CAGradientLayer介绍
CAGradientLayer根据官方的解释是渐变层在其背景色上绘制颜色渐变，即是填充层的形状（包括圆角），通俗的来说就是根据shape形状来绘制渐变色。对于iOS开发者来说，如果不需要太酷炫的渐变效果，这CAGradientLayer类完全可以胜任一般渐变效果的实现，不管是两种颜色还是三种颜色渐变，都能很好的实现。不需要下载其他功能复杂的第三方，几句代码就可以实现渐变效果，简直了。
渐变色示例
View添加渐变色
UIView *colorView = [[UIView alloc] init];
colorView.frame = CGRectMake(0, 0,[UIScreen mainScreen].bounds.size.width,[UIScreen mainScreen].bounds.size.height);
colorView.center = CGPointMake([UIScreen mainScreen].bounds.size.width/2.0, [UIScreen mainScreen].bounds.size.height/2.0);
// colorView.center = CGPointMake(375/2.0, 667/2.0-100);
[self.view addSubview