Java 使用SurfaceView时锁定画布失败且画布为空

Android SurfaceView的问题。 我想在SurfaceView中画一个圆，但在SimpleDraw方法中canvas是空的。为什么？ 否1，画布为空。 第二，在ClickListener中运行的SimpleDraw方法是正确的，canvas不是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);
    }

}

---

}区别在于时间

No.1尝试在画布创建之前绘制画布，但失败。调用

sfh.lockCanvas（…）

失败并返回

null

，而不是

Canvas

No.2尝试在画布创建后进行绘制，因此它可以正常工作

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（此）

创建画布后，使用新的
SimpleDraw
添加3个
@Override
方法。
我被你的问题弄糊涂了，你能澄清一下“2号，SimpleDraw方法在ClickListener中运行，是对的，画布不是空的，为什么？”因为调用
SimpleDraw（）
onClick
方法中的
被注释掉并且不会运行？我是中国人，英语不好。No 2可以运行。No 1和No 2有什么不同。
surfaceCreated（…）
将被调用，但这需要
sfh.addCallback（这个）
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) {

}