CAGradientLayer *layer = [CAGradientLayer layer];
layer.frame = CGRectMake(100, 100, 200, 200);
layer.position = self.view.center;
[self.view.layer addSublayer:layer];
layer.colors = @[(__bridge id)[UIColor redColor].CGColor,(__bridge id)[UIColor greenColor].CGColor,(__bridge id)[UIColor blueColor].CGColor,(__bridge id)[UIColor yellowColor].CGColor];
// layer.locations = @[@(0.2),@(0.4),@(0.6),@(0.8)];
layer.startPoint = CGPointMake(0, 0);
layer.endPoint = CGPointMake(1, 0);