ios - ペイントアプリiosの消しゴムを実装する必要があります

Question

消しゴムについていろいろ調べました。消しゴムを白く塗りたくありません。消しゴムを使うときは白く塗らずに、塗った色だけを消したいです。ここに、着色のための私のコードがあります:

  - (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {

        if(isButtonPressed){
            mouseSwiped = NO;
            UITouch *touch = [touches anyObject];
            lastPoint = [touch locationInView:self.view];

        }

    }

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {

    if(isButtonPressed){
        mouseSwiped = YES;
        UITouch *touch = [touches anyObject];
        CGPoint currentPoint = [touch locationInView:self.view];

        UIGraphicsBeginImageContext(self.view.frame.size);
        [self.tempDrawImage.image drawInRect:CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height)];
        CGContextMoveToPoint(UIGraphicsGetCurrentContext(), lastPoint.x, lastPoint.y);
        CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), currentPoint.x, currentPoint.y);
        CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
        CGContextSetLineWidth(UIGraphicsGetCurrentContext(), brush );
        CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), red, green, blue, alpha);

        CGContextSetBlendMode(UIGraphicsGetCurrentContext(),kCGBlendModeNormal);

        CGContextStrokePath(UIGraphicsGetCurrentContext());
        self.tempDrawImage.image = UIGraphicsGetImageFromCurrentImageContext();
        [self.tempDrawImage setAlpha:opacity];
        UIGraphicsEndImageContext();

        lastPoint = currentPoint;
    }

}

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {

    if(isButtonPressed){
        if(!mouseSwiped) {
            UIGraphicsBeginImageContext(self.view.frame.size);
            [self.tempDrawImage.image drawInRect:CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height)];
            CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
            CGContextSetLineWidth(UIGraphicsGetCurrentContext(), brush);
            CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), red, green, blue, opacity);
            CGContextMoveToPoint(UIGraphicsGetCurrentContext(), lastPoint.x, lastPoint.y);
            CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), lastPoint.x, lastPoint.y);
            CGContextStrokePath(UIGraphicsGetCurrentContext());
            CGContextFlush(UIGraphicsGetCurrentContext());
            self.tempDrawImage.image = UIGraphicsGetImageFromCurrentImageContext();
            UIGraphicsEndImageContext();

            CGContextSetBlendMode(UIGraphicsGetCurrentContext(), kCGBlendModeClear);

        }
     UIGraphicsBeginImageContext(self.mainImage.frame.size);
        [self.mainImage.image drawInRect:CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height) blendMode:kCGBlendModeNormal alpha:1.0];
        [self.tempDrawImage.image drawInRect:CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height) blendMode:kCGBlendModeNormal alpha:opacity];
        self.mainImage.image = UIGraphicsGetImageFromCurrentImageContext();
        self.tempDrawImage.image = nil;
        UIGraphicsEndImageContext();
    }
}

消しゴムを実装するにはどうすればよいですか?