CrystalBall app - fade in randomly doesn't work

    package com.atDesign.crystalball;

import android.app.Activity;
import android.graphics.drawable.AnimationDrawable;
import android.hardware.Sensor;
import android.hardware.SensorManager;
import android.media.MediaPlayer;
import android.media.MediaPlayer.OnCompletionListener;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;
import android.view.animation.AlphaAnimation;
import android.widget.ImageView;
import android.widget.TextView;
import com.atDesign.crystalball.R;
import com.atDesign.crystalball.ShakeDetector.OnShakeListener;

public class MainActivity extends Activity {
    public static final String TAG = MainActivity.class.getName();

    private CrystalBall mCrystalBall = new CrystalBall();
    private TextView mAnswerLabel;
    private ImageView mCrystalBallImage;
    private SensorManager mSensorManager;
    private Sensor mAccelerometer;
    private ShakeDetector mShakeDetector;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        //Assign the Views from the layout file
        mAnswerLabel = (TextView) findViewById(R.id.textView1);
        mCrystalBallImage = (ImageView) findViewById(R.id.imageView1);

        //System Services and the Accelerometer
        mSensorManager = (SensorManager) getSystemService(SENSOR_SERVICE);
        mAccelerometer = mSensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);
        mShakeDetector = new ShakeDetector(new OnShakeListener() {

            @Override
            public void onShake() {
                handleNewAnswer();

            }
        });    

        //Toast.makeText(this, "Yay!  Our Activity was created!",Toast.LENGTH_LONG).show();
        /*Toast welcomeToast = Toast.makeText(this, "Look at me up here!", Toast.LENGTH_LONG);
        welcomeToast.setGravity(Gravity.TOP, 0, 0);
        welcomeToast.show();*/

        Log.d("TAG", "We're logging from the onCreate() method!");
    }

    @Override
    public void onResume(){
        super.onResume();
        mSensorManager.registerListener(mShakeDetector, mAccelerometer, 
                SensorManager.SENSOR_DELAY_UI);
    }

    @Override
    public void onPause(){
        super.onPause();
        mSensorManager.unregisterListener(mShakeDetector);

    }

    private void animateCrystalBall() {
        mCrystalBallImage.setImageResource(R.drawable.ball_animation);
        AnimationDrawable ballAnimation = (AnimationDrawable) mCrystalBallImage.getDrawable();
        if (ballAnimation.isRunning()){
            ballAnimation.stop();
        }
        ballAnimation.start();
    }

    private void animateAnswer(){
        AlphaAnimation fadeInAnimation = new AlphaAnimation(0.0f, 1.0f);
        fadeInAnimation.setDuration(1500);
        fadeInAnimation.setFillAfter(true);

        mAnswerLabel.setAnimation(fadeInAnimation);
    }

    private void playSound(){
        MediaPlayer player = MediaPlayer.create(this, R.raw.crystal_ball);
        player.start();
        player.setOnCompletionListener(new OnCompletionListener() {

            @Override
            public void onCompletion(MediaPlayer mp) {
                mp.release();

            }
        });
    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    private String handleNewAnswer() {
        String answer = mCrystalBall.getAnAnswer();

        animateCrystalBall();
        animateAnswer();
        playSound();
        return answer;
    }

}