UIView: how to do non-destructive drawing?

Question

My original question:

I\'m creating a simple drawing application and need to be able to draw over existing, previously drawn content in my d

Answer 1

On optimizing drawRect

Try this:

- (void)drawRect:(CGRect)rect {
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGImageRef cgImage = CGBitmapContextCreateImage(drawingContext);
    CGContextClipToRect(context, rect);
    CGContextDrawImage(context, CGRectMake(0, 0, self.frame.size.width, self.frame.size.height), cgImage);
    CGImageRelease(cgImage);
}

After that you should also comment these lines in your code:

//CGContextTranslateCTM(context, 0, size.height);
//CGContextScaleCTM(context, 1.0, -1.0);

Also created a separate question to be sure that it's the optimal way.

Answer 2

This prevents your view from being erased before drawRect is done:

[self.layer setNeedsDisplay];

Also, I find it is better to do all the drawing in the drawRect method (unless you have a good reason not to). Drawing offscreen and transferring takes more time and adds more complexity then simply drawing everything once.

Answer 3

It is fairly common to draw everything in an offscreen image, and simply display this image when drawing the screen. You can read: Creating a Bitmap Graphics Context.

Answer 4

This seems like a better method than I've been using. That is, in a touches event I make a copy of the view about to be updated. Then in drawRect I take that image and draw it to the view and make my other view changes at the same time.

But this seems inefficient but the only way I figured out how to do it.