Android App中實現簡單的刮刮卡抽獎效果的實例詳解

主要思想：
將一個view設計成多層：背景層，含中獎資訊等；
遮蓋層，用於刮獎，使用關聯一個Bitmap的Canvas
在該Bitmap上，使用它的canvas.drawPath的api來處理手勢滑動(類似刮獎的動作)
使用paint.setXfermode 來進行消除手勢滑動區域

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 連續畫筆銜接時：  ROUND 以圓弧

        BEVEL 以直線

Paint.Cap 指定對於線和路徑(lines and paths) 的開始和結束點的處理方式：

    ROUND  with the center at the end of the path 半圓

        SQUARE  with the center at the end of the path 方形

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