Detailed explanation of examples of implementing simple scratch card lottery effects in Android App

Main idea:
Design a view into multiple layers: background layer, including winning information, etc.;
Covering layer, used for scratching prizes, using Canvas associated with a Bitmap
On the Bitmap, Use its canvas.drawPath api to handle gesture sliding (similar to scratching actions)
Use paint.setXfermode to eliminate gesture sliding areas

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 When connecting continuous brushes:

MITER Connect at an acute angle on the outer edge

ROUND Use an arc

BEVEL Use a straight line

Paint.Cap specifies the line How to handle the start and end points of lines and paths:

BUTT ends with the path without going beyond it

ROUND with the center at the end of the path Semicircle

SQUARE with the center at the end of the path Square

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