android - NullPointerException on lockCanvas()

我正在为android开发一个小游戏（测试），但是我不理解用surfaceview制作一个好的定时器线程的逻辑。
我遇到了[surfaceview object].getholder（）.lockCanvas（空）的问题。问题是lockCanvas返回的值无效（在本例中为空）。
我不能抓住这个例外，因为我…不能！：
这是我的代码：

package game;

import game.logics.SceneGroup;
import game.scenes.TeamScene;
import java.util.Timer;
import java.util.TimerTask;
import android.app.Activity;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.os.Bundle;
import android.view.SurfaceHolder;
import android.view.SurfaceView;
import android.view.Window;
import android.view.WindowManager;

// Definição da Atividade
public class GameActivity extends Activity {
    // Definição do Ambiente
    public class GameSurface extends SurfaceView
                                     implements SurfaceHolder.Callback {
        public GameSurface(final Context context) {
            super(context);

            this.getHolder().addCallback(this);
            this.setFocusable(true);
        }

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

        @Override
        public void surfaceCreated(final SurfaceHolder arg0) {
            GameActivity.this.running = true;
        }

        @Override
        public void surfaceDestroyed(final SurfaceHolder arg0) {
            GameActivity.this.running = false;
        }

        public void update(final Canvas canvas) {
            GameActivity.this.scenes.update(canvas);
        }
    }

    class GameTask extends TimerTask {
        @Override
        public void run() {
            if(GameActivity.this.running == false) {
                return;
            }

            final SurfaceHolder holder =
                                GameActivity.this.surface.getHolder();

            Canvas canvas = null;
            try {
                canvas = holder.lockCanvas();
                canvas.drawColor(Color.BLACK);

                synchronized(holder) {
                    GameActivity.this.surface.update(canvas);
                }
            }
            finally {
                if(canvas != null) {
                    holder.unlockCanvasAndPost(canvas);
                }
            }
        }
    }

    private GameSurface     surface;
    private SceneGroup      scenes;

    private Timer           timer;
    private TimerTask       task;
    private boolean         running = false;

    @Override
    public void onCreate(final Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        this.requestWindowFeature(Window.FEATURE_NO_TITLE);
        this.getWindow().setFlags(
                            WindowManager.LayoutParams.FLAG_FULLSCREEN,
            WindowManager.LayoutParams.FLAG_FULLSCREEN);

        // ### SOLVE: PROBLEM START HERE ###
        this.surface = new GameSurface(this);

        this.scenes = new SceneGroup(this);
        this.scenes.add(new TeamScene(this));

        this.timer = new Timer();
        this.task = new GameTask();

        // ### SOLVE: PROBLEM END HERE ###
        // ### Invalid new instance of GameSurface ###
        this.setContentView(new GameSurface(this));
    }

    @Override
    public void onPause() {
        super.onPause();
        this.timer.cancel();
    }

    @Override
    public void onResume() {
        super.onResume();
        this.timer.scheduleAtFixedRate(this.task, 0, 33);
    }
}

我做得对吗？
我能做什么来修正我的代码？

最佳答案

你正在创建游戏表面两次。
曾经在这里：

this.surface = new GameSurface(this);

这里再说一遍：

this.setContentView(new GameSurface(this));

大概应该是同一个？