CefApp和CefClient的作用

CefApp

在cefsimple中，提到了一個cefapp的類型，這個類型是一個接口類，主要目的是提供獲取三種handler的接口

///
// Implement this interface to provide handler implementations. Methods will be
// called by the process and/or thread indicated.
///
/*--cef(source=client,no_debugct_check)--*/
class CefApp : public virtual CefBaseRefCounted {
 public:
  ///
  // Provides an opportunity to view and/or modify command-line arguments before
  // processing by CEF and Chromium. The |process_type| value will be empty for
  // the browser process. Do not keep a reference to the CefCommandLine object
  // passed to this method. The CefSettings.command_line_args_disabled value
  // can be used to start with an empty command-line object. Any values
  // specified in CefSettings that equate to command-line arguments will be set
  // before this method is called. Be cautious when using this method to modify
  // command-line arguments for non-browser processes as this may result in
  // undefined behavior including crashes.
  ///
  /*--cef(optional_param=process_type)--*/
  virtual void OnBeforeCommandLineProcessing(
      const CefString& process_type,
      CefRefPtr<CefCommandLine> command_line) {}

  ///
  // Provides an opportunity to register custom schemes. Do not keep a reference
  // to the |registrar| object. This method is called on the main thread for
  // each process and the registered schemes should be the same across all
  // processes.
  ///
  /*--cef()--*/
  virtual void OnRegisterCustomSchemes(
      CefRawPtr<CefSchemeRegistrar> registrar) {}

  ///
  // Return the handler for resource bundle events. If
  // CefSettings.pack_loading_disabled is true a handler must be returned. If no
  // handler is returned resources will be loaded from pack files. This method
  // is called by the browser and render processes on multiple threads.
  ///
  /*--cef()--*/
  virtual CefRefPtr<CefResourceBundleHandler> GetResourceBundleHandler() {
    return NULL;
  }

  ///
  // Return the handler for functionality specific to the browser process. This
  // method is called on multiple threads in the browser process.
  ///
  /*--cef()--*/
  virtual CefRefPtr<CefBrowserProcessHandler> GetBrowserProcessHandler() {
    return NULL;
  }

  ///
  // Return the handler for functionality specific to the render process. This
  // method is called on the render process main thread.
  ///
  /*--cef()--*/
  virtual CefRefPtr<CefRenderProcessHandler> GetRenderProcessHandler() {
    return NULL;
  }
};

GetResourceBundleHandler``GetBrowserProcessHandler``GetRenderProcessHandler分別可以得到對應回調handler，如果繼承了這三種handler就能在這些時機處理自己的回調業務，舉個例子，例如CefBrowserProcessHandler，可以實現browser進程的回調，用於執行browser進程生命周期中的重要回調。

///
// Class used to implement browser process callbacks. The methods of this class
// will be called on the browser process main thread unless otherwise indicated.
///
/*--cef(source=client,no_debugct_check)--*/
class CefBrowserProcessHandler : public virtual CefBaseRefCounted {
 public:
  ///
  // Called on the browser process UI thread immediately after the CEF context
  // has been initialized.
  ///
  /*--cef()--*/
  virtual void OnContextInitialized() {}

  ///
  // Called before a child process is launched. Will be called on the browser
  // process UI thread when launching a render process and on the browser
  // process IO thread when launching a GPU or plugin process. Provides an
  // opportunity to modify the child process command line. Do not keep a
  // reference to |command_line| outside of this method.
  ///
  /*--cef()--*/
  virtual void OnBeforeChildProcessLaunch(
      CefRefPtr<CefCommandLine> command_line) {}

  ///
  // Called on the browser process IO thread after the main thread has been
  // created for a new render process. Provides an opportunity to specify extra
  // information that will be passed to
  // CefRenderProcessHandler::OnRenderThreadCreated() in the render process. Do
  // not keep a reference to |extra_info| outside of this method.
  ///
  /*--cef()--*/
  virtual void OnRenderProcessThreadCreated(
      CefRefPtr<CefListValue> extra_info) {}

