(List<String> description) {
+    super.debugFillDescription(description);
+    description.add('offstage: $offstage');
+  }
 }
 
 /// A widget that attempts to size the child to a specific aspect ratio.

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

@@ -8,6 +8,9 @@ import 'package:flutter/foundation.dart';
 import 'framework.dart';
 import 'table.dart';
 
+/// Log the dirty widgets that are built each frame.
+bool debugPrintRebuildDirtyWidgets = false;
+
 Key _firstNonUniqueKey(Iterable<Widget> widgets) {
   Set<Key> keySet = new HashSet<Key>();
   for (Widget widget in widgets) {

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

@@ -1236,6 +1236,11 @@ class BuildOwner {
         int dirtyCount = _dirtyElements.length;
         int index = 0;
         while (index < dirtyCount) {
+          assert(() {
+            if (debugPrintRebuildDirtyWidgets)
+              debugPrint('Rebuilding ${_dirtyElements[index].widget}');
+            return true;
+          });
           _dirtyElements[index].rebuild();
           index += 1;
           if (dirtyCount < _dirtyElements.length) {

packages/flutter_test/lib/src/matchers.dart

@@ -136,11 +136,11 @@ class _FindsWidgetMatcher extends Matcher {
   }
 }
 
-bool _hasAncestorOfType(Finder finder, Type targetType) {
+bool _hasAncestorMatching(Finder finder, bool predicate(Widget widget)) {
   expect(finder, findsOneWidget);
   bool result = false;
   finder.evaluate().single.visitAncestorElements((Element ancestor) {
-    if (ancestor.widget.runtimeType == targetType) {
+    if (predicate(ancestor.widget)) {
       result = true;
       return false;
     }
@@ -149,11 +149,22 @@ bool _hasAncestorOfType(Finder finder, Type targetType) {
   return result;
 }
 
+bool _hasAncestorOfType(Finder finder, Type targetType) {
+  return _hasAncestorMatching(finder, (Widget widget) => widget.runtimeType == targetType);
+}
+
 class _IsOffStage extends Matcher {
   const _IsOffStage();
 
   @override
-  bool matches(Finder finder, Map<dynamic, dynamic> matchState) => _hasAncestorOfType(finder, OffStage);
+  bool matches(Finder finder, Map<dynamic, dynamic> matchState) {
+    return _hasAncestorMatching(finder, (Widget widget) {
+      if (widget.runtimeType != OffStage)
+        return false;
+      OffStage offstage = widget;
+      return offstage.offstage;
+    });
+  }
 
   @override
   Description describe(Description description) => description.add('offstage');
@@ -163,7 +174,20 @@ class _IsOnStage extends Matcher {
   const _IsOnStage();
 
   @override
-  bool matches(Finder finder, Map<dynamic, dynamic> matchState) => !_hasAncestorOfType(finder, OffStage);
+  bool matches(Finder finder, Map<dynamic, dynamic> matchState) {
+    expect(finder, findsOneWidget);
+    bool result = true;
+    finder.evaluate().single.visitAncestorElements((Element ancestor) {
+      Widget widget = ancestor.widget;
+      if (widget.runtimeType == OffStage) {
+        OffStage offstage = widget;
+        result = !offstage.offstage;
+        return false;
+      }
+      return true;
+    });
+    return result;
+  }
 
   @override
   Description describe(Description description) => description.add('onstage');