Mimic CALayer shadow properties found in iPhone OS 3.2 for OS 3.1

The CALayer shadow properties like shadowOffset, shadowRadius, shadowColor are not available in iPhone OS versions below 3.2 and I'm wondering how I could mimic that functionality for use with 3.1 and below.

I want to use this to be able to add drop shadows to UIViews in a clean way so that the shadows are drawn at layer level somehow, and not by drawing it in a view's -(void)drawRect:(CGRect)rect method which requires to sh开发者_如何学JAVArink the actual views frame to accomodate for the shadow. (This shrinking approach have been proposed in the other UIView drop shadow related questions I found here on SO).

I was thinking a layered approach would be cleaner. For example I tried creating subclassing CALayer to which I added a separate shadow layer as a sublayer, but then that would be drawn on top of whatever was draw in the drawRect: method of the UIView that had the main layer as backing layer.

I've also tried implementing the subclass CALayer's drawInContext: something like this,

- (void)drawInContext:(CGContextRef)ctx {
// code to draw shadow for a frame the size of the layer's frame        
[super drawInContext:ctx];
}

But then the shadow is still clipped to the current clipping bounding box of the context, which seems to be the layers own frame.

I also had some idea of redirecting the drawing of the main layer to a sublayer, which would be placed above another sublayer which had the shadow drawn onto it. Then I would probably get rid of the clipping and the shadow would be farthest away. But I couldn't really wrap my head around how I would do that, and it doesn't really feel like a clean approach.

Any ideas on how to go about this? Just to make clear how my UIView drop shadow related question is different from the other ones I found here on SO; I do not want to shrink the actual drawing frame of a UIView to accomodate for a shadow. I want it to somehow be on a separate layer in the background, whithout beeing clipped.

You could create an image simulating a shadow and just scale that, probably automatically by giving it a pointer to any view. It would then automatically adjust itself and position below that in whatever superview that view has.

This works pretty nicely. Shadow should be about 50% of screen size, so the average expansion / compression is pretty invisible. If it has a big radius you really can't tell if it's scaled or not.