Calling drawInRect on an UIImageView's image: which rect to use? - objective-c

Question

I'm trying to write a finger paint type program using Quartz graphics. The problem I'm having is when the screen rotates. The dots and lines I draw "creep" away from where I'm touching. With each successive touch, the previous dot creeps away. This does not happen in either of the two portrait modes, but it does in both landscape modes.
I may be mistaken but I think my problem is with this call:
[drawImage.image drawInRect:CGRectMake(0, 0, self.view.frame.size.width , self.view.frame.size.height)];
drawImage is a UIImageView in my main view controller's nib file.
I've also tried using the following:
[drawImage.image drawInRect:CGRectMake(0, 0, drawImage.frame.size.width, drawImage.frame.size.height )];
[drawImage.image drawInRect:drawImage.bounds];
[drawImage.image drawInRect:self.view.bounds];
I guess I really have no idea whether to use self or my UIImageView object for this call.
I have tried both, in both the drawInRect call and where I set up my drawing context (being consistent).
I set up the context like this in viewDidLoad (to try to make a persistant context that I don't have to reset things in each time I draw - using push and pop context).
//---------------- set up a persistent context for drawing -----------------
UIGraphicsBeginImageContext(self.view.frame.size);
context = UIGraphicsGetCurrentContext();
CGContextSetLineCap(context, kCGLineCapRound);
CGContextSetLineWidth(context, curWidth);
CGContextSetRGBStrokeColor(context, curRed, curGreen, curBlue, curAlpha);
UIGraphicsPushContext(context);
I tried to get rid of the context I set up on rotation and reinitialize it when the view rotated, thinking this was the problem, but I'm not sure I did it right.

Answer 1

You shouldn't be drawing that image yourself directly. Even if you set up your own context and draw into it correctly, you've got to somehow get it flushed to the screen. This is why UIKit sets up a context for you before it calls drawRect: on views. It's ready to put things on the screen and it just needs to know what to put there.
You can set up your own image context and draw into it in touchesBegan:, etc., but what you should do then is turn that into an image using UIGraphicsGetImageFromCurrentImageContext, call [myImageView setImage:theImageIJustGot], and then [myImageView setNeedsDisplay]. That signals UIKit that you have new things to put on the screen, and UIKit will take care of the rest.