Implement RenderImage and Image for Sky

sky/sdk/lib/framework/rendering/render_image.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 'dart:sky' as sky;
+import 'package:sky/framework/net/image_cache.dart' as image_cache;
+import 'render_node.dart';
+import 'render_box.dart';
+
+class RenderImage extends RenderBox {
+
+  RenderImage(String src, num width, num height) {

[abarth-chromium, 2015/06/03 00:07:54]

s/num/double/

We're pretty committed to render tree sizes being doubles at this point.

[jackson, 2015/06/03 17:41:42]

Acknowledged.

+    configure(src, width, height);
+  }
+
+  sky.Image _image;
+  sky.Size requestedSize;
+  final sky.Paint _paint = new sky.Paint();
+
+  void configure(String src, num width, num height) {

[eseidel, 2015/06/02 23:46:01]

Should this be private?

+    sky.Size newSize = new sky.Size(width, height);

[abarth-chromium, 2015/06/03 00:07:54]

Probably the caller should pass in the sky.Size instead of passing in height and
width separately.

+    if (requestedSize != newSize) {
+      requestedSize = newSize;
+      markNeedsLayout();
+      markNeedsPaint();

[abarth-chromium, 2015/06/03 00:07:54]

No need to markNeedsPaint here.  markNeedsLayout will trigger a paint if this
changes size.

+    }
+    image_cache.load(src, (result) {
+      if (_image != result) {
+        _image = result;
+        updateSizeFromImage();
+        markNeedsLayout();

[abarth-chromium, 2015/06/03 00:07:54]

Shouldn't updateSizeFromImage do the work of markNeedsLayout if the relevant
sizing information changed?  Otherwise we'll do a layout for no reason.

+        markNeedsPaint();
+      }
+    });
+  }
+
+  void updateSizeFromImage() {

[eseidel, 2015/06/02 23:46:01]

Should this be private?

+    // If both height and width are specified, we don't care about image dimensions
+    if (requestedSize.width != null && requestedSize.height != null)
+      size = constraints.constrain(requestedSize);

[abarth-chromium, 2015/06/03 00:07:53]

How do you know |constraints| is populated at this point?  This code is called
from a network callback.  We might not have been through layout yet.

Also, you don't want to set |size| outside of a |performLayout| function.  The
position and size of other RenderNodes might depend on the size of this node. 
That's why layout is a coordinated walk of the tree.

+
+    // If there's no image, we can't size ourselves
+    if (_image == null)
+      return;
+
+    // If neither height nor width are specified, use inherent image dimensions
+    // If only one dimension is specified, adjust the other dimension to
+    // maintain the aspect ratio
+    if (requestedSize.width == null) {
+      if (requestedSize.height == null) {
+        size = constraints.constrain(new sky.Size(_image.width, _image.height));
+      } else {
+        num width = requestedSize.height * _image.width / _image.height;
+        size = constraints.constrain(new sky.Size(width, requestedSize.height));
+      }
+    } else if (requestedSize.height == null) {
+      num height = requestedSize.width * _image.height / _image.width;
+      size = constraints.constrain(new sky.Size(requestedSize.width, height));
+    } else {
+      size = constraints.constrain(requestedSize);
+    }
+  }
+
+  void performLayout() {
+    updateSizeFromImage();
+  }
+
+  void paint(RenderNodeDisplayList canvas) {
+    if (_image == null) return;
+    double widthScale = size.width / _image.width;
+    double heightScale = size.height / _image.height;
+    canvas.save();
+    canvas.scale(widthScale, heightScale);

[abarth-chromium, 2015/06/03 00:07:54]

We should skip the save/scale/restore if size.width == _image.width &&
size.height == _image.height.  That's the common case as save/scale/restore is
relatively expensive.

+    canvas.drawImage(_image, 0.0, 0.0, _paint);

[abarth-chromium, 2015/06/03 00:07:54]

I don't think it's worth caching the paint on this object.  If we need to cache
empty paint objects, we can have a global static one that everyone can share
instead of storing a separate one on every RenderImage object in the tree.

+    canvas.restore();
+  }
+}