我的Android应用程序的gameLoop线程在退出时崩溃

问题是，当我进入游戏并单击“主页”按钮时，gameLoop线程（我想）被弄乱了，然后弹出“不幸的是，“应用程序名称”已停止”。

我制作了一个非常简单的程序来解决此问题。该程序所做的全部工作就是在屏幕上显示一个数字，并在您触摸屏幕时增加它。当我在GameLoopThread中注释掉view.onDraw（c）时，一切似乎运行良好。

错误信息：

FATAL EXEPTION: Thread-23207
java.lang.NullPointerExeption
at com.example.crashtest.GameView.onDraw(GameView.java:61)
at com.example.crashtest.GameLoopThread.run(GameLoopThread.java:34)

这是代码：

MainActivity.java

package com.example.crashtest;

import android.os.Bundle;
import android.app.Activity;
import android.content.res.Configuration;
import android.view.KeyEvent;
import android.view.Window;

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        requestWindowFeature(Window.FEATURE_NO_TITLE);
        setContentView(new GameView(this));
    }

    @Override
    public void onConfigurationChanged(Configuration newConfig) {
        super.onConfigurationChanged(newConfig);
    }

    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        switch (keyCode) {
            case KeyEvent.KEYCODE_BACK:
                return true;

            case KeyEvent.KEYCODE_MENU:
                return true;

            case KeyEvent.KEYCODE_VOLUME_UP:
                return true;

            case KeyEvent.KEYCODE_VOLUME_DOWN:
                return true;

            default:
                return false;
        }
    }
}

GameView.java

package com.example.crashtest;

import android.annotation.SuppressLint;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Paint.Align;
import android.view.MotionEvent;
import android.view.SurfaceHolder;
import android.view.SurfaceView;

public class GameView extends SurfaceView { 
    private SurfaceHolder holder;
    private GameLoopThread gameLoopThread;

    private int num = 0;

    public GameView(final Context context) {
        super(context);

        holder = getHolder();
        holder.addCallback(new SurfaceHolder.Callback() {

            @Override
            public void surfaceDestroyed(SurfaceHolder holder) {

                   boolean retry = true;
                   gameLoopThread.setRunning(false);
                   while (retry) {
                          try {
                                gameLoopThread.join();
                                retry = false;
                          } catch (InterruptedException e) {}
                   }
            }

            @Override
            public void surfaceCreated(SurfaceHolder holder) {

                makeThread();
                gameLoopThread.start();
                gameLoopThread.setRunning(true);
            }

            @Override
            public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {

            }
        });
    }

    private void makeThread() {
        gameLoopThread = new GameLoopThread(this);
    }

    @SuppressLint({ "WrongCall", "DrawAllocation" })
    @Override
    protected void onDraw(Canvas canvas) {
        // Draw black background - Write variable 'num' on the screen
        canvas.drawColor(Color.BLACK);

        Paint paint = new Paint();
        paint.setARGB(255, 0, 255, 0);
        paint.setTextSize(50);
        paint.setTextAlign(Align.CENTER);

        canvas.drawText(Integer.toString(num), getWidth() / 2, getHeight() / 2, paint);
    }

    public boolean onTouchEvent(MotionEvent event) {
        if(event.getAction() == MotionEvent.ACTION_DOWN) {
            // Increase 'num' with 1 every touch
            num++;
        }

        return super.onTouchEvent(event);
    }
}

GameLoopThread.java

package com.example.crashtest;

import android.annotation.SuppressLint;
import android.graphics.Canvas;

public class GameLoopThread extends Thread {
      static final long FPS = 10;
       private GameView view;
       public boolean running = false;
       public boolean pause = false;

       public GameLoopThread(GameView view) {
             this.view = view;
       }

       public void setRunning(boolean run) {
             running = run;
       }

       @SuppressLint("WrongCall")
       @Override
       public void run() {
           long ticksPS = 1000 / FPS;
           long startTime = 0;
           long sleepTime = 0;

             while (running) {
                    Canvas c = null;
                    startTime = System.currentTimeMillis();

                    try {
                           c = view.getHolder().lockCanvas();
                           synchronized (view.getHolder()) {
                               view.onDraw(c);
                           }
                    } finally {
                           if (c != null) {
                                  view.getHolder().unlockCanvasAndPost(c);
                           }
                    }

                    sleepTime = ticksPS-(System.currentTimeMillis() - startTime);
                    try {
                           if (sleepTime > 0)
                                  sleep(sleepTime);
                           else
                                  sleep(10);
                    } catch (Exception e) {}
             }
       }
}