//自定義view繼承view

 public class XfermodeView extends View {

private Context context;
private Path path;
private Canvas mcanvas;
private Paint mpanit;
private Bitmap bgBitmap;
private Bitmap fgBitmap;

public XfermodeView(Context context) {
    this(context, null);
}

public XfermodeView(Context context, AttributeSet attrs) {
    super(context, attrs);
   // this.context=context;
    initView();
}

private void initView() {
    mpanit = new Paint();
    mpanit.setAlpha(0);//透明度設定為0顯示檫除效果,
    //portduff進行圖層混合時也會計算透明通道的值,混合後會顯示底層的圖片
    mpanit.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_IN));
    //使用dis_in模式將路徑繪製到到前面覆蓋的圖層上面
    mpanit.setStyle(Paint.Style.STROKE);
    mpanit.setStrokeWidth(50);
    mpanit.setStrokeCap(Paint.Cap.ROUND);//讓筆觸連線處圓滑
    mpanit.setStrokeJoin(Paint.Join.ROUND);//讓筆觸連線處圓滑
    path = new Path();
    bgBitmap = BitmapFactory.decodeResource(getResources(), R.mipmap.lock_screen_houma);//設定背景圖片
    fgBitmap = Bitmap.createBitmap(bgBitmap.getWidth(), bgBitmap.getHeight(), Bitmap.Config.ARGB_8888);
    mcanvas = new Canvas(fgBitmap);//根據背景圖片繪製灰色區域
    mcanvas.drawColor(Color.GRAY);//設定畫布顏色
}

@Override
public boolean onTouchEvent(MotionEvent event) {
    switch (event.getAction()) {
        case MotionEvent.ACTION_DOWN:
            path.reset();
            path.moveTo(event.getX(), event.getY());
            break;
        case MotionEvent.ACTION_MOVE:
            path.lineTo(event.getX(), event.getY());//繪製線條
            break;

    }
    mcanvas.drawPath(path, mpanit);
    invalidate();
    return true;
}

@Override
protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);
    canvas.drawBitmap(bgBitmap,0,0,null);//預設畫筆繪製背景
    canvas.drawBitmap(fgBitmap,0,0,null);//繪製灰色區域

}
 

}