Added a ResourceMessageFilter for handling resource messages on another thread.

ppapi/host/resource_message_filter.h

Index: ppapi/host/resource_message_filter.h
diff --git a/ppapi/host/resource_message_filter.h b/ppapi/host/resource_message_filter.h
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+++ b/ppapi/host/resource_message_filter.h
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+// Copyright (c) 2012 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 PPAPI_HOST_RESOURCE_MESSAGE_FILTER_H_
+#define PPAPI_HOST_RESOURCE_MESSAGE_FILTER_H_
+
+#include "base/memory/ref_counted.h"
+#include "ppapi/c/pp_stdint.h"
+#include "ppapi/host/host_message_context.h"
+#include "ppapi/host/ppapi_host_export.h"
+#include "ppapi/host/resource_message_handler.h"
+
+namespace base {
+class MessageLoopProxy;
+class TaskRunner;
+}
+
+namespace IPC {
+class Message;
+}
+
+namespace ppapi {
+namespace host {
+
+class ResourceHost;
+
+// This is the base class of resource message filters that can handle resource
+// messages on another thread. ResourceHosts can handle most messages
+// directly, but if they need to handle something on a different thread it is
+// inconvenient. This class makes handling that case easier. This class is
+// similar to a BrowserMessageFilter but for resource messages. Note that the
+// liftetime of a ResourceHost is managed by a PpapiHost and may be destroyed
+// before or while your message is being processed on another thread.
+// If this is the case, the message handler will always be called but a reply
+// may not be sent back to the host.
+//
+// To handle a resource message on another thread you should implement a
+// subclass as follows:
+// class MyMessageFilter : public ResourceMessageFilter {
+//  protected:
+//   virtual scoped_refptr<base::TaskRunner> OverrideTaskRunnerForMessage(
+//       const IPC::Message& message) OVERRIDE {
+//     if (message.type() == MyMessage::ID)
+//       return BrowserThread::GetMessageLoopProxyForThread(BrowserThread::UI);
+//     return NULL;
+//   }
+//
+//   virtual int32_t OnResourceMessageReceived(
+//       const IPC::Message& msg,
+//       HostMessageContext* context) OVERRIDE {
+//     IPC_BEGIN_MESSAGE_MAP(MyMessageFilter, msg)
+//       PPAPI_DISPATCH_HOST_RESOURCE_CALL(MyMessage, OnMyMessage)
+//     IPC_END_MESSAGE_MAP()
+//     return PP_ERROR_FAILED;
+//   }
+//
+//  private:
+//   int32_t OnMyMessage(ppapi::host::HostMessageContext* context, ...) {
+//     // Will be run on the UI thread.
+//   }
+// }
+//
+// The filter should then be added in the resource host using:
+// AddFilter(make_scoped_refptr(new MyMessageFilter));
+class PPAPI_HOST_EXPORT ResourceMessageFilter
+    : public ResourceMessageHandler,
+      public base::RefCountedThreadSafe<ResourceMessageFilter> {
+ public:
+  // This object must be constructed on the same thread that a reply message
+  // should be sent, i.e. the IO thread when constructed in the browser process
+  // or the main thread when constructed in the renderer process. Since
+  // ResourceMessageFilters are usually constructed in the constructor of the
+  // owning ResourceHost, this will almost always be the case anyway.
+  ResourceMessageFilter();
+  // Test constructor. Allows you to specify the message loop which will be used
+  // to dispatch replies on.
+  ResourceMessageFilter(
+      scoped_refptr<base::MessageLoopProxy> reply_thread_message_loop_proxy);
+
+  // Called when a filter is added to a ResourceHost.
+  void OnFilterAdded(ResourceHost* resource_host);
+  // Called when a filter is removed from a ResourceHost.
+  void OnFilterDestroyed();
+
+  // This will dispatch the message handler on the target thread. It returns
+  // true if the message was handled by this filter and false otherwise.
+  virtual bool HandleMessage(const IPC::Message& msg,
+                             HostMessageContext* context) OVERRIDE;
+
+  // This can be called from any thread.
+  virtual void SendReply(const ReplyMessageContext& context,
+      const IPC::Message& msg) OVERRIDE;
+
+ protected:
+  friend class base::RefCountedThreadSafe<ResourceMessageFilter>;
+  virtual ~ResourceMessageFilter();
+
+  // If you want the message to be handled on another thread, return a non-null
+  // task runner which will target tasks accordingly.
+  virtual scoped_refptr<base::TaskRunner> OverrideTaskRunnerForMessage(
+      const IPC::Message& message);
+
+ private:
+  // This method is posted to the target thread and runs the message handler.
+  void DispatchMessage(const IPC::Message& msg,
+                       HostMessageContext context);
+
+  // Message loop to send resource message replies on. This will be the message
+  // loop proxy of the IO thread for the browser process or the main thread for
+  // the renderer process.
+  scoped_refptr<base::MessageLoopProxy> reply_thread_message_loop_proxy_;
+
+  // Non-owning pointer to the resource host owning this filter. Should only be
+  // accessed from the thread which sends messages to the plugin resource (i.e.
+  // the IO thread for the browser process or the main thread for the renderer).
+  // This will be NULL upon creation of the filter and is set to the owning
+  // ResourceHost when |OnFilterAdded| is called. When the owning ResourceHost
+  // is destroyed, |OnFilterDestroyed| is called and this will be set to NULL.
+  ResourceHost* resource_host_;
+
+  DISALLOW_COPY_AND_ASSIGN(ResourceMessageFilter);
+};
+
+}  // namespace host
+}  // namespace ppapi
+
+#endif  // PPAPI_HOST_RESOURCE_MESSAGE_FILTER_H_