添加颜色渐变

 [self.layer addSublayer:[self shadowAsInverse]];

- (CAGradientLayer *)shadowAsInverse

{

    CAGradientLayer *newShadow = [[[CAGradientLayer alloc] init] autorelease];

    CGRect newShadowFrame =self.frame;

    newShadow.frame = newShadowFrame;

    //添加渐变的颜色组合

    newShadow.colors = [NSArray arrayWithObjects:(id)[UIColor whiteColor].CGColor,(id)[UIColor grayColor].CGColor,nil];

    return newShadow;

}