Error painting on iOS app

Question

I'm developing an iOs 5 app where you can paint on a UIImageView inside a subview of the main view. When you paint the first 30px of the line doesn't follow your stroke. But what is really rare is that if you size up the ImageView and make it's size as all the screen 460x320 (although you don't see the full ImageView because is bigger than the view where it's inside.) it works fine. So maybe has a solution. That's my code:

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {   
    mouseSwiped = NO;
    UITouch *touch = [touches anyObject];

    if ([touch tapCount] == 2) {
        drawImage.image = nil;
        return;
    }

    lastPoint = [touch locationInView:self.view];
    lastPoint.y -= 10;
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
    mouseSwiped = YES;
    UITouch *touch = [touches anyObject];   
    CGPoint currentPoint = [touch locationInView:self.drawImage];
    currentPoint.y -= 10; // only for 'kCGLineCapRound'
    UIGraphicsBeginImageContext(self.drawImage.frame.size);

    [drawImage.image drawInRect:CGRectMake(0, 0, drawImage.frame.size.width, drawImage.frame.size.height)]; //originally self.frame.size.width, self.frame.size.height)];
    CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound); //kCGLineCapSquare, kCGLineCapButt, kCGLineCapRound
    CGContextSetLineWidth(UIGraphicsGetCurrentContext(), 5); // for size
    CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), 0.0, 1.0, 0.0, 1.0); //values for R, G, B, and Alpha
    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)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {      
    UITouch *touch = [touches anyObject];

    if ([touch tapCount] == 2) {
        drawImage.image = nil;
        return;
    }
    if(!mouseSwiped) {
        //if color == green
        UIGraphicsBeginImageContext(self.drawImage.frame.size);
        [drawImage.image drawInRect:CGRectMake(0, 0, drawImage.frame.size.width, drawImage.frame.size.height)]; //originally self.frame.size.width, self.frame.size.height)];
        CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound); //kCGLineCapSquare, kCGLineCapButt, kCGLineCapRound
        CGContextSetLineWidth(UIGraphicsGetCurrentContext(), 5);
        CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), 0.0, 1.0, 0.0, 1.0);
        CGContextMoveToPoint(UIGraphicsGetCurrentContext(), lastPoint.x, lastPoint.y);
        CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), lastPoint.x, lastPoint.y);
        CGContextStrokePath(UIGraphicsGetCurrentContext());
        CGContextFlush(UIGraphicsGetCurrentContext());
        drawImage.image = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
    }
}

Answer 1

The problem is that the first touchesMoved: is delayed by the API. I'm not sure anybody has been able to work around this issue yet.

Please file a bug report at https://feedbackassistant.apple.com/

The real problem in your case is a simple mistake, though: You're using two different coordinate systems in touchesBegan: and touchesMoved:.

touchesBegan:

lastPoint = [touch locationInView:self.view];
                                 //////////

touchesMoved:

CGPoint currentPoint = [touch locationInView:self.drawImage];
                                            ///////////////