Why canvas = null in my case?

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Java

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Android

public class MyActivity extends Activity {
    /**
     * Called when the activity is first created.
     */
    Button btn;
    SurfaceView sv;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        btn = (Button)findViewById(R.id.button1);
        sv =  (SurfaceView)findViewById(R.id.surfaceView1);
        sv.setOnTouchListener( new SurfaceView.OnTouchListener(){
            private boolean moving = false;//stupid state
            @Override
            public boolean onTouch(View v, MotionEvent event) {
                switch( event.getAction() ){
                    case MotionEvent.ACTION_DOWN:
                        final int x = (int)event.getX();
                        final int y = (int)event.getY();
                        final SurfaceHolder mHolder;
                        mHolder = sv.getHolder();
                        Canvas canvas = null;
                        canvas = mHolder.lockCanvas();
                        Paint mPaint = new Paint();
                        mPaint.setColor(Color.WHITE);
                        try{
                        canvas.drawText("some text", 50, 50, mPaint);
                        }catch (Exception xx)
                        {
                            xx.toString();//canvas = null
                        }
                        return true;
                    case MotionEvent.ACTION_MOVE:
                        if( moving ){
                            final int x_new = (int)event.getX();
                            final int y_new = (int)event.getY();
                                }

                        return true;
                    case MotionEvent.ACTION_UP:
                        moving = false;
                        return true;
                }
                return false;
            }
        });
    }

I just want to draw some info when I touched or moved. I don't want to use OnDraw method because it eat CPU.
Edited as was suggested:

@Override
protected void onPause()
{
    mHolder = sv.getHolder();
    canvas = null;
    canvas = mHolder.lockCanvas();
}

But it not helped, this func doesn't executes, canvas still = null.
I also tried this, in button OnClick:

mPaint = new Paint();
mPaint.setColor(Color.WHITE);
mHolder = sv.getHolder();
canvas = null;
canvas = mHolder.lockCanvas();
try{
   canvas.drawText("some text", 100, 100, mPaint);
}catch (Exception xx)
{
  xx.toString();
}

- When I first click on button canvas != null, but after canvas.drawText execution go in nowhere, when I make second click - canvas = null. My brain corrode.