How to realize a callback in android?

问题

Maybe my knowledge about this topic is so small, that the word "callback" is not right here, sorry about that.

If I start an thread in any class/activity/other thread and after finishing I want it to execute some other code from the instance (class/activity/other thread) which started the thread, how do I realize that?

Currently, I do it like this:

Here is pseudocode.

// Method inside class/activity/other thread
private void startThread(){
    MyThread thread = new MyThread(this);
    thread.start();
}

And inside the Thread

public class MyThread extends Thread{
    private (class/activity/other thread) instanceAbove;

    public MyThread ( (class/activity/other thread) instanceAbove){
        this.instanceAbove = instanceAbove;
    }

    public void run(){
        //Do some stuff
        finish();
    }

    public void finish(){
        instanceAbove.MyThreadFinishMethod();
    }

}

I think this is not a good way, but can you give me an example?

Using interfaces is working for AlertDialogs, because I can use onAppend(). However, in this case, I don't know how I could use them.

回答1:

You can easily use an interface and handle the callback accordingly from your calling activity/class/other thread. Here is an example.

Define an interface like the following first.

public interface ThreadFinishListener {
    void onFinish();
}

Let us assume you are instantiating a Thread from an Activity. Hence implement the listener in your Activity like the following.

public class MainActivity extends AppCompatActivity implements ThreadFinishListener {
    // .... Other functions in your activity 

    @Override
    void onFinish() {
        // Do something when the Thread is finished and returned back to the activity
        // This is actually your callback function. 
    }
}

Now modify the constructor of your Thread class so that you can pass the listener to it.

public class MyThread extends Thread {
    private ThreadFinishListener listener; 

    public MyThread(ThreadFinishListener listener){
       this.listener = listener;
    }

    public void run(){
        //Do some stuff
        finish();
    }

    public void finish() {
        // This will invoke the function call in your activity
        listener.onFinish();
    }
}

Now while initializing your Thread, you might want to pass the listener to it as follows.

MyThread thread = new MyThread(this);
thread.run();

I hope that suffices.

回答2:

use AsyncTask:

private class MyAsyncTask extends AsyncTask<Void, Void, Void> {

    @Override
    protected void onPreExecute() {
      super.onPreExecute();
      //before start code in background 
    }

    @Override
    protected Void doInBackground(Void... voids) {
    //Do some stuff in background
      return null;
    }

    @Override
    protected void onPostExecute(Void aVoid) {
      super.onPostExecute(aVoid);
      //after code finished in background 
    }
}

And then call, new MyAsyncTask().execute();

来源：https://stackoverflow.com/questions/57620618/how-to-realize-a-callback-in-android

标签

java

android

multithreading

callback