Post a Message from Java to JS

android_webview/common/aw_message_port_messages.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.
 
 // Multiply-included file, no traditional include guard.
 #include <vector>
 
 #include "base/basictypes.h"
 #include "ipc/ipc_message_macros.h"
 
 #define IPC_MESSAGE_START AwMessagePortMsgStart
 
 //-----------------------------------------------------------------------------
 // MessagePort messages
 // These are messages sent from the browser to the renderer process.
 
-// Tells the renderer to convert the sent message from a WebSerializeScript
-// format to a base::ListValue. Due to the complexities of renderer/browser
-// relation, this can only be done in renderer for now.
-IPC_MESSAGE_ROUTED3(AwMessagePortMsg_Message,
+// Normally the postmessages are exchanged between the renderers and the message
+// itself is opaque to the browser process. The format of the message is a
+// WebSerializesScriptValue.  A WebSerializedScriptValue is a blink structure
+// and can only be serialized/deserialized in renderer. Further, we could not
+// have Blink or V8 on the browser side due to their relience on static
+// variables.
+//
+// For posting messages from Java (Webview) to JS, we pass the browser/renderer
+// boundary an extra time and convert the messages to a type that browser can
+// use. Since WebView is single-process this is not terribly expensive, but
+// if we can do the conversion at the browser, then we can drop this code.
+
+// Important Note about multi-process situation: Webview is single process so
+// such a conversion does not increase the risk due to untrusted renderers.
+// However, in a multi-process scenario, the renderer that does the conversion
+// can be different then the renderer that receives the message. There are
+// 2 possible solutions to deal with this:
+// 1. Do the conversion at the browser side by writing a new serializer
+// deserializer for WebSerializedScriptValue
+// 2. Do the conversion at the content layer, at the renderer at the time of
+// receiveing the message. This may need adding new flags to indicate that
+// message needs to be converted. However, this is complicated due to queing
+// at the browser side and possibility of ports being shipped to a different
+// renderer or browser delegate.
+
+
+// Tells the renderer to convert the message from a WebSerializeScript
+// format to a base::ListValue. This IPC is used for messages that are
+// incoming to Android webview from JS.
+IPC_MESSAGE_ROUTED3(AwMessagePortMsg_WebToAppMessage,
                     int /* recipient message port id */,
                     base::string16 /* message */,
                     std::vector<int> /* sent message port_ids */)
+
+// Tells the renderer to convert the message from a String16
+// format to a WebSerializedScriptValue. This IPC is used for messages that
+// are outgoing from Webview to JS.
+// TODO(sgurun) when we start supporting other types, use a ListValue instead
+// of string16
+IPC_MESSAGE_ROUTED3(AwMessagePortMsg_AppToWebMessage,
+                    int /* recipient message port id */,
+                    base::string16 /* message */,
+                    std::vector<int> /* sent message port_ids */)
 
 //-----------------------------------------------------------------------------
 // These are messages sent from the renderer to the browser process.
 
-// Response to AwMessagePortMessage_ConvertMessage
-IPC_MESSAGE_ROUTED3(AwMessagePortHostMsg_ConvertedMessage,
+// Response to AwMessagePortMessage_WebToAppMessage
+IPC_MESSAGE_ROUTED3(AwMessagePortHostMsg_ConvertedWebToAppMessage,
                     int /* recipient message port id */,
                     base::ListValue /* converted message */,
                     std::vector<int> /* sent message port_ids */)
+
+// Response to AwMessagePortMessage_AppToWebMessage
+IPC_MESSAGE_ROUTED3(AwMessagePortHostMsg_ConvertedAppToWebMessage,
+                    int /* recipient message port id */,
+                    base::string16 /* converted message */,
+                    std::vector<int> /* sent message port_ids */)