在指定的CEF线程上执行任务

CEF有几种线程，比如TID_UI、比如TID_RENDER，每种都有特定的含义，具体类别定义在cef_types.h中，摘录如下：

typedef enum {
// BROWSER PROCESS THREADS -- Only available in the browser process.

  ///
  // The main thread in the browser. This will be the same as the main
  // application thread if CefInitialize() is called with a
  // CefSettings.multi_threaded_message_loop value of false.
  ///
  TID_UI,

  ///
  // Used to interact with the database.
  ///
  TID_DB,

  ///
  // Used to interact with the file system.
  ///
  TID_FILE,

  ///
  // Used for file system operations that block user interactions.
  // Responsiveness of this thread affects users.
  ///
  TID_FILE_USER_BLOCKING,

  ///
  // Used to launch and terminate browser processes.
  ///
  TID_PROCESS_LAUNCHER,

  ///
  // Used to handle slow HTTP cache operations.
  ///
  TID_CACHE,

  ///
  // Used to process IPC and network messages.
  ///
  TID_IO,

// RENDER PROCESS THREADS -- Only available in the render process.

  ///
  // The main thread in the renderer. Used for all WebKit and V8 interaction.
  ///
  TID_RENDERER,
} cef_thread_id_t;

关于这些线程的说明，看上面代码里的注释，另外亦可以参开这里：https://bitbucket.org/chromiumembedded/cef/wiki/GeneralUsage#markdown-header-threads。

我们要想在某个线程上执行任务，比如你在Browser进程里想弹出个窗口，就要在TID_UI线程上来执行你的代码。类似这种任务，可以通过CefTask来完成。很简单，cef_task.h里有CefTask接口的声明，我们继承它，然后再使用CefPostTask即可。

代码类似下面：

#include "include/cef_task.h"

void doSomethingOnUIThread()
{
    MessageBox(...);
}

class SomeTask : public CefTask
{
    IMPLEMENT_REFCOUNTING(SomeTask);
public:
    void Execute()
    {
        doSomethingOnUIThread();
    }
};

void doSomething()
{
    if (CefCurrentlyOn(TID_UI))
    {
        doSomethingOnUIThread();
    }
    else
    {
        CefRefPtr<SomeTask> t = new SomeTask;
        CefPostTask(TID_UI, t);
    }
}

如果你不想自己写一个类实现CefTask接口，CEF还有一些辅助手段来帮到你。先看下面的代码片段：

if (!CefCurrentlyOn(TID_UI))
{
    // Execute on the UI thread.
    CefPostTask(TID_UI,
        base::Bind(&ClientHandler::CloseAllBrowsers, this, force_close));
    return;
}

这是CefSimple里的，它使用了base::Bind来生成一个Functor，用法类似STL的bind，包含”include/base/cef_bind.h”头文件即可。

其他参考文章详见我的专栏：【CEF与PPAPI开发】。