PorterDuffXfermode xfermode = new 
PorterDuffXfermode(PorterDuff.Mode.SRC_IN);

canvas.saveLayer( left, top, right, bottom, paint, saveFlags)

/**
 * 设置图层
 */
int layer = canvas.saveLayer(0,0,w,h,paint,Canvas.ALL_SAVE_FLAG);
//绘制背景图片
canvas.drawBitmap(bitmap,0,0,paint);
//设置 xformode 模式
paint.setXfermode(xfermode);
//绘制矩形
paint.setColor(Color.RED);
RectF rectF = new RectF(0,y,bitmap.getWidth(),bitmap.getHeight());
canvas.drawRect(rectF,paint);
//最后设置为空
paint.setXfermode(null);
canvas.restoreToCount(layer);

paint.setXfermode(xfermode);

canvas.restoreToCount(layer);

public class XformodeView extends View {

 Paint paint ;

 //屏幕宽高
 int w;
 int h;
 //定义一个矩形的高度变化
 float y;

 //xformode 的 类型 选择
 PorterDuffXfermode xfermode = new PorterDuffXfermode(PorterDuff.Mode.SRC_IN);
 //图片
 Bitmap bitmap;

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

 public XformodeView(Context context, @Nullable AttributeSet attrs) {
 this(context,attrs,0);
 }

 public XformodeView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
 super(context, attrs, defStyleAttr);

 //画笔
 paint = new Paint();
 paint.setAntiAlias(true);
 paint.setDither(true);

 init(context);
 }

 /**
 * 初始化
 * @param context
 */
 public void init(Context context){
 //获得屏幕宽高
 WindowManager wm = (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);
 w = wm.getDefaultDisplay().getWidth();
 h = wm.getDefaultDisplay().getHeight();
 //加载bitmap 图片
 bitmap = BitmapFactory.decodeResource(context.getResources(), R.mipmap.xxx);
 //开始动画
 animator();
 }

 /**
 * 测量view
 */
 @Override
 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
 super.onMeasure(widthMeasureSpec, heightMeasureSpec);

 setMeasuredDimension(bitmap.getWidth(),bitmap.getHeight());
 }

 @Override
 protected void onDraw(Canvas canvas) {
 super.onDraw(canvas);

 /**
  * 绘制 图片 剪切 画布 控制图片显示
  */
// Path path = new Path();
// path.moveTo(0,y);
//
// path.lineTo(bitmap.getWidth(),y);
// path.lineTo(bitmap.getWidth(),bitmap.getHeight());
// path.lineTo(0,bitmap.getHeight());
// canvas.clipPath(path);

 /**
  * 设置图层
  */
 int layer = canvas.saveLayer(0,0,w,h,paint,Canvas.ALL_SAVE_FLAG);
 //绘制背景图片
 canvas.drawBitmap(bitmap,0,0,paint);
 //设置 xformode 模式
 paint.setXfermode(xfermode);
 //绘制矩形
 paint.setColor(Color.RED);
 RectF rectF = new RectF(0,y,bitmap.getWidth(),bitmap.getHeight());
 canvas.drawRect(rectF,paint);
 //最后设置为空
 paint.setXfermode(null);
 canvas.restoreToCount(layer);
 }

 /**
 * 动画
 */
 public void animator(){

 ValueAnimator animator = ValueAnimator.ofFloat(bitmap.getHeight(),0);
 animator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
  @Override
  public void onAnimationUpdate(ValueAnimator animation) {
  y = (float) animation.getAnimatedValue();
  invalidate();
  }
 });
 animator.setDuration(3000);
 animator.start();
 }
}