Use the baseline information exposed by C++ to pipe baseline data through RenderBox.

sky/examples/rendering/baseline.dart

Index: sky/examples/rendering/baseline.dart
diff --git a/sky/examples/rendering/baseline.dart b/sky/examples/rendering/baseline.dart
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+++ b/sky/examples/rendering/baseline.dart
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+// 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 'dart:sky' as sky;
+
+import 'package:sky/painting/text_style.dart';
+import 'package:sky/rendering/block.dart';
+import 'package:sky/rendering/box.dart';
+import 'package:sky/rendering/object.dart';
+import 'package:sky/rendering/paragraph.dart';
+import 'package:sky/rendering/sky_binding.dart';
+
+RenderBox getBox(double lh) {
+  RenderParagraph paragraph = new RenderParagraph(
+    new InlineStyle(
+      new TextStyle(),
+      [
+        new InlineText('test'),
+        new InlineStyle(
+          new TextStyle(
+            color: const Color(0xFF0000A0),
+            fontFamily: 'serif',
+            fontSize: 50.0,
+            height: lh
+          ),
+          [new InlineText('مرحبا Hello')]
+        )
+      ]
+    )
+  );
+  return new RenderPadding(
+    padding: new EdgeDims.all(10.0),
+    child: new RenderConstrainedBox(
+      additionalConstraints: new BoxConstraints.tightFor(height: 200.0),
+      child: new RenderDecoratedBox(
+        decoration: new BoxDecoration(
+          backgroundColor: const Color(0xFFFFFFFF)
+        ),
+        child: new RenderPadding(
+          padding: new EdgeDims.all(10.0),
+          child: new RenderCustomPaint(
+            child: paragraph,
+            callback: (canvas, size) {
+              double baseline = paragraph.getDistanceToBaseline(TextBaseline.alphabetic);
+              double w = paragraph.getMaxIntrinsicWidth(new BoxConstraints.loose(size));
+              double h = paragraph.getMaxIntrinsicHeight(new BoxConstraints.loose(size));
+              Path path = new Path();
+              path.moveTo(0.0, 0.0);
+              path.lineTo(w, 0.0);
+              path.moveTo(0.0, baseline);
+              path.lineTo(w, baseline);
+              path.moveTo(0.0, h);
+              path.lineTo(w, h);
+              Paint paint = new Paint();
+              paint.color = const Color(0xFFFF9000);
+              paint.setStyle(sky.PaintingStyle.stroke);
+              paint.strokeWidth = 3.0;
+              canvas.drawPath(path, paint);  
+            }
+          )
+        )
+      )
+    )
+  );
+}
+
+void main() {
+  RenderBox root = new RenderBlock(children: [
+    new RenderConstrainedBox(
+      additionalConstraints: new BoxConstraints.tightFor(height: 50.0)
+    ),
+    getBox(1.0),
+    getBox(null),
+  ]);
+  var b = new SkyBinding(root: root);
+  // b.onFrame = b.debugDumpRenderTree;
+}