UIView - CAGradientLayer

    CAGradientLayer *layer = [[CAGradientLayer alloc] init];
    layer.frame = self.bounds;
    //渐变转折点
    layer.locations = @[@0, @0.25, @0.75, @1];
    //渐变颜色，个数与locations一致
    layer.colors = [NSArray arrayWithObjects:(id)[UIColor clearColor].CGColor, (id)[UIColor blackColor].CGColor, (id)[UIColor blackColor].CGColor, (id)[UIColor clearColor].CGColor, nil];
    
    //起始位置和结束位置，控制渐变的变化方向
    layer.startPoint = CGPointMake(0, 0.5);
    layer.endPoint = CGPointMake(1, 0.5);
    
    self.layer.mask = layer;