主要用到了Layer上的渐变层，核心代码如下，关于CALayer的使用可参考
UIColor *colorOne = [UIColor colorWithRed:(33/255.0) green:(33/255.0) blue:(33/255.0) alpha:1.0];
UIColor *colorTwo = [UIColor colorWithRed:(33/255.0) green:(33/255.0) blue:(33/255.0) alpha:0.0];
NSArray *colors = [NSArray arrayWithObjects:(id)colorOne.CGColor, colorTwo.CGColor, nil];
NSNumber *stopOne = [NSNumber numberWithFloat:0.0];
NSNumber *stopTwo = [NSNumber numberWithFloat:1.0];
NSArray *locations = [NSArray arrayWithObjects:stopOne, stopTwo, nil];
CAGradientLayer *headerLayer = [CAGradientLayer layer];
headerLayer.colors = colors;
headerLayer.locations = locations;
headerLayer.frame = _banner_shadow.bounds;
[_banner_shadow.layer insertSublayer:headerLayer atIndex:0];
