Android Canvas locking throws IllegalArgumentException

Question

I am trying to implement my own custom SurfaceView that, when touched, draws a circle at the point where the user touched the screen. However, when I call mSurfaceHolder.lockCanvas() I get an exception. Something along the lines of an illegal argument whenever the canvas locks. Sample code is posted below.

public class TapArea extends SurfaceView implements SurfaceHolder.Callback {
    private static final int TAP_RADIUS = 4;
    private boolean mLoaded = false;
    private Paint mTapPaint;
    private SurfaceHolder mSurfaceHolder;

    protected OnTouchListener mTouchEvent = new OnTouchListener() {
        @Override
        public boolean onTouch(View arg0, MotionEvent arg1) {
            if (!mLoaded)
                return false;

            Canvas c = null;
            c = mSurfaceHolder.lockCanvas();
            drawTap(c, arg1);

            return true;
        }
    };

    public TapArea(Context context, AttributeSet attrs) {
        super(context, attrs);

        this.setOnTouchListener(mTouchEvent);
        mHandler = new Handler();
        mSurfaceHolder = getHolder();
        mSurfaceHolder.addCallback(this);
        mTapPaint = new Paint();
    }

    public void drawTap(Canvas canvas, MotionEvent tap) {
        canvas.drawCircle(tap.getX(), tap.getY(), TAP_RADIUS, mTapPaint);
    }

    @Override
    public void surfaceChanged(SurfaceHolder arg0, int arg1, int arg2, int arg3) {
        // TODO Auto-generated method stub

    }

    @Override
    public void surfaceCreated(SurfaceHolder arg0) {
        // TODO Auto-generated method stub
        mLoaded = true;
    }

    @Override
    public void surfaceDestroyed(SurfaceHolder arg0) {
        // TODO Auto-generated method stub

    }
}

Below are the error logs I keep getting:

01-17 00:19:44.703: E/Surface(9731): Surface::lock failed, already locked
01-17 00:19:44.796: E/SurfaceHolder(9731): Exception locking surface
01-17 00:19:44.796: E/SurfaceHolder(9731): java.lang.IllegalArgumentException
01-17 00:19:44.796: E/SurfaceHolder(9731):  at android.view.Surface.lockCanvasNative(Native Method)
01-17 00:19:44.796: E/SurfaceHolder(9731):  at android.view.Surface.lockCanvas(Surface.java:314)
01-17 00:19:44.796: E/SurfaceHolder(9731):  at android.view.SurfaceView$3.internalLockCanvas(SurfaceView.java:762)
01-17 00:19:44.796: E/SurfaceHolder(9731):  at android.view.SurfaceView$3.lockCanvas(SurfaceView.java:741)
01-17 00:19:44.796: E/SurfaceHolder(9731):  at com.frequency.FreqTapArea$2.onTouch(FreqTapArea.java:54)
01-17 00:19:44.796: E/SurfaceHolder(9731):  at android.view.View.dispatchTouchEvent(View.java:3897)
01-17 00:19:44.796: E/SurfaceHolder(9731):  at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:869)
01-17 00:19:44.796: E/SurfaceHolder(9731):  at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:869)
01-17 00:19:44.796: E/SurfaceHolder(9731):  at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:869)
01-17 00:19:44.796: E/SurfaceHolder(9731):  at com.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:1737)
01-17 00:19:44.796: E/SurfaceHolder(9731):  at com.android.internal.policy.impl.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1153)
01-17 00:19:44.796: E/SurfaceHolder(9731):  at android.app.Activity.dispatchTouchEvent(Activity.java:2096)
01-17 00:19:44.796: E/SurfaceHolder(9731):  at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:1721)
01-17 00:19:44.796: E/SurfaceHolder(9731):  at android.view.ViewRoot.deliverPointerEvent(ViewRoot.java:2200)
01-17 00:19:44.796: E/SurfaceHolder(9731):  at android.view.ViewRoot.handleMessage(ViewRoot.java:1884)
01-17 00:19:44.796: E/SurfaceHolder(9731):  at android.os.Handler.dispatchMessage(Handler.java:99)
01-17 00:19:44.796: E/SurfaceHolder(9731):  at android.os.Looper.loop(Looper.java:130)
01-17 00:19:44.796: E/SurfaceHolder(9731):  at android.app.ActivityThread.main(ActivityThread.java:3835)
01-17 00:19:44.796: E/SurfaceHolder(9731):  at java.lang.reflect.Method.invokeNative(Native Method)
01-17 00:19:44.796: E/SurfaceHolder(9731):  at java.lang.reflect.Method.invoke(Method.java:507)
01-17 00:19:44.796: E/SurfaceHolder(9731):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:847)
01-17 00:19:44.796: E/SurfaceHolder(9731):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:605)
01-17 00:19:44.796: E/SurfaceHolder(9731):  at dalvik.system.NativeStart.main(Native Method)

Help would be greatly appreciated.

Answer 1

You need to unlock the canvas after drawing on it. The right secuence is:

1. get de canvas calling mSurfaceHolder.lockCanvas();
2. draw on canvas.
3. unlock canvas calling mSurfaceHolder.unlockCanvasAndPost(c);

In your code could be:

public boolean onTouch(View arg0, MotionEvent arg1) {
        if (!mLoaded)
            return false;

        Canvas c = mSurfaceHolder.lockCanvas();
        drawTap(c, arg1);
        mSurfaceHolder.unlockCanvasAndPost(c);

        return true;
    }