以红色渐变为透明为例

CAGradientLayer*layer = [CAGradientLayer layer];

UIColor* inputColor0 = [UIColor colorWithRed:248./255. green:40./255. blue:31./255. alpha:1];

UIColor* inputColor1 = [UIColor colorWithRed:248./255. green:40./255. blue:31./255. alpha:0];

layer.colors = @[(__bridgeid)inputColor0.CGColor, (__bridgeid)inputColor1.CGColor];

//(0,0) (1.0,0)表示水平方向渐变,(0,0)(0,1.0)代表竖直方向渐变

CGPoint inputPoint0 = CGPointMake(0,0);

CGPoint inputPoint1 = CGPointMake(1.0,0);

layer.startPoint= inputPoint0;

layer.endPoint= inputPoint1;

//frame要和view本身的frame相同

layer.frame=self.selfFrame;

[self.layer insertSublayer:layer atIndex:0];

运行效果:

给UIView设置渐变透明背景