Implementing an eraser in an Android drawing app - black trail and then transparent

Question

I have an drawing app for Android and I am currently trying to add a real eraser to it. Before, I had just used white paint for an eraser, but that won\'t do anymore since n

Answer 1

I could suggest you to read the official sample of FingerPaint.java
It exactly matches what you are trying to achieve here.

To not show the trail when you erase content, take a look at the onDraw() method and the eraserMode variable:

@Override
protected void onDraw(Canvas canvas) {
    canvas.drawColor(0xFFAAAAAA);
    canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint);
    if (!eraserMode) {
        canvas.drawPath(mPath, mPaint);
    }
}

boolean eraserMode = false;

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    eraserMode = false;
    mPaint.setXfermode(null);
    mPaint.setAlpha(0xFF);
    switch (item.getItemId()) {
        /*...*/
        case ERASE_MENU_ID:
            // Add this line
            eraserMode = true;
            mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
            return true;
        /*...*/
    }
    return super.onOptionsItemSelected(item);
}