Android Eraser in paint canvas does not erase

Question

I have the following code to activate/deactivate the eraser:

public PorterDuffXfermode clear = new PorterDuffXfermode(PorterDuff.Mode.CLEAR);    
eraseB.setOnClickListener(new OnClickListener() {

                    @Override
                    public void onClick(View v) {
                        if (!eraser) {
                            eraser = true;
                            eraseB.setImageResource(R.drawable.erase_on);
                            paint = new Paint(Paint.DITHER_FLAG);
                            paint.setColor(0x00000000);
                            paint.setAlpha(0x00);
                            paint.setXfermode(clear);
                            paint.setStyle(Paint.Style.STROKE);
                            paint.setStrokeJoin(Paint.Join.ROUND);
                            paint.setStrokeCap(Paint.Cap.ROUND);
                            paint.setStrokeWidth(stroke);
                            paintv.setPaint(paint);
                        } else {
                            eraser = false;
                            eraseB.setImageResource(R.drawable.erase);
                            paint = new Paint(Paint.DITHER_FLAG);
                            paint.setDither(true);
                            paint.setXfermode(null);
                            paint.setColor(Color.RED);
                            paint.setStyle(Paint.Style.STROKE);
                            paint.setStrokeJoin(Paint.Join.ROUND);
                            paint.setStrokeCap(Paint.Cap.ROUND);
                            paint.setStrokeWidth(stroke);
                            paintv.setPaint(paint);
                        }
                    }
                });

setPaint is from my customView:

public void setPaint(Paint paint) {
    this.paint = paint;
    LogService.log("in setPaint", "paint = " + paint);
}

and onDraw I use:

canvas.drawPath(mPath, paint);

If I deactivate the eraser, it will draw with a red line, but instead, if I activate the eraser, instead of erasing, it will draw a black line. How can I fix this

Answer 1

Canvas does not support eraser while Bitmap does.

Basic workaround flow:

1. Create another canvas

2. Create a bitmap

3. Set that bitmap to that canvas

   public void init(int width, int height) {
       Log.i(TAG,"init with "+width+"x"+height);
       foreground = Bitmap.createBitmap(width, height, Config.ARGB_8888);
       cacheCanvas = new Canvas();
       cacheCanvas.setBitmap(foreground);
   }
   

4. Record the touches on that bitmap, including paint and eraser

   public boolean onTouchEvent(MotionEvent event) {
       float eventX = event.getX();
       float eventY = event.getY();
   
       switch (event.getAction()) {
       case MotionEvent.ACTION_DOWN:
           currentStroke = new Stroke();
           currentStroke.color = paint;
           currentStroke.path.moveTo(eventX, eventY);
           currentStroke.path.lineTo(eventX, eventY);
   
           synchronized (strokes) {
               strokes.add(currentStroke);
           }
           lastTouchX = eventX;
           lastTouchY = eventY;
           // There is no end point yet, so don't waste cycles invalidating.
           return true;
   
       case MotionEvent.ACTION_MOVE:
       case MotionEvent.ACTION_UP:
           // Start tracking the dirty region.
           resetDirtyRect(eventX, eventY);
   
           // When the hardware tracks events faster than they are delivered,
           // the
           // event will contain a history of those skipped points.
           int historySize = event.getHistorySize();
           for (int i = 0; i < historySize; i++) {
               float historicalX = event.getHistoricalX(i);
               float historicalY = event.getHistoricalY(i);
           expandDirtyRect(historicalX, historicalY);
               if (i == 0) {
                   lastX = historicalX;
                   lastY = historicalY;
                   currentStroke.path.lineTo(historicalX, historicalY);
               } else {
                   currentStroke.path.quadTo(lastX, lastY,
                       (historicalX + lastX) / 2,
                       (historicalY + lastY) / 2);
               }
           }
   
       // After replaying history, connect the line to the touch point.
           if(historySize==0){
               long duration=event.getEventTime()-event.getDownTime();
               float offset=0.1f;
               if(duration<300){
                   offset=50.0f/duration;
               }
               currentStroke.path.lineTo(eventX+offset, eventY+offset);
           }else{
               currentStroke.path.lineTo(eventX, eventY);
           }
           synchronized (strokes) {
               strokes.add(currentStroke);
           }
   
           break;
   
       default:
       return false;
       }
   
   // Include half the stroke width to avoid clipping.
       float width = paint.getStrokeWidth() / 2;
       invalidate((int) (dirtyRect.left - width),
           (int) (dirtyRect.top - width), (int) (dirtyRect.right + width),
           (int) (dirtyRect.bottom + width));
   
       lastTouchX = eventX;
       lastTouchY = eventY;
   
       return true;
       }
   

5. Draw that bitmap on the canvas of your View

   protected void onDraw(Canvas canvas) {
       synchronized (strokes) {
           if (strokes.size() > 0) {
               for (Stroke s : strokes) {
                   cacheCanvas.drawPath(s.path, s.color);
               }
               canvas.drawBitmap(foreground, 0, 0, null);
               strokes.clear();
           }
       }
   }
   

Answer 2

If you are drawing on solid color background, use the paint of same color in brush to that of background and the brush will behave like an eraser.