How to invoke CefV8Context::Eval() method in browser process?

Question

I want to invoke CefV8Context::Eval function and get the returned value in browser process's UI thread. But the CEF3 C++ API Docs states that V8 handles can only be accessed from the thread on which they are created. Valid threads for creating a V8 handle include the render process main thread (TID_RENDERER) and WebWorker threads. Is that means I should use the inter-process communication (CefProcessMessage) to invoke that method and get the return value? If so, how to do this in synchronous mode?

Answer 1

Short answer: CefFrame::ExecuteJavaScript for simple requests will work. For more complex ones, you have to give up one level of synchronousness or use a custom message loop.

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What I understand you want to do is to execute some Javascript code as part of your native App's UI Thread. There are two possibilities:

1. It's generic JS code, doesn't really access any variables or functions in your JS, and as such has not context. This means Cef can just spin up a new V8 context and execute your code - see CefFrame::ExecuteJavaScript(). To quote the examples on CEF's JS Integration link:

   CefRefPtr browser = ...; CefRefPtr frame = browser->GetMainFrame(); frame->ExecuteJavaScript("alert('ExecuteJavaScript works!');", frame->GetURL(), 0);

2. It's JS code with a context. In this case, read on.

Yes - CEF is designed such that only the RenderProcess has access to the V8 engine, you'll have to use a CefProcessMessage to head over to the Browser and do the evaluation there. You sound like you already know how to do that. I'll link an answer of mine for others who don't and may stumble upon this later: Background process on the native function at Chromium Embedded Framework

The CEFProcessMessage from Browser to Render processes is one place where the request has to be synchronized.

So after your send your logic over to the render process, you'll need to do the actual execution of the javascript code. That, thankfully, is quite easy - the same JS integration link goes on to say:

Native code can execute JS functions by using the ExecuteFunction() and ExecuteFunctionWithContext() methods

The best part - the execution seems to be synchronous (I say seems to, since I can't find concrete docs on this). The usage in the examples illustrates this:

if (callback_func_->ExecuteFunctionWithContext(callback_context_, NULL, args, retval, exception, false)) {
  if (exception.get()) {
    // Execution threw an exception.
  } else {
    // Execution succeeded.
  }
}

You'll notice that the second line assumes that the first has finished execution and that the results of said execution are available to it. So, The CefV8Value::ExecuteFunction() call is by nature synchronous.

So the question boils down to - How do I post a CefProcessMessage from Browser to Renderer process synchronously?. Unfortunately, the class itself is not set up to do that. What's more, the IPC Wiki Page explicitly disallows it:

Some messages should be synchronous from the renderer's perspective. This happens mostly when there is a WebKit call to us that is supposed to return something, but that we must do in the browser. Examples of this type of messages are spell-checking and getting the cookies for JavaScript. Synchronous browser-to-renderer IPC is disallowed to prevent blocking the user-interface on a potentially flaky renderer.

Is this such a big deal? Well, I don't really know since I've not come across this need - to me, it's ok since the Browser's message loop will keep spinning away waiting for something to do, and receive nothing till your renderer sends a process message with the results of JS. The only way the browser gets something else to do is when some interaction happens, which can't since the renderer is blocking.

If you really definitely need synchronousness, I'd recommend that you use your custom MessageLoop which calls CefDoMessageLoopWork() on every iteration. That way, you can set a flag to suspend loop work until your message is received from renderer. Note that CefDoMessageLoopWork() and CefRunMessageLoop() are mutually exclusive and cannot work with each other - you either manage the loop yourself, or let CEF do it for you.

That was long, and covers most of what you might want to do - hope it helps!