Android：在透明色中用手指在位图上绘制

Question

我有一个带有位图的ImageView。我想要的：当用户用手指在位图上绘制时，此部分应该变为透明（将像素Alpha值设置为0），您可以在图像视图下看到视图。 我可以使用ImageView吗？还是应该实现自定义视图？

我该如何认识到这一点？ （只是大致）

Answer 1

您可以使用Canvas绘图。使用可变位图创建自定义视图。 擦除可以通过设置喷漆的对象是这样来实现：

Paint paint = new Paint(); 
paint.setStyle(Paint.Style.FILL); 
paint.setColor(Color.TRANSPARENT); 
paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR)); 

实现绘图：

@Override 
    protected void onSizeChanged(int w, int h, int oldw, int oldh) { 
     super.onSizeChanged(w, h, oldw, oldh); 

     if (w > 0 && h > 0) { 


      // Set up canvas - bitmap can be initialized with your Image 
      mBitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888); 
      mCanvas = new Canvas(mBitmap); 

     } 
    } 

    @Override 
    protected void onDraw(Canvas canvas) { 
     super.onDraw(canvas); 

     if (mBitmap == null || mCanvas == null || mPath == null) { 
      return; 
     } 
     mCanvas.drawPath(mPath, mPaint); 
     canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint); 

     canvas.drawPath(circlePath, circlePaint); 
    } 


    private float mX, mY; 
    private static final float TOUCH_TOLERANCE = 4; 

    private void touchStart(float x, float y) { 
     mPath.reset(); 
     mPath.moveTo(x, y); 
     mX = x; 
     mY = y; 

    } 

    private void touchMove(float x, float y) { 
     float dx = Math.abs(x - mX); 
     float dy = Math.abs(y - mY); 
     if (dx >= TOUCH_TOLERANCE || dy >= TOUCH_TOLERANCE) { 
      mPath.quadTo(mX, mY, (x + mX)/2, (y + mY)/2); 

      mX = x; 
      mY = y; 

      circlePath.reset(); 
      circlePath.addCircle(mX, mY, 30, Path.Direction.CW); 
     } 
    } 

    private void touchUp() { 
     mPath.lineTo(mX + 1, mY + 1); 
     circlePath.reset(); 
     // commit the path to our offscreen 
     mCanvas.drawPath(mPath, mPaint); 
     // kill this so we don't double draw 
     mPath.reset(); 
    } 

    @Override 
    public boolean onTouchEvent(MotionEvent event) { 
     if (!isEnabled()) { 
      return true; 
     } 
     float x = event.getX(); 
     float y = event.getY(); 

     switch (event.getAction()) { 
      case MotionEvent.ACTION_DOWN: 
       touchStart(x, y); 
       invalidate(); 
       break; 
      case MotionEvent.ACTION_MOVE: 
       touchMove(x, y); 
       invalidate(); 
       break; 
      case MotionEvent.ACTION_UP: 
       touchUp(); 
       invalidate(); 
       break; 
     } 
     return true; 
    }