android - 如何删除在 Canvas 上绘制的线条？

Question

我正在使用 Canvas 类在 Canvas 上绘制线条。现在，我想以与使用橡皮擦在笔记本中类似的方式删除在 Canvas 中绘制的线条。我经历了几个例子，但对我没有任何作用。

如果有人知道这个问题的解决方案，请你帮我解决这个问题吗？

Java 代码:

public class DrawView extends View implements OnTouchListener 
{
        private Canvas      m_Canvas;
        
        private Path        m_Path;
        
        private Paint       m_Paint;
        
        ArrayList<Pair<Path, Paint>> paths = new ArrayList<Pair<Path, Paint>>();
        
        ArrayList<Pair<Path, Paint>> undonePaths = new ArrayList<Pair<Path, Paint>>(); 
        
        private float mX, mY;
        
        private static final float TOUCH_TOLERANCE = 4;
        
        private Bitmap          bitmapToCanvas;
        
        private CanvasManager   m_CanvasManagerObject;
        
        private Paint   mBitmapPaint;
        
        public DrawView(Context context)
        {
            super(context);
            setFocusable(true);
            setFocusableInTouchMode(true);      
            this.setOnTouchListener(this);
            
            onCanvasInitialization();
        }    
        
        public void onCanvasInitialization()
        {
            m_Paint = new Paint(Paint.DITHER_FLAG);
            m_Paint.setAntiAlias(true);
            m_Paint.setDither(true);
            m_Paint.setColor(Color.parseColor("#37A1D1"));
            m_Paint.setStyle(Paint.Style.STROKE);
            m_Paint.setStrokeJoin(Paint.Join.ROUND);
            m_Paint.setStrokeCap(Paint.Cap.ROUND);
            m_Paint.setStrokeWidth(2);                          
            m_Path = new Path();
            
            mBitmapPaint = new Paint(Paint.DITHER_FLAG);
        }
        
        @Override
        protected void onSizeChanged(int w, int h, int oldw, int oldh) 
        {       
            super.onSizeChanged(w, h, oldw, oldh);
    
            bitmapToCanvas = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
            m_Canvas = new Canvas(bitmapToCanvas);
        }
        
        @Override
        protected void onDraw(Canvas canvas)
        {           
            canvas.drawBitmap(bitmapToCanvas, 0f, 0f, mBitmapPaint);    
            canvas.drawPath(m_Path, m_Paint);
        }
        
        public boolean onTouch(View arg0, MotionEvent event) 
        {
            float x = event.getX();
            float y = event.getY();

            switch (event.getAction()) 
            {
                case MotionEvent.ACTION_DOWN:
                touch_start(x, y);
                invalidate();
                break;
                case MotionEvent.ACTION_MOVE:
                touch_move(x, y);
                invalidate();
                break;
                case MotionEvent.ACTION_UP:
                touch_up();
                invalidate();
                break;
            }
            
            return true;
        }

        private void touch_start(float x, float y) 
        {
            m_Path.reset();
            m_Path.moveTo(x, y);
            mX = x;
            mY = y;
        }
        
        private void touch_move(float x, float y) 
        {
            float dx = Math.abs(x - mX);
            float dy = Math.abs(y - mY);
            if (dx >= TOUCH_TOLERANCE || dy >= TOUCH_TOLERANCE) 
            {
                m_Path.quadTo(mX, mY, (x + mX)/2, (y + mY)/2);
                mX = x;
                mY = y;
            }
        }
        
        private void touch_up() 
        {
            m_Path.lineTo(mX, mY);
                    
            // commit the path to our offscreen
            m_Canvas.drawPath(m_Path, m_Paint);
            
            // kill this so we don't double draw                    
            Paint newPaint = new Paint(m_Paint); // Clones the mPaint object
            paths.add(new Pair<Path, Paint>(m_Path, newPaint));
            m_Path = new Path();
        }
        
        public void onClickEraser() 
        { 
            
        }
                
}

Answer 1

如果您有纯色背景，您需要做的就是将Paint 颜色设置为您的背景色。例如，如果你有一个白色背景，你可以这样做:

paint.setColor(Color.White);

但是，如果您需要删除具有透明背景的线条，您可以试试这个:

为了使用透明颜色进行绘制，您必须使用Paint setXfermode，这仅在您将位图设置到 Canvas 时才有效。如果您按照以下步骤操作，您应该会得到想要的结果。

1. 创建 Canvas 并设置其位图。

   mCanvas = new Canvas();
   mBitmap= Bitmap.createBitmap(scrw, scrh, Config.ARGB_8888);
   mCanvas.setBitmap(mBitmap);
   

2. 当您想要删除某些内容时，您只需使用 setXfermode。

   public void onClickEraser() 
   { 
      if (isEraserOn)
         mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
      else
         mPaint.setXfermode(null);
   }
   

3. 现在您应该可以使用透明颜色绘制:

   mCanvas.drawPath(path, mPaint);