Question

I am writing a Cocoa application for OS X, where the user can draw squares on an NSView instance by clicking with the mouse. Currently I am making the squares disappear after 2 seconds, using the performSelector:withObject:afterDelay: method of NSObject, to force a redraw of the view, with no square included.

However, instead of just disappearing, I would like the squares to fade out gradually. I’ve tried using an NSTimer to periodically force a redraw, with the opacity of the square decreasing to 0 over 2 seconds, but this seems rather inelegant and probably inefficient, especially if I have a lot of squares.

Is there an idiomatic way to do this?

UPDATE: just to clarify, I want each square drawn in the view to have an independent fade starting from the point at which it’s drawn, I’m not looking to fade out the entire view.

Answer 1

The solution I’ve ended up using is to create a CALayer instance for each square, rather than using NSRectFill to draw the squares. The opacity of each CALayer instance can then be independently animated using a CABasicAnimation instance. E.g.

CABasicAnimation *animation = [CABasicAnimation animationWithKeyPath:@"opacity"];
[animation setFromValue:[NSNumber numberWithFloat:1.]];
[animation setToValue:[NSNumber numberWithFloat:0.]];
[animation setDuration:2.];
[layer setOpacity:0.];
[layer addAnimation:animation forKey:@"opacity"];