Android / Java：如何从类引用MainActivity.java

Question

Sorry for the question, probably it is answered within a few minutes. I'm new to Android App development and have been searching for an answer for about 2 hours, but I don't find a solution.

So, this is my problem: I created a MainActivity with a very simple layout, only one ToggleButton to start/stop some sound. I got it working with calling the MediaPlayer from within the MainActivity-Class. Now I want to put the MediaPlayer-Handling into a separate class, such that it can be called from a widget as well. When rising a Toast or calling a MediaPlayer-Method, I need to refer to the MainActivity, which was (in the MainActivity itself) "this". But I don't know how to refer to the instance of the MainActivity.

The code is as follows:

package com.heavyloadreverse;

//import java.io.IOException;

import android.app.Activity;
//import android.media.MediaPlayer;
import android.os.Bundle;
import android.view.View;
//import android.widget.Toast;
import android.widget.ToggleButton;

public class MainActivity extends Activity {

    //private MediaPlayer mp;
    private Sound snd;
    private ToggleButton btn;   

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        btn = (ToggleButton) findViewById(R.id.btn_OnOff);
        snd = new Sound();
        snd.mp_create(MainActivity.this);
    }

    public void onToggleClicked(View v) {       
        // Perform action on clicks
        if (((ToggleButton) v).isChecked()) {            
            snd.mp_start();
        } else {            
            snd.mp_stop();
        }
    }

    /*********************************************************************************
    public void mp_create() {
        mp = MediaPlayer.create(this, R.raw.truckreverse);
    }

    public void mp_start () {
        Toast.makeText(this, R.string.start, Toast.LENGTH_SHORT).show();
        // start the sound  
        mp.setLooping(true);
        mp.start();
    }

    public void mp_stop () {
        Toast.makeText(this, R.string.stop, Toast.LENGTH_SHORT).show();
        // stop the sound
        mp.stop();
        try {
            mp.prepare();
        } catch (IllegalStateException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

    public void mp_init() {
        btn.setChecked(false); 
    }
    **********************************************************************************/

    public void btn_init() {
        btn.setChecked(false); 
    }   

    @Override
    public void onStart() {
        super.onStart();
    }

    @Override
    public void onRestart() {
        super.onRestart();      
        btn_init();     
    }

    @Override
    public void onResume() {
        super.onResume();
        btn_init();
    }

    @Override
    public void onPause() {
        super.onPause();
        snd.mp_stop();
    }

    @Override
    public void onStop() {
        super.onStop();
        snd.mp_stop();
    }

    @Override
    public void onDestroy() {
        super.onDestroy(); 
        snd.mp_stop();
    }

    @Override
    public void onSaveInstanceState(Bundle savedInstanceState) {
        super.onSaveInstanceState(savedInstanceState);      
    }

}

The class for the MediaPlayer-Handling:

package com.heavyloadreverse;

import java.io.IOException;

import android.app.Application;
import android.media.MediaPlayer;
import android.widget.Toast;
import com.heavyloadreverse.R;

public class Sound extends Application {

    private MediaPlayer mp; 

    public void mp_create (MainActivity main) {
        Toast.makeText(main.this, "test", Toast.LENGTH_SHORT).show();
        mp = new MediaPlayer();
        try {
            mp = MediaPlayer.create(this, R.raw.truckreverse); 
        } catch (IllegalStateException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (RuntimeException e) {
            e.printStackTrace();
        }       
    }

    public void mp_start () {
        Toast.makeText(MainActivity.this, R.string.start, Toast.LENGTH_SHORT).show();
        // start the sound  
        try {
            mp.setLooping(true);
            mp.start();
        } catch (IllegalStateException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (RuntimeException e) {
            e.printStackTrace();
        }       
    }

    public void mp_stop () {
        //Toast.makeText(this, R.string.stop, Toast.LENGTH_SHORT).show();       
        try {
            // stop the sound
            mp.stop();
            mp.prepare();
        } catch (IllegalStateException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (RuntimeException e) {
            e.printStackTrace();
        }
    }

}

Toast.makeText(this, "test", Toast.LENGTH_SHORT).show();

--> raises a runtime-error when executing:

--> 03-12 20:23:18.412: E/AndroidRuntime(862): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.heavyloadreverse/com.heavyloadreverse.MainActivity}: java.lang.NullPointerException

Toast.makeText(main.this, "test", Toast.LENGTH_SHORT).show();

--> error in code:

--> *Multiple markers at this line - main cannot be resolved to a type - Line breakpoint:Sound [line: 15] - mp_create(MainActivity)*

Toast.makeText(MainActivity.this, "test", Toast.LENGTH_SHORT).show();

--> error in code:

--> No enclosing instance of the type MainActivity is accessible in scope

What do I have to do in order to make the Toast- and MediaPlayer-Calls in "Sound.java" working?

Thanks a lot in advance.

Sven

Answer 1

Option 1 Add 'Context' as a parameter on 'Sound'

public class Sound{
    private Context mContext;
    Sound(Context context){
        mContext = context;
    }

    ...
    Toast.makeText(mContext, text, length).show();
    ...
}

When you create Sound from activity you will do it like new Sound(this);

Option 2

Define an interface in Sound to provide callbacks

public class Sound {
    interface OnSoundListener{
        public void onSoundStarted();
        public void onSoundStopped();
    }
}

And your main activity will look like

public class MainActivity implements Sound.OnSoundListener{
    @Override
    public void onSoundStarted(){
        //your toast here
    }
}

Personally I prefer the second one, that way you can separate logic from UI.

Answer 2

Not sure if this work, only an idea.

Firs of all extend your Sound class from your MainActivity

public class Sound extends MainActivity {

second, this is the code I use for Toast to work:

Toast.makeText(MainActivity.this,"Your Text Here",Toast.LENGTH_LONG).show();

Answer 3

For Toast this is what you need to do:

Toast toast=Toast.makeText(this, "Hello toast", 2000);

     toast.show();

Check this tutorial tutorial if it helps.