drawing in the UIImageView moves up after every stroke - ios

Question

I am following the tutorial to create a simple drawing pad app (http://www.raywenderlich.com/18840/how-to-make-a-simple-drawing-app-with-uikit#comments)
The app has a single UIViewController and a UIImageView in it. After implementing touchBegin, touchMoved, and touchEnd, I could draw but after every stroke ends, the whole picture moves up. So after several strokes, the whole picture will be distorted and squeezed to the very top of the screen. Any suggestions? Thank you very much.
Here is the code:
- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
self.mouseSwiped = NO;
UITouch *touch = [touches anyObject];
self.lastPoint =[touch locationInView:self.view];
}
- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
self.mouseSwiped = YES;
//find out the position where the touch event happened
UITouch *touch = [touches anyObject];
CGPoint currentPoint = [touch locationInView:self.view];
//Setup the drawing context and prepare to draw something
UIGraphicsBeginImageContext(self.view.frame.size);
//Create a drawing area that is of the same size as the view dimensions
[self.mainImage.image drawInRect:CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height)];
//Set the drawing starting point
CGContextMoveToPoint(UIGraphicsGetCurrentContext(), self.lastPoint.x, self.lastPoint.y);
//Setup a line from the starting point to specified point, i.e current point in this case
CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), currentPoint.x, currentPoint.y);
//Set the drawing style
CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
CGContextSetLineWidth(UIGraphicsGetCurrentContext(), self.brush);
CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), self.red, self.green, self.blue, 1);
CGContextSetBlendMode(UIGraphicsGetCurrentContext(), kCGBlendModeNormal);
//Paint the line
CGContextStrokePath(UIGraphicsGetCurrentContext());
self.mainImage.image=UIGraphicsGetImageFromCurrentImageContext();
[self.mainImage setAlpha:self.opacity];
UIGraphicsEndImageContext();
self.lastPoint = currentPoint;
}
- (void) touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
if(!self.mouseSwiped){
UIGraphicsBeginImageContext(self.view.frame.size);
[self.mainImage.image drawInRect:
CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height)];
CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
CGContextSetLineWidth(UIGraphicsGetCurrentContext(), self.brush);
CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(),
self.red, self.green, self.blue, self.opacity);
CGContextMoveToPoint(UIGraphicsGetCurrentContext(), self.lastPoint.x, self.lastPoint.y);
CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), self.lastPoint.x, self.lastPoint.y);
CGContextStrokePath(UIGraphicsGetCurrentContext());
CGContextFlush(UIGraphicsGetCurrentContext());
self.mainImage.image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
}
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:self.opacity];
self.mainImage.image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
}

Answer 1

You probably have a mismatch between the size of the 2 views you're using, so as you move between the 2 different coordinate spaces you get a scaling effect.