How do I draw a shadow under a UIView?

I'm trying to draw a shadow under the bottom edge of a UIView in Cocoa Touch. I understand that I should use CGContextSetShadow() to draw the shadow, but the Quartz 2D programming guide is a little vague:

1. Save the graphics state.
2. Call the function CGContextSetShadow, passing the appropriate values.
3. Perform all the drawing to which you want to apply shadows.
4. Restore the graphics state

I've tried the following in a UIView subclass:

- (void)drawRect:(CGRect)rect {
    CGContextRef currentContext = UIGraphicsGetCurrentContext();
    CGContextSaveGState(currentContext);
    CGContextSetShadow(currentContext, CGSizeMake(-15, 20), 5);
    CGContextRestoreGState(currentContext);
    [super drawRect: rect];
}

..but this doesn't work for me and I'm a bit stuck about (a) where to go next and (b) if there's anything I need to do to my UIView to make this work?

A by far easier approach is to set some layer attributes of the view on initialization:

self.layer.masksToBounds = NO;
self.layer.shadowOffset = CGSizeMake(-15, 20);
self.layer.shadowRadius = 5;
self.layer.shadowOpacity = 0.5;

You need to import QuartzCore.

#import <QuartzCore/QuartzCore.h>

self.layer.masksToBounds = NO;
self.layer.cornerRadius = 8; // if you like rounded corners
self.layer.shadowOffset = CGSizeMake(-15, 20);
self.layer.shadowRadius = 5;
self.layer.shadowOpacity = 0.5;

This will slow down the application. Adding the following line can improve performance as long as your view is visibly rectangular:

self.layer.shadowPath = [UIBezierPath bezierPathWithRect:self.bounds].CGPath;