Accessing the V8 context from other threads

[alextusinean]

I'm trying to modify a V8 value from my browser's main frame V8 context (by executing the UpdateLoadState function), but it fails: (from debug.log)

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[0223/182019.775:FATAL:cef_ref_counted.h(325)] Assert failed: ptr_ != 0.

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void UpdateLoadState(std::wstring text, int percent)
{
   auto obj = browser->GetMainFrame()->GetV8Context()->GetGlobal()->GetValue("app")->GetValue("loadState");
   if (text != L"") {
      obj->SetValue("text", CefV8Value::CreateString(text), V8_PROPERTY_ATTRIBUTE_NONE);
   }

   if (percent != -1) {
      obj->SetValue("percent", CefV8Value::CreateInt(percent), V8_PROPERTY_ATTRIBUTE_NONE);
   }

   CefV8ValueList arguments;
   obj->GetValue("update")->ExecuteFunction(NULL, arguments);
}

The assert fails only when UpdateLoadState is executed from a std::thread so I've looked over the Wiki and found out that all V8 execution must take place on the main thread.
My question is: How could I use V8 in other threads?

[magreenblatt]

Calling into V8 from other threads is not supported.

[alextusinean]

Calling into V8 from other threads is not supported.

I understand that, but what would be a way to do it? I've also tried using process messages but the same error occurs.
I'm not that experienced in C++ and CEF.

[magreenblatt]

You can post a task to the TID_RENDERER thread, and call Enter/Exit on the CefV8Context object.

[alextusinean]

You can post a task to the TID_RENDERER thread, and call Enter/Exit on the CefV8Context object.

I've managed to do this:

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void Manager::_UpdateLoadState(std::wstring text, int percent)
{
   auto ctx = browser->GetMainFrame()->GetV8Context();
   ctx->Enter();
   
   auto loadState = browser->GetMainFrame()->GetV8Context()->GetGlobal()->GetValue("app")->GetValue("loadState");
   if (text != L"") {
      loadState->SetValue("text", CefV8Value::CreateString(text), V8_PROPERTY_ATTRIBUTE_NONE);
   }
   
   if (percent != -1) {
      loadState->SetValue("percent", CefV8Value::CreateInt(percent), V8_PROPERTY_ATTRIBUTE_NONE);
   }
   
   CefV8ValueList arguments;
   loadState->GetValue("update")->ExecuteFunction(NULL, arguments);
   
   ctx->Exit();
}

void Manager::UpdateLoadState(std::wstring text, int percent)
{
   if (!CefCurrentlyOn(TID_RENDERER))
      CefPostTask(TID_RENDERER, base::Bind(Manager::_UpdateLoadState, this, text, percent));
   else
      _UpdateLoadState(text, percent);
}

The things in V8 seems to get updated but after _UpdateLoadState runs, the browser freezes.

[magreenblatt]

What does your “update” JS function do?

[alextusinean]

What does your “update” JS function do?

The browser is "running" a ReactJS app and the update function changes some html elements to the loadState's text and percent values. It is defined like this in my component:

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app.loadState.update = () => {
  this.forceUpdate();
};

Thought, I don't think the browser freezes because of the update function's execution. Even if I leave _UpdateLoadState's body blank (the function that does the V8 context stuff), the browser still freezes.

[alextusinean]

bump?

[magreenblatt]

What, specifically, freezes? Do you mean the web content stops responding? What about the rest of the UI (windows, menus, etc)? Can you navigate to a new URL?

[alextusinean]

What, specifically, freezes? Do you mean the web content stops responding? What about the rest of the UI (windows, menus, etc)? Can you navigate to a new URL?

I'm not sure what happened, but yes, the web content would stop responding and 1-2 seconds later, the browser would turn blank.
Now it seems to be fixed, not sure what I did . Thanks anyways.