Android: How to use a Handler handleMessage and access the reference to the MainActivity

问题

I have a service which receives a command from the Internet and launches a background thread. This thread is passed a handler from the service (the service is bounded and passed the handler) and sends a message to the handler to take a picture. I'm stuck on the implementation of the handler.

static Handler handler = new Handler() {

        @Override
        public void handleMessage(Message msg) {
            //TODO: Handle different types of messages
            mCamera.takePicture(null, null, MainActivity.this);
        }
};

Questions:

Does Handler need to be static? Without static, I get "This Handler class should be static or leaks might occur"
Does mCamera need to be static? I've been told to make mCamera static, but why is this necessary? Is there a way to setup takePicture without making mCamera static?
What's the proper way to pass the reference to MainActivity? Right now I get the error: "No enclosing instance of the type MainActivity is accessible in scope"

回答1:

You can make a class (Activity/Service) implement Handler.Callback and create a new Handler for it via new Handler(this).

回答2:

You can change your code as follows:

    static Handler handler = new Handler() {

            MainActivity mActivity;

            @Override
            public void handleMessage(Message msg) {
                //TODO: Handle different types of messages
                if(mActivity != null) {
                    mActivity.mCamera.takePicture(null, null, mActivity);
                }
            }
    };

    void MainActivity::onCreate(Bundle savedState) {
         ...
         handler.mActivity = this;
    }

    void MainActivity::onDestroy() {
        ...
        handler.mActivity = null;
    }

回答3:

Here is a good explanation why handler should be static:

This Handler class should be static or leaks might occur: IncomingHandler

About your other question, if you make Handler static you should also make all the fields that you use inside it static.

回答4:

You can use Message member obj and pass the desired object to the handler.

static Handler handler = new Handler() {

        @Override
        public void handleMessage(Message msg) {
            //TODO: Handle different types of messages
            //TODO: handle cast exception
            final MainActivity activity = (MainActivity) msg.obj;
            final Camera camera = activity.getCamera();
            camera.takePicture(null, null, activity);
        }
};

来源：https://stackoverflow.com/questions/17140391/android-how-to-use-a-handler-handlemessage-and-access-the-reference-to-the-main

标签

android

android-handler