iOS开发 UILable 文字颜色从上到下渐变 显示不出来oc代码

UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake(0, 0, 100, 100)]; CAGradientLayer *gradientLayer = [CAGradientLayer layer]; gradientLayer.frame = label.bounds; gradientLayer.colors = @[(__bridge id)[UIColor redColor].CGColor, (__bridge id)[UIColor blueColor].CGColor]; gradientLayer.startPoint = CGPointMake(0.5, 0); gradientLayer.endPoint = CGPointMake(0.5, 1); [label.layer addSublayer:gradientLayer]; [self.view addSubview:label];