iOS 颜色渐变

UILabel  * label  = [[UILabel alloc]initWithFrame:CGRectMake(0,(ScreenWidth/2)-30, ScreenWidth/2-2.5, 30)];

CAGradientLayer *layer = [CAGradientLayer new];

        layer.colors = @[(__bridge id)[UIColor clearColor].CGColor, (__bridge id)[UIColor grayColor].CGColor];//渐变初始值，和结束值

        layer.startPoint = CGPointMake(0,0 );

        layer.endPoint = CGPointMake(0, 1);

        layer.frame = CGRectMake(0, 0, label.frame.size.width, label.frame.size.height);

        [label.layer addSublayer:layer];

        label.textAlignment  =   1;

        label.textColor      =   [UIColor whiteColor];

        label.text  =  @"show you text";