PorterDuffXfermode ——實現刮刮卡效果

利用PorterDuffXfermode實現，圖例如下，需要注意的是，PorterDuffXfermode設置的是兩個圖層交集區域的顯示方式，dst是先畫的圖形，src是後畫的圖形。

其中最常用的就是通過DST_IN,SRC_IN模式來實現將一個矩形圖片編程圓角圖片或者圓形圖片的效果。先利用一個普通畫筆畫一個Mask遮罩層，再用帶PorterDuffXfermode的畫筆講圖像畫在遮罩層上，這樣就可以通過上面所說的效果來混合兩個圖像了。

 mBitmap = BitmapFactory.decodeResource(getResources(),R.drawable.test);
   mOut = Bitmap.createBitmap(mBitmap.getWidth(), mBitmap.getHeight(), Bitmap.Config.ARGB_8888);
    Canvas canvas = new Canvas(mOut);
   paint = new Paint();
    paint.setAntiAlias(true);
    canvas.drawRoundRect(0, 0, mBitmap.getWidth(), mBitmap.getHeight(), 80, 80, paint);
    paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC_IN));
    canvas.drawBitmap(mBitmap,0,0,paint);

下面實現刮刮卡效果，如下圖

完整代碼如下：

package com.example.administrator.myapplication;

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Path;
import android.graphics.PorterDuff;
import android.graphics.PorterDuffXfermode;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;

/**
 * Created by Administrator on 2015/11/25 0025.
 */
public class XfermodeView extends View {

    Bitmap mBgBitmap,mFgBitmap;
    Paint mPaint;
    Canvas mCanvas;
    Path mPath;

    public XfermodeView(Context context) {
        super(context);
        init();
    }

    public XfermodeView(Context context, AttributeSet attrs) {
        super(context, attrs);
        init();
    }

/*
*   讓它的筆觸和連接處能更加圓滑一點，即Paint.Join.ROUND和Paint.Cap.ROUND屬性
*   最關鍵的一步是：需要將畫筆的透明度設置爲0，這樣才能顯示出擦除的效果，因爲在使用PorterDuffXfermode進行圖層混合時，並不是簡單地進行圖層的計算，同時也會去計算透明度通道的值
*
*/
    private void init() {
        mPaint = new Paint();
        mPaint.setAlpha(0);
        mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_IN));
        mPaint.setStyle(Paint.Style.STROKE);
        mPaint.setStrokeJoin(Paint.Join.ROUND);
        mPaint.setStrokeWidth(50);
        mPaint.setStrokeCap(Paint.Cap.ROUND);
        mPath = new Path();
        mBgBitmap = BitmapFactory.decodeResource(getResources(),R.drawable.test);
        mFgBitmap = Bitmap.createBitmap(mBgBitmap.getWidth(), mBgBitmap.getHeight(), Bitmap.Config.ARGB_8888);
        mCanvas = new Canvas(mFgBitmap);
        mCanvas.drawColor(Color.GRAY);

    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
                switch (event.getAction()){
                    case MotionEvent.ACTION_DOWN:
                        mPath.reset();
                        mPath.moveTo(event.getX(),event.getY());
                        break;
                    case MotionEvent.ACTION_MOVE:
                        mPath.lineTo(event.getX(),event.getY());
                        break;
                }

        mCanvas.drawPath(mPath, mPaint);
        invalidate();
        return true;
    }

    @Override
    protected void onDraw(Canvas canvas) {
            canvas.drawBitmap(mBgBitmap,0,0,null);
            canvas.drawBitmap(mFgBitmap,0,0,null);

    }
}