  ///
  // Return the handler for printing on Linux. If a print handler is not
  // provided then printing will not be supported on the Linux platform.
  ///
  /*--cef()--*/
  virtual CefRefPtr<CefPrintHandler> GetPrintHandler() { return NULL; }

  ///
  // Called from any thread when work has been scheduled for the browser process
  // main (UI) thread. This callback is used in combination with CefSettings.
  // external_message_pump and CefDoMessageLoopWork() in cases where the CEF
  // message loop must be integrated into an existing application message loop
  // (see additional comments and warnings on CefDoMessageLoopWork). This
  // callback should schedule a CefDoMessageLoopWork() call to happen on the
  // main (UI) thread. |delay_ms| is the requested delay in milliseconds. If
  // |delay_ms| is <= 0 then the call should happen reasonably soon. If
  // |delay_ms| is > 0 then the call should be scheduled to happen after the
  // specified delay and any currently pending scheduled call should be
  // cancelled.
  ///
  /*--cef()--*/
  virtual void OnScheduleMessagePumpWork(int64 delay_ms) {}
};

上述代碼就是CefBrowserProcessHandler提供的接口，可以看到，當上下文初始化后，子進程啟動前等等時機都有。在cefsimple中，還用OnContextInitialized函數來CreateBrowser，是十分重要的handler。

結合cefsimple來看，cefapp的具體作用。

// Implement application-level callbacks for the browser process.
class SimpleApp : public CefApp, public CefBrowserProcessHandler {
 public:
  SimpleApp();

  // CefApp methods:
  virtual CefRefPtr<CefBrowserProcessHandler> GetBrowserProcessHandler()
      OVERRIDE {
    return this;
  }

  // CefBrowserProcessHandler methods:
  virtual void OnContextInitialized() OVERRIDE;

 private:
  // Include the default reference counting implementation.
  IMPLEMENT_REFCOUNTING(SimpleApp);
};

十分簡單，還將CefApp和CefBrowserProcessHandler多繼承了，並且在OnContextInitialized中，調用了CreateBrowser。

從cefclient和cefsimple這倆demo來看，在構建app的時候CefApp主要用在倆地方，CefExecuteProcess和CefInitialize。從上面的代碼可以看出CefApp和監視進程級別的handler一起工作所以需要的時機會比較早。

CefClient

與CefApp相對應的還有CefClient，這個CefClient指的是一個類，不是指的那個demo程序，和CefClient類似，他也是提供了很多獲取handler的接口

class CefClient : public virtual CefBaseRefCounted {
 public:
  ///
  // Return the handler for context menus. If no handler is provided the default
  // implementation will be used.
  ///
  /*--cef()--*/
  virtual CefRefPtr<CefContextMenuHandler> GetContextMenuHandler() {
    return NULL;
  }

  ///
  // Return the handler for dialogs. If no handler is provided the default
  // implementation will be used.
  ///
  /*--cef()--*/
  virtual CefRefPtr<CefDialogHandler> GetDialogHandler() { return NULL; }

  ///
  // Return the handler for browser display state events.
  ///
  /*--cef()--*/
  virtual CefRefPtr<CefDisplayHandler> GetDisplayHandler() { return NULL; }
...

這些handler的都是基於功能的回調，應用開發者應該提供對應的handler實現，然后提供CefClient獲取對應handler實體。

因為是基於功能的回調，所以在瀏覽器沒起來的時候，也沒有啥功能。因此CefClient初始化和傳入的時機會比CefApp要晚一些，會在CreateBrowser的時候傳入。

在cefsimple中，也是采用了多繼承的方式實現，因為很簡單

class SimpleHandler : public CefClient,
                      public CefDisplayHandler,
                      public CefLifeSpanHandler,
                      public CefLoadHandler {
 public:
  explicit SimpleHandler(bool use_views);
  ~SimpleHandler();