如何在 UIView 下绘制阴影？

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?

In your current code, you save the GState of the current context, configure it to draw a shadow .. and the restore it to what it was before you configured it to draw a shadow. Then, finally, you invoke the superclass's implementation of drawRect: .

Any drawing that should be affected by the shadow setting needs to happen after

CGContextSetShadow(currentContext, CGSizeMake(-15, 20), 5);

but before

CGContextRestoreGState(currentContext);

So if you want the superclass's drawRect: to be 'wrapped' in a shadow, then how about if you rearrange your code like this?

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