How to implement erase/undo in this method - iphone

Question

I have successfully implemented touchesMoved and it is drawing line properly now I am facing problem in implementing erase method, please assist me where I am lacking.
- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
mouseSwiped = YES;
UITouch *touch = [touches anyObject];
CGPoint currentPoint = [touch locationInView:self.view];
currentPoint.y -= 20;
UIGraphicsBeginImageContext(self.view.frame.size);
[drawImage.image drawInRect:CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height)];
CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
CGContextSetLineWidth(UIGraphicsGetCurrentContext(), 5.0);
CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), 1.0, 0.0, 0.0, 1.0);
CGContextBeginPath(UIGraphicsGetCurrentContext());
CGContextMoveToPoint(UIGraphicsGetCurrentContext(), lastPoint.x, lastPoint.y);
CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), currentPoint.x, currentPoint.y);
CGContextStrokePath(UIGraphicsGetCurrentContext());
drawImage.image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
lastPoint = currentPoint;
mouseMoved++;
if (mouseMoved == 10) {
mouseMoved = 0;
}
}
- (void) Erase
{
//NSSet *touches;
//UITouch *touch; //= (NSSet *)[touch anyObject];
CGPoint currentPoint; //= [touch locationInView:self.view];
currentPoint.y += 20;
if(mouseSwiped) {
UIGraphicsBeginImageContext(self.view.frame.size);
[drawImage.image drawInRect:CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height)];
CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
CGContextSetLineWidth(UIGraphicsGetCurrentContext(), 5.0);
CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), 1.0, 0.0, 0.0, 1.0);
CGContextMoveToPoint(UIGraphicsGetCurrentContext(), lastPoint.x, lastPoint.y);
CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), lastPoint.x, lastPoint.y);
CGContextStrokePath(UIGraphicsGetCurrentContext());
CGContextFlush(UIGraphicsGetCurrentContext());
drawImage.image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
}
mouseMoved--;
}
I have implemented my erases successfully in touches moved but when I tap one so it erases but on mouse over and tap left it stop erasing. Then need to click(tap) again for erasing at new point. So how can I continously erases on touches moved,
- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
if (eraseBtnSelected == YES) {
if ([touch tapCount] == 1) {
[self EraseButton];
}
}
}

Answer 1

Don't do any drawing in your event-handling methods. Ever. Use them to edit a list of things to draw, and do all of your drawing in -draw... methods like -drawRect:.
To erase something, just delete it from your list of things to draw, and redraw the view or layer in question.

Answer 2

CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), 1.0, 1.0, 1.0, 1.0);
this line of code works if background colour is white if other then it is a problem

Answer 3

change following line
CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), 1.0, 0.0, 0.0, 1.0);
to
CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), 1.0, 1.0, 1.0, 1.0);