Use FrameTreeNode ID as frameId in extension APIs

extensions/browser/extension_api_frame_id_map.h

+// 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.
+
+#ifndef EXTENSIONS_BROWSER_EXTENSION_API_FRAME_ID_MAP_H_
+#define EXTENSIONS_BROWSER_EXTENSION_API_FRAME_ID_MAP_H_
+
+#include "base/callback.h"
+
+namespace content {
+class RenderFrameHost;
+class WebContents;
+}  // namespace content
+
+namespace extensions {
+
+// Extension frame IDs are exposed through the chrome.* APIs and have the
+// following characteristics:
+// - The top-level frame has ID 0.
+// - Any child frame has a positive ID.
+// - A non-existant frame has ID -1.
+// - They are only guaranteed to be unique within a tab.
+// - Multiple RenderFrameHosts may be mapped to the same ID.

[Devlin, 2015/12/12 14:25:00]

it's probably superfluous with above, but let's be explicit and say "mapped to
the same ID within different tabs".

[robwu, 2015/12/14 20:55:42]

This is not what I meant, so I'll clarify it even more. The extension frame ID
is derived from the FrameTreeNode ID , and this ID is constant for the lifetime
of a frame. But the RenderFrameHost for this frame can change over time, and
consequently result in a different RFH for the given FrameTreeNode ID.

+struct ExtensionApiFrameId {
+  static const int kInvalidFrameId = -1;
+
+  ExtensionApiFrameId();
+  explicit ExtensionApiFrameId(content::RenderFrameHost* rfh);
+
+  // The name "ExtensionApiFrameId" refers to |frame_id|.
+  int frame_id;
+
+  // The extension frame ID of the parent frame ID is also included because
+  // some extension APIs need both IDs.
+  int parent_frame_id;
+};
+
+// This class provides a mapping from a (render_process_id, frame_routing_id)
+// pair that maps a RenderFrameHost to an extension frame ID.
+class ExtensionApiFrameIdMap {
+ public:
+  using FrameIdCallback = base::Callback<void(const ExtensionApiFrameId&)>;
+
+  // Runs |callback| with the result that is equivalent to calling GetFrameId()
+  // on the UI thread. If the result was cached the callback is immediately run.
+  // Otherwise the callback is queued and run in order when the result becomes
+  // available. Queued callbacks are not called when the class is destroyed.
+  // Can only be called on the IO thread.
+  static void GetFrameIdOnIO(int render_process_id,
+                             int frame_routing_id,
+                             const FrameIdCallback& callback);
+
+  // Get the frameId for a RenderFrameHost identified by |render_process_id| and
+  // |frame_routing_id|. These methods always returns the same value when called
+  // with the same parameters.
+  // Can only be called on the UI thread.
+  static const ExtensionApiFrameId& GetFrameId(int render_process_id,
+                                               int frame_routing_id);
+  static const ExtensionApiFrameId& GetFrameId(content::RenderFrameHost* rfh);
+
+  // Find a RenderFrameHost for a given extension frame ID and WebContents.
+  // Returns nullptr if not found.
+  static content::RenderFrameHost* GetRenderFrameHostById(
+      content::WebContents* web_contents,
+      int frame_id);
+};
+
+}  // namespace extensions
+
+#endif  // EXTENSIONS_BROWSER_EXTENSION_API_FRAME_ID_MAP_H_