背景知识

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
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;
import android.view.ViewConfiguration;
import android.view.WindowManager;
/**
 * 红包效果
 *
 * @author tonywang
 */
public class RedPacketView extends View {
 private int mScaleTouchSlop;
 //红包内容图片
 private Bitmap mFgBitmap;
 //红包遮挡物图片
 private Bitmap mBgBitmap;
 //刮红包路径
 private Path mPath;
 //用于来遮挡物图片和手指路径相交效果的canvas
 private Canvas mCanvas;
 //之前的x坐标
 private float preX;
	//之前的y坐标
 private float preY;
	//采用Xfermode的paint
 private Paint mPaint;
 public RedPacketView(Context context) {
 super(context);
 init(context);
 }
 public RedPacketView(Context context, AttributeSet attrs) {
 super(context, attrs);
 init(context);
 }
 private void init(Context context) {
	 //获得系统的最小有效滑动距离,习惯上采用2倍距离
 ViewConfiguration viewConfiguration = ViewConfiguration.get(context);
 mScaleTouchSlop = 2 * viewConfiguration.getScaledTouchSlop();
 WindowManager wm = (WindowManager) getContext()
 .getSystemService(Context.WINDOW_SERVICE);
 int screenW = wm.getDefaultDisplay().getWidth();
 int screenH = wm.getDefaultDisplay().getHeight();
 mPath = new Path();
 mPaint = new Paint(Paint.ANTI_ALIAS_FLAG | Paint.DITHER_FLAG);
 mPaint.setARGB(128, 25, 0, 0);
 //这里就是设置PorterDuffXfermode,模式指定为DST_IN
 mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_IN));
 mPaint.setStyle(Paint.Style.STROKE);
 //为了效果更贴近手指滑动,设置连接处为圆形
 mPaint.setStrokeJoin(Paint.Join.ROUND);
 mPaint.setStrokeCap(Paint.Cap.ROUND);
 //宽度为50
 mPaint.setStrokeWidth(50);
 // 生成前景图Bitmap
 mFgBitmap = Bitmap.createBitmap(screenW, screenH, Bitmap.Config.ARGB_8888);
 // 将其注入画布
 mCanvas = new Canvas(mFgBitmap);
 // 绘制画布背景为中性灰,这个也是我们的源图
 mCanvas.drawColor(0xFF808080);
 // 获取背景底图Bitmap
 mBgBitmap = BitmapFactory.decodeResource(context.getResources(), R.drawable.prize_none);
 // 缩放背景底图Bitmap至屏幕大小
 mBgBitmap = Bitmap.createScaledBitmap(mBgBitmap, screenW, screenH, true);
 }
 @Override
 protected void onDraw(Canvas canvas) {
 //绘制背景
 canvas.drawBitmap(mBgBitmap, 0, 0, null);
 //绘制前景
 canvas.drawBitmap(mFgBitmap, 0, 0, null);
 //画路径,绘制目标图
 mCanvas.drawPath(mPath, mPaint);
 }
 @Override
 public boolean onTouchEvent(MotionEvent event) {
 float x = event.getX();
 float y = event.getY();
 switch (event.getAction()) {
 case MotionEvent.ACTION_DOWN:
 mPath.reset();
 mPath.moveTo(x, y);
 preX = x;
 preY = y;
 break;
 case MotionEvent.ACTION_MOVE:
 float dx = Math.abs(x - preX);
 float dy = Math.abs(y - preY);
 if (dx > mScaleTouchSlop || dy > mScaleTouchSlop) {
 mPath.quadTo(preX, preY, (x + preX) / 2, (y + preY) / 2);
 preX = x;
 preY = y;
 }
 break;
 }
 //千万注意加上invalidate,使view重绘
 invalidate();
 return true;
 }
}