//先声明

CAGradientLayer *_headerLayer;

//确定要渐变的颜色,切记颜色顺序和透明度要一一对应

UIColor *colorOne = [UIColor colorWithRed:(0.0/255.0)  green:(0.0/255.0)  blue:(0.0/255.0)  alpha:0.7];

UIColor *colorTwo = [UIColor colorWithRed:(0.0/255.0)  green:(0.0/255.0)  blue:(0.0/255.0)  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];

if (!_headerLayer) {

//crate gradient layer

_headerLayer = [CAGradientLayer layer];

_headerLayer.colors = colors;

_headerLayer.locations = locations;

}

_headerLayer.frame = CGRectMake(0, 0, [UIScreen mainScreen].bounds.size.width, 20.0);

[self.viewController.view.layer addSublayer:_headerLayer];