Android App中实现简单的刮刮卡抽奖效果的实例详解

主要思想：
将一个view设计成多层：背景层，含中奖信息等；
遮盖层，用于刮奖，使用关联一个bitmap的canvas
在该bitmap上，使用它的canvas.drawpath的api来处理 手势滑动(类似刮奖的动作)
使用paint.setxfermode 来进行消除手势滑动区域

public class guaview extends view { 
 
  private bitmap mbitmap; //遮盖的图层 
  private canvas mcanvas; //绘制遮盖图层 
  private paint mouterpaint; 
  private path mpath; 
  private float mlastx; 
  private float mlasty; 
 
  private bitmap mcoverbitmap; //遮盖图 
  private int mwidth, mheight; 
  private paint minnerpaint; 
  private string minfo; 
 
 
  public guaview(context context) { 
    this(context, null); 
  } 
 
  public guaview(context context, attributeset attrs) { 
    super(context, attrs); 
 
    init(); 
  } 
 
  private void init() { 
    mpath = new path(); 
    mouterpaint = new paint(); 
    minnerpaint = new paint(); 
    mcoverbitmap = bitmapfactory.decoderesource(getresources(), r.drawable.fg_guaguaka); 
 
    minfo = "￥ 5 0 0"; 
  } 
 
  @override 
  protected void onmeasure(int widthmeasurespec, int heightmeasurespec) { 
    super.onmeasure(widthmeasurespec, heightmeasurespec); 
    mwidth = mcoverbitmap.getwidth(); 
    mheight = mcoverbitmap.getheight(); 
    setmeasureddimension(mwidth, mheight); 
 
    mbitmap = bitmap.createbitmap(mwidth, mheight, bitmap.config.argb_8888); 
    mcanvas = new canvas(mbitmap); 
    mcanvas.drawbitmap(mcoverbitmap, 0, 0, null); 
 
    setouterpaint(); 
    setinnerpaint(); 
 
  } 
 
  private void setinnerpaint() { 
    minnerpaint.setcolor(color.red); 
    minnerpaint.setstyle(paint.style.stroke); 
    minnerpaint.setstrokecap(paint.cap.round); 
    minnerpaint.setstrokejoin(paint.join.round); 
    minnerpaint.setantialias(true); 
    minnerpaint.setdither(true); //防抖 
    minnerpaint.setstrokewidth(5); 
    minnerpaint.settextsize(100); 
    minnerpaint.settextalign(paint.align.center); 
  } 
 
  private void setouterpaint() { 
    mouterpaint.setcolor(color.green); 
    mouterpaint.setstyle(paint.style.stroke); 
    mouterpaint.setstrokecap(paint.cap.round); 
    mouterpaint.setstrokejoin(paint.join.round); 
    mouterpaint.setantialias(true); 
    mouterpaint.setdither(true); //防抖 
    mouterpaint.setstrokewidth(20); 
  } 
 
  @override //path 
  public boolean ontouchevent(motionevent event) { 
    float x = event.getx(); 
    float y = event.gety(); 
    switch (event.getaction()) { 
      case motionevent.action_down: 
        mlastx = x; 
        mlasty = y; 
        mpath.moveto(x, y); 
        break; 
      case motionevent.action_move: 
        float deltax = math.abs(x - mlastx); 
        float deltay = math.abs(y - mlasty); 
        if (deltax > 5 || deltay > 5) { 
          mpath.lineto(x, y); 
        } 
        mlastx = x; 
        mlasty = y; 
        break; 
      case motionevent.action_up: 
        break; 
    } 
    invalidate();//调用ondraw 
    return true; 
  } 
 
  @override 
  protected void ondraw(canvas canvas) { 
    super.ondraw(canvas); 
 
    canvas.drawcolor(color.parsecolor("#bbbbbb")); //背景底色 灰色 
    canvas.drawtext(minfo, mwidth / 2, mheight / 4 * 3, minnerpaint); //绘制文本 
    canvas.drawbitmap(mbitmap, 0, 0, null); //绘制mbitmap  这是一个可变的bitmap，通过mcanvas绘制，首先绘制了mcoverbitmap 
    drawpath(); 
 
  } 
 
  private void drawpath() { 
    //使用该mode：dst和src相交后， 只保留dst，且除去相交的部份 
    mouterpaint.setxfermode(new porterduffxfermode(porterduff.mode.dst_out)); 
    mcanvas.drawpath(mpath, mouterpaint); 
  } 
} 

paint.join 连续画笔衔接时：

•         miter 在外边缘以一个锐角连接
•         round 以圆弧
•         bevel 以直线

paint.cap 指定对于 线和路径(lines and paths) 的开始和结束点的处理方式：

•         butt  ends with the path  不超越它
•         round  with the center at the end of the path 半圆
•         square  with the center at the end of the path 方形