Android活动在切换应用程序时崩溃

Question

When pushing the Android activity to the background (pressing the app switcher or home button), the application crashes immediately ("Unfortunately, Apps has stopped.").

My activity looks like this:

public class MyActivity extends Activity {
    MyView myView;

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

The view in question uses a thread for drawing:

public class MyView extends SurfaceView implements SurfaceHolder.Callback
{
    protected Engine engine;

    protected SurfaceHolder surfaceHolder;
    protected Context context;

    private PaintThread thread;

    void initView() {
        // Initialize our screen holder
        SurfaceHolder holder = getHolder();
        holder.addCallback( this);

        // Get screen size
        DisplayMetrics displayMetrics = new DisplayMetrics();
        ((Activity) getContext()).getWindowManager().getDefaultDisplay().getMetrics(displayMetrics);

        // Initialize engine
        engine = new Engine(displayMetrics.widthPixels, displayMetrics.heightPixels);
        engine.init(context);

        thread = new PaintThread(holder, context, new Handler(), engine);
        setFocusable(true);
    }

    public MyView(Context context, AttributeSet attrs, int defStyle){
        super(context, attrs, defStyle);
        this.context = context;
        initView();
    }

    public MyView(Context context, AttributeSet attrs){
        super(context, attrs);
        this.context = context;
        initView();
    }

    @Override
    public void surfaceChanged(SurfaceHolder arg0, int arg1, int arg2, int arg3) {}

    @Override
    public void surfaceDestroyed(SurfaceHolder arg0) {
        Log.i("app", "here!");
        boolean retry = true;

        thread.state = PaintThread.PAUSED;
        while (retry) {
            try {
                thread.join();
                retry = false;
            } catch (InterruptedException e) {}
        }
    }

    @Override
    public void surfaceCreated(SurfaceHolder arg0) {
        if (thread.state == PaintThread.PAUSED){
            thread = new PaintThread(getHolder(), context, new Handler(), engine);
            thread.start();
        } else {
            thread.start();
        }
    }
}

And the thread works like this:

public class PaintThread extends Thread {
    private SurfaceHolder surfaceHolder;
    private Handler handler;
    private Context context;

    private Engine engine;

    public int state = 1;

    public final static int RUNNING = 1;
    public final static int PAUSED = 2;

    public PaintThread(SurfaceHolder surfaceHolder, Context context, Handler handler,
                       Engine engine) {
        this.surfaceHolder = surfaceHolder;
        this.handler = handler;
        this.context = context;

        this.engine = engine;
    }

    @Override
    public void run() {
        long previousUpdate = System.nanoTime();
        long beforeTime;
        long passedTime;
        long accumulator = 0;
        long dt = 1000/60;

        while (state == RUNNING) {
            beforeTime = System.nanoTime();
            passedTime = TimeUnit.MILLISECONDS.convert(beforeTime - previousUpdate, TimeUnit.NANOSECONDS);
            previousUpdate = beforeTime;

            accumulator += passedTime;

            while (accumulator >= dt) {
                engine.update(dt);
                accumulator -= dt;
            }

            Canvas c = null;
            try {
                c = surfaceHolder.lockCanvas(null);

                synchronized (surfaceHolder) {
                    c.drawColor(Color.WHITE);
                    engine.draw(c);
                }
            } finally {
                if (c != null) {
                    surfaceHolder.unlockCanvasAndPost(c);
                }
            }
        }
    }
}

The log message in the view its surfaceDestroyed method is being displayed when one of those buttons is pressed but the app crashes anyway with the following message:

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.graphics.Canvas.drawColor(int)' on a null object reference

The affected line is from the PaintThread its run method:

c.drawColor(Color.WHITE);

It seems like lockCanvas is at that point not returning a Canvas we can work with. This seems to be intended behavior as the canvas is put in the background when one of those buttons is pressed.

But, this all happens just before the thread is paused. Is there anything wrong with the setup that this happens or how can this be prevented?

Answer 1

一个if语句，在使用c之前检查c是否为null？

Answer 2

I just faced a similar situation to OP, and my fix was to put an If statement around codes that draw on canvas.

 if (canvas != null) {
      // draw on canvas
 }

Hope that helps anyone who reads this!