Add debug flag `debugProfileLayoutsEnabled`. (#63652)

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

@@ -121,8 +121,10 @@ bool debugPrintMarkNeedsPaintStacks = false;
 ///
 /// See also:
 ///
-///  * [debugProfilePaintsEnabled], which does something similar for
-///    painting but using the timeline view.
+///  * [debugProfileLayoutsEnabled], which does something similar for layout
+///    but using the timeline view.
+///  * [debugProfilePaintsEnabled], which does something similar for painting
+///    but using the timeline view.
 ///  * [debugPrintRebuildDirtyWidgets], which does something similar for widgets
 ///    being rebuilt.
 ///  * The discussion at [RendererBinding.drawFrame].
@@ -134,6 +136,21 @@ bool debugPrintLayouts = false;
 /// enabled by the test framework.
 bool debugCheckIntrinsicSizes = false;
 
+/// Adds [dart:developer.Timeline] events for every [RenderObject] layout.
+///
+/// For details on how to use [dart:developer.Timeline] events in the Dart
+/// Observatory to optimize your app, see:
+/// <https://fuchsia.googlesource.com/topaz/+/master/shell/docs/performance.md>
+///
+/// See also:
+///
+///  * [debugPrintLayouts], which does something similar for layout but using
+///    console output.
+///  * [debugProfileBuildsEnabled], which does something similar for widgets
+///    being rebuilt.
+///  * [debugProfilePaintsEnabled], which does something similar for painting.
+bool debugProfileLayoutsEnabled = false;
+
 /// Adds [dart:developer.Timeline] events for every [RenderObject] painted.
 ///
 /// This is only enabled in debug builds. The timing information this exposes is
@@ -146,11 +163,11 @@ bool debugCheckIntrinsicSizes = false;
 ///
 /// See also:
 ///
-///  * [debugPrintLayouts], which does something similar for layout but using
-///    console output.
 ///  * [debugProfileBuildsEnabled], which does something similar for widgets
 ///    being rebuilt, and [debugPrintRebuildDirtyWidgets], its console
 ///    equivalent.
+///  * [debugProfileLayoutsEnabled], which does something similar for layout,
+///    and [debugPrintLayouts], its console equivalent.
 ///  * The discussion at [RendererBinding.drawFrame].
 ///  * [RepaintBoundary], which can be used to contain repaints when unchanged
 ///    areas are being excessively repainted.

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

@@ -1674,6 +1674,9 @@ abstract class RenderObject extends AbstractNode with DiagnosticableTreeMixin im
   /// implemented here) to return early if the child does not need to do any
   /// work to update its layout information.
   void layout(Constraints constraints, { bool parentUsesSize = false }) {
+    if (!kReleaseMode && debugProfileLayoutsEnabled)
+      Timeline.startSync('$runtimeType',  arguments: timelineArgumentsIndicatingLandmarkEvent);
+
     assert(constraints != null);
     assert(constraints.debugAssertIsValid(
       isAppliedConstraint: true,
@@ -1727,6 +1730,9 @@ abstract class RenderObject extends AbstractNode with DiagnosticableTreeMixin im
         _debugDoingThisResize = false;
         return true;
       }());
+
+      if (!kReleaseMode && debugProfileLayoutsEnabled)
+        Timeline.finishSync();
       return;
     }
     _constraints = constraints;
@@ -1789,6 +1795,9 @@ abstract class RenderObject extends AbstractNode with DiagnosticableTreeMixin im
     }());
     _needsLayout = false;
     markNeedsPaint();
+
+    if (!kReleaseMode && debugProfileLayoutsEnabled)
+      Timeline.finishSync();
   }
 
   /// If a subclass has a "size" (the state controlled by `parentUsesSize`,

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

@@ -100,9 +100,11 @@ bool debugPrintGlobalKeyedWidgetLifecycle = false;
 ///
 /// See also:
 ///
-///  * [debugProfilePaintsEnabled], which does something similar but for
-///    painting, and [debugPrintRebuildDirtyWidgets], which does something similar
-///    but reporting the builds to the console.
+///  * [debugPrintRebuildDirtyWidgets], which does something similar but
+///    reporting the builds to the console.
+///  * [debugProfileLayoutsEnabled], which does something similar for layout,
+///    and [debugPrintLayouts], its console equivalent.
+///  * [debugProfilePaintsEnabled], which does something similar for painting.
 bool debugProfileBuildsEnabled = false;
 
 /// Show banners for deprecated widgets.