ios 颜色渐变效果

#import <QuartzCore/QuartzCore.h>

CAGradientLayer *gradient = [CAGradientLayer layer];
gradient.frame = rect;
gradient.colors = [NSArray arrayWithObjects:(id)[UIColor blackColor].CGColor, 
                                            (id)[UIColor grayColor].CGColor, 
                                            (id)[UIColor blackColor].CGColor,nil];
[self.layer insertSublayer:gradient atIndex:0];