iOS Core Graphics How to erase a drawing

Question

I am following this tutorial for my doodle application.

http://www.raywenderlich.com/18840/how-to-make-a-simple-drawing-app-with-uikit

his erase function is using white color drawing. But I am using image background and when I erase, I really need to erase.

I have tried

CGContextSetBlendMode(UIGraphicsGetCurrentContext(), kCGBlendModeClear);

But it's not working. Here is my code:

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    _mouseSwiped = NO;
    UITouch *touch = [touches anyObject];
    _lastPoint = [touch locationInView:self.tempImageView];
}

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

    _mouseSwiped = YES;
    UITouch *touch = [touches anyObject];
    CGPoint currentPoint = [touch locationInView:self.tempImageView];

    UIGraphicsBeginImageContext(self.tempImageView.frame.size);
    [self.tempImageView.image drawInRect:CGRectMake(0, 0, self.tempImageView.frame.size.width, self.tempImageView.frame.size.height)];
    CGContextMoveToPoint(UIGraphicsGetCurrentContext(), _lastPoint.x, _lastPoint.y);
    CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), currentPoint.x, currentPoint.y);
    CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
    CGContextSetLineWidth(UIGraphicsGetCurrentContext(), _width );


    CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), _red, _green, _blue, 1.0);

    if (_isErasing) {
        CGContextSetBlendMode(UIGraphicsGetCurrentContext(), kCGBlendModeClear);
    }
    else {
        CGContextSetBlendMode(UIGraphicsGetCurrentContext(),kCGBlendModeNormal);
    }

    CGContextStrokePath(UIGraphicsGetCurrentContext());
    self.tempImageView.image = UIGraphicsGetImageFromCurrentImageContext();
    [self.tempImageView setAlpha:_alpha];
    UIGraphicsEndImageContext();

    _lastPoint = currentPoint;
}

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

    CGSize size = self.tempImageView.frame.size;

    if(!_mouseSwiped) {
        UIGraphicsBeginImageContext(self.tempImageView.frame.size);
        [self.tempImageView.image drawInRect:CGRectMake(0, 0, size.width, size.height)];
        CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
        CGContextSetLineWidth(UIGraphicsGetCurrentContext(), _width);


        CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), _red, _green, _blue, _alpha);
        CGContextMoveToPoint(UIGraphicsGetCurrentContext(), _lastPoint.x, _lastPoint.y);
        CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), _lastPoint.x, _lastPoint.y);
        CGContextStrokePath(UIGraphicsGetCurrentContext());
        CGContextFlush(UIGraphicsGetCurrentContext());
        self.tempImageView.image = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
    }




    UIGraphicsBeginImageContext(self.drawImageView.frame.size);
    [self.drawImageView.image drawInRect:CGRectMake(0, 0, size.width, size.height) blendMode:kCGBlendModeNormal alpha:1.0];

    if (_isErasing) {
        CGContextSetBlendMode(UIGraphicsGetCurrentContext(), kCGBlendModeClear);
    }

    [self.tempImageView.image drawInRect:CGRectMake(0, 0, self.tempImageView.frame.size.width, self.tempImageView.frame.size.height) blendMode:kCGBlendModeNormal alpha:_alpha];


    self.drawImageView.image = UIGraphicsGetImageFromCurrentImageContext();
    self.tempImageView.image = nil;
    UIGraphicsEndImageContext();
}

Answer 1

solved this problem by swapping tempImageView and drawImageView when touch began if it's erasing

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    _mouseSwiped = NO;
    UITouch *touch = [touches anyObject];
    _lastPoint = [touch locationInView:self.tempImageView];

    if (_isErasing) {
        self.tempImageView.image = self.drawImageView.image;
        self.drawImageView.image = nil;
    }

}



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

    _mouseSwiped = YES;
    UITouch *touch = [touches anyObject];
    CGPoint currentPoint = [touch locationInView:self.tempImageView];

    UIGraphicsBeginImageContext(self.tempImageView.frame.size);
    [self.tempImageView.image drawInRect:CGRectMake(0, 0, self.tempImageView.frame.size.width, self.tempImageView.frame.size.height)];
    CGContextMoveToPoint(UIGraphicsGetCurrentContext(), _lastPoint.x, _lastPoint.y);
    CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), currentPoint.x, currentPoint.y);
    CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
    CGContextSetLineWidth(UIGraphicsGetCurrentContext(), _width );


    if (_isErasing) {
        CGContextSetBlendMode(UIGraphicsGetCurrentContext(), kCGBlendModeClear);
    }
    else {
        CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), _red, _green, _blue, 1.0);
        CGContextSetBlendMode(UIGraphicsGetCurrentContext(),kCGBlendModeNormal);
    }

