Create manager to track OffscreenCanvasSurfaceImpl instances

third_party/WebKit/public/platform/modules/offscreencanvas/offscreen_canvas_surface.mojom

 // Copyright 2016 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.
 
 module blink.mojom;
 
 import "cc/ipc/compositor_frame.mojom";
 import "cc/ipc/mojo_compositor_frame_sink.mojom";
 import "cc/ipc/surface_id.mojom";
 import "cc/ipc/surface_sequence.mojom";
 
 interface OffscreenCanvasSurface {
   // TODO(619138): Make creation of surface id Async after implementation
   // change on surface_id generator that makes client side be able to generate
   // unique surface_id.id_namespace alone.
   [Sync]
-  GetSurfaceId() => (cc.mojom.SurfaceId surface_id);
+  GetSurfaceId(uint32 canvas_id) => (cc.mojom.SurfaceId surface_id);

[dcheng, 2016/11/08 18:30:23]

Help me understand =)

What is canvas id? why do we need to identify these with canvas ids?

[xlai (Olivia), 2016/11/10 15:45:11]

Canvas ID is a unique identifier for an HTMLCanvasElement. Actually, it is just
a unique ID for every DOM object.

We need to identify different canvas ID because we want to handle the case when
there are multiple HTMLCanvasElement transfer control
to Offscreen. In this case, the browser side needs to distinguish which
HTMLCanvasElement is making a request to the browser.

[dcheng, 2016/11/11 08:36:40]

In a Mojo world though, I would expect the endpoints to be associated by virtue
of having a connected interface. We shouldn't need numeric IDs to associate
different endpoints.

 
   Require(cc.mojom.SurfaceId surface_id, cc.mojom.SurfaceSequence sequence);
   Satisfy(cc.mojom.SurfaceSequence sequence);
 };
 
 interface OffscreenCanvasCompositorFrameSinkProvider {
-  CreateCompositorFrameSink(cc.mojom.SurfaceId surface_id,
+  CreateCompositorFrameSink(uint32 canvas_id,
+                            cc.mojom.SurfaceId surface_id,
                             cc.mojom.MojoCompositorFrameSinkClient client,
                             cc.mojom.MojoCompositorFrameSink& sink);
 };