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;