Painting on UIImageView with better performance - iphone

Question

I have a view in which I paint a dotted line when the user moves their finger across the screen of the iPad/iPhone. I draw the line between a point stored as LastLocation, and the point that is the CurrentLocation. The drawings are to be persistant on the screen.
This happens every time the touchMoved event is fired, and ends up letting me draw a dotted line tracing where the person has dragged their finger.... just like a painting application.
I have a function that gets called that does the following when the touch event is fired.
The view contains a UIImageView called drawImage. I use UIImageView as a means of persisting the lines drawn. This clearly isnt how people usually do paint applications, as it is quite slow.
Any insight in to a better way to do persistant painting using the CGContextRef calls would be appreciated.
/* Enter bitmap graphics drawing context */
UIGraphicsBeginImageContext(frame.size);
/* Draw the previous frame back in to the bitmap graphics context */
[drawImage.image drawInRect:CGRectMake(0, 0, drawImage.frame.size.width, drawImage.frame.size.height)]; //originally self.frame.size.width, self.frame.size.height)];
/* Draw the new line */
CGContextRef ctx = UIGraphicsGetCurrentContext();
/* Set line draw color to green, rounded cap end and width of 5 */
CGContextSetLineDash(ctx, 0, dashPattern, 2);
CGContextSetStrokeColorWithColor(ctx, color);
CGContextSetLineCap(ctx, kCGLineCapRound); //kCGLineCapSquare, kCGLineCapButt, kCGLineCapRound
CGContextSetLineWidth(ctx, 5.0); // for size
/* Start the new path point */
CGContextMoveToPoint(ctx, LastLocation.x, LastLocation.y);
CGContextAddLineToPoint(ctx, Current.x, Current.y);
CGContextStrokePath(ctx);
/* Push the updated graphics back to the image */
drawImage.image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
The 'drawImage.image drawInRect' call is extremely slow, and is in essence redrawing the entire image.
Is there a faster way of doing this? I have seen drawing code like this in a few places on blogs for painting, but it's just a bit slow.
Would love to hear some thoughts on the topic.

Answer 1

There is no need to composite the image and line manually. Have the view that draws the line above another UIImageView that draws the image. Let the system do the compositing and draw the image.
In your code, just do the stuff between the two drawImage lines in the drawRect: method of the line drawing view.
-(void) drawRect:(CGRect)dirty {
CGContextRef ctx = UIGraphicsGetCurrentContext();
/* Set line draw color to green, rounded cap end and width of 5 */
CGContextSetLineDash(ctx, 0, dashPattern, 2);
CGContextSetStrokeColorWithColor(ctx, color);
CGContextSetLineCap(ctx, kCGLineCapRound); //kCGLineCapSquare, kCGLineCapButt, kCGLineCapRound
CGContextSetLineWidth(ctx, 5.0); // for size
/* Start the new path point */
CGContextMoveToPoint(ctx, LastLocation.x, LastLocation.y);
CGContextAddLineToPoint(ctx, Current.x, Current.y);
CGContextStrokePath(ctx);
}
When one end of the line moves, save the new point and mark the line drawing view as needing to display. So both Current and LastLocation should be members of the line drawing view, and in the setter method for each, call setNeedsDisplay.
Make sure clearsContextBeforeDrawing is YES and opaque is NO for the line drawing view.