debugPaintPointersEnabled mode

Implements a mode that highlights RenderBoxes while events are being
routed through them.

Also, moves the size painting to after paint, so that opaque boxes
don't obscure the debugging lines.

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

@@ -5,6 +5,7 @@
 import 'dart:math' as math;
 import 'dart:ui' as ui;
 
+import 'package:flutter/gestures.dart';
 import 'package:flutter/painting.dart';
 import 'package:vector_math/vector_math_64.dart';
 
@@ -620,11 +621,28 @@ abstract class RenderBox extends RenderObject {
   /// The returned paint bounds are in the local coordinate system of this box.
   Rect get paintBounds => Point.origin & size;
 
+  int _debugActivePointers = 0;
+  void handleEvent(InputEvent event, HitTestEntry entry) {
+    super.handleEvent(event, entry);
+    assert(() {
+      if (debugPaintPointersEnabled) {
+        if (event.type == 'pointerdown')
+          _debugActivePointers += 1;
+        if (event.type == 'pointerup' || event.type == 'pointercancel')
+          _debugActivePointers -= 1;
+        markNeedsPaint();
+      }
+      return true;
+    });
+  }
+
   void debugPaint(PaintingContext context, Offset offset) {
     if (debugPaintSizeEnabled)
       debugPaintSize(context, offset);
     if (debugPaintBaselinesEnabled)
       debugPaintBaselines(context, offset);
+    if (debugPaintPointersEnabled)
+      debugPaintPointers(context, offset);
   }
   void debugPaintSize(PaintingContext context, Offset offset) {
     Paint paint = new Paint()
@@ -657,6 +675,13 @@ abstract class RenderBox extends RenderObject {
       context.canvas.drawPath(path, paint);
     }
   }
+  void debugPaintPointers(PaintingContext context, Offset offset) {
+    if (_debugActivePointers > 0) {
+      Paint paint = new Paint()
+       ..color = new Color(debugPaintPointersColorValue | ((0x04000000 * depth) & 0xFF000000));
+      context.canvas.drawRect(offset & size, paint);
+    }
+  }
 
   String debugDescribeSettings(String prefix) => '${super.debugDescribeSettings(prefix)}${prefix}size: ${ hasSize ? size : "MISSING" }\n';
 }

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

@@ -30,6 +30,12 @@ ui.Color debugPaintLayerBordersColor = const ui.Color(0xFFFF9800);
 /// Causes RenderObjects to paint warnings when painting outside their bounds.
 bool debugPaintBoundsEnabled = false;
 
+/// Causes RenderBox objects to flash while they are being tapped
+bool debugPaintPointersEnabled = false;
+
+/// The color to use when reporting pointers.
+int debugPaintPointersColorValue = 0x00BBBB;
+
 /// The color to use when painting RenderError boxes in checked mode.
 ui.Color debugErrorBoxColor = const ui.Color(0xFFFF0000);
 

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

@@ -1014,7 +1014,6 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
       _debugDoingThisPaint = true;
       debugLastActivePaint = _debugActivePaint;
       _debugActivePaint = this;
-      debugPaint(context, offset);
       if (debugPaintBoundsEnabled) {
         context.canvas.save();
         context.canvas.clipRect(paintBounds.shift(offset));
@@ -1031,6 +1030,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
       _debugReportException('paint', e, stack);
     }
     assert(() {
+      debugPaint(context, offset);
       if (debugPaintBoundsEnabled)
         context.canvas.restore();
       _debugActivePaint = debugLastActivePaint;
@@ -1082,8 +1082,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
   // EVENTS
 
   /// Override this function to handle events that hit this render object
-  void handleEvent(InputEvent event, HitTestEntry entry) {
-  }
+  void handleEvent(InputEvent event, HitTestEntry entry) { }
 
 
   // HIT TESTING