I'm creating a simple app where when the user presses a button, a series of lines will be drawn on the screen and the user will be able to see these lines drawn in real time (almost like an animation).

My code looks something like this (has been simplified):

UIGraphicsBeginImageContext(CGSizeMake(300,300));
CGContextRef context = UIGraphicsGetCurrentContext();

for (int i = 0; i < 100; i++ ) {
    CGContextMoveToPoint(context, i, i);
    CGContextAddLineToPoint(context, i+20, i+20);
    CGContextSetStrokeColorWithColor(context, [[UIColor blackColor] CGColor]);
    CGContextStrokePath(context);
}

UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();

UIGraphicsEndImageContext();

My problem is that:

1) As soon as the user presses the button, the UIThread blocks until the drawing is done.

2) I can't get the lines to be drawn on the screen one at a time - I've tried setting the UIImage directly inside the loop and also tried setting a layer content inside the loop.

How do I get around these problems?