    CGContextStrokePath(UIGraphicsGetCurrentContext());
    self.tempImageView.image = UIGraphicsGetImageFromCurrentImageContext();
    [self.tempImageView setAlpha:_alpha];
    UIGraphicsEndImageContext();

    _lastPoint = currentPoint;
}





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

    CGSize size = self.tempImageView.frame.size;

    if(!_mouseSwiped) {
        UIGraphicsBeginImageContext(self.tempImageView.frame.size);
        [self.tempImageView.image drawInRect:CGRectMake(0, 0, size.width, size.height)];
        CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
        CGContextSetLineWidth(UIGraphicsGetCurrentContext(), _width);


        if (_isErasing) {
            CGContextSetBlendMode(UIGraphicsGetCurrentContext(), kCGBlendModeClear);
        }
        else {
            CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), _red, _green, _blue, _alpha);
            CGContextSetBlendMode(UIGraphicsGetCurrentContext(),kCGBlendModeNormal);
        }

        CGContextMoveToPoint(UIGraphicsGetCurrentContext(), _lastPoint.x, _lastPoint.y);
        CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), _lastPoint.x, _lastPoint.y);
        CGContextStrokePath(UIGraphicsGetCurrentContext());
        CGContextFlush(UIGraphicsGetCurrentContext());
        self.tempImageView.image = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
    }




    UIGraphicsBeginImageContext(self.drawImageView.frame.size);
    [self.drawImageView.image drawInRect:CGRectMake(0, 0, size.width, size.height) blendMode:kCGBlendModeNormal alpha:1.0];
    [self.tempImageView.image drawInRect:CGRectMake(0, 0, size.width, size.height) blendMode:kCGBlendModeNormal alpha:_alpha];
    self.drawImageView.image = UIGraphicsGetImageFromCurrentImageContext();
    self.tempImageView.image = nil;
    UIGraphicsEndImageContext();
}

Answer 2

If anybody is still intereted this is how it worked for me in Swift 3. Thanks for the help OMGPOP

override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
    swiped = false
    hideAllButtons()
    if let touch = touches.first {
        lastPoint = touch.location(in: self.view)
    }

    if isErasing {
        self.tempImageView.image = self.mainImageView.image
        self.mainImageView.image = nil
    }
}

override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
    swiped = true
    if let touch = touches.first {
        let currentPoint = touch.location(in: view)
        drawLineFrom(fromPoint: lastPoint, toPoint: currentPoint)

        lastPoint = currentPoint
    }

}

override func touchesEnded(_ touches: Set<UITouch>, with event: UIEvent?) {
    showAllButtons()
    if !swiped {
        drawLineFrom(fromPoint: lastPoint, toPoint: lastPoint)
    }

    UIGraphicsBeginImageContext(mainImageView.frame.size)
    mainImageView.image?.draw(in: CGRect(x: 0, y: 0, width: view.frame.size.width, height: view.frame.size.height), blendMode: .normal, alpha: 1.0)
    tempImageView.image?.draw(in:  CGRect(x: 0, y: 0, width: view.frame.size.width, height: view.frame.size.height), blendMode: .normal, alpha: opacity)
    mainImageView.image = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()

    tempImageView.image = nil
}

// Draws a line between two points on the view
func drawLineFrom(fromPoint: CGPoint, toPoint: CGPoint) {
    UIGraphicsBeginImageContext(view.frame.size)
    let context = UIGraphicsGetCurrentContext()
    tempImageView.image?.draw(in: CGRect(x:0, y:0, width: view.frame.size.width, height: view.frame.size.height))

    context?.move(to: fromPoint)
    context?.addLine(to: toPoint)

    context?.setLineCap(.round)
    context?.setLineWidth(brushWidth)

    if isErasing {
        context?.setBlendMode(.clear)
    } else {
        context?.setStrokeColor(red: red, green: green, blue: blue, alpha: 1.0)
        context?.setBlendMode(.normal)
    }
    context?.strokePath()

    tempImageView.image = UIGraphicsGetImageFromCurrentImageContext()
    tempImageView.alpha = opacity
    UIGraphicsEndImageContext()

}