java - 使用 SurfaceView 时锁定 Canvas 失败且 Canvas 为空

Question

Android SurfaceView的问题。 我想在SurfaceView中画一个Circle，但是在SimpleDraw方法中canvas为null。为什么？ 否 1， Canvas 为空。 否2，SimpleDraw方法在ClickListener中运行，是对的，canvas不为null，为什么？

1.public class SurfaceTestActivity extends Activity {
SurfaceView sfv;
SurfaceHolder sfh;

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    sfv=(SurfaceView)findViewById(R.id.surface);
    sfh=sfv.getHolder();
    SimpleDraw();

}

void SimpleDraw(){
    Canvas canvas = sfh.lockCanvas(new Rect(0, 0, 300,
            getWindowManager().getDefaultDisplay().getHeight()));
    Paint mPaint = new Paint();
    mPaint.setColor(Color.GREEN);
    mPaint.setStrokeWidth(2);
    canvas.drawCircle(150, 150, 80, mPaint);
    sfh.unlockCanvasAndPost(canvas);
    }

}

2.public class SurfaceTestActivity extends Activity {
Button simpleButton;
SurfaceView sfv;
SurfaceHolder sfh;

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    simpleButton=(Button)findViewById(R.id.simple);
    sfv=(SurfaceView)findViewById(R.id.surface);
    sfh=sfv.getHolder();
    simpleButton.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub
            //SimpleDraw();
        }
    });
}

void SimpleDraw(){
    Canvas canvas = sfh.lockCanvas(new Rect(0, 0, 300,
            getWindowManager().getDefaultDisplay().getHeight()));
    Paint mPaint = new Paint();
    mPaint.setColor(Color.GREEN);
    mPaint.setStrokeWidth(2);
    canvas.drawCircle(150, 150, 80, mPaint);
    sfh.unlockCanvasAndPost(canvas);
    }

}

Answer 1

区别在于时间。

没有。 1 尝试在创建 Canvas 之前绘制 Canvas ，但失败。对 sfh.lockCanvas(...) 的调用失败并返回 null，而不是 Canvas。

没有。 2 创建 Canvas 后尝试绘制，因此可以正常工作。

这样做:

public class SurfaceTestActivity extends Activity implements SurfaceHolder.Callback {
SurfaceView sfv;
SurfaceHolder sfh;

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    sfv=(SurfaceView)findViewById(R.id.surface);
    sfh=sfv.getHolder();
    sfh.addCallback(this);

}

void SimpleDraw(){
    Canvas canvas = sfh.lockCanvas(new Rect(0, 0, 300,
            getWindowManager().getDefaultDisplay().getHeight()));
    Paint mPaint = new Paint();
    mPaint.setColor(Color.GREEN);
    mPaint.setStrokeWidth(2);
    canvas.drawCircle(150, 150, 80, mPaint);
    sfh.unlockCanvasAndPost(canvas);
}



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

@Override
public void surfaceCreated(SurfaceHolder holder) {
    // draw AFTER surface created!
    simpleDraw();

}

@Override
public void surfaceDestroyed(SurfaceHolder holder) {

}

变化:

添加SurfaceHolder.Callback

添加sfh.addCallback(this);

在创建 Canvas 后，使用新的 SimpleDraw 添加 3 个 @Override 方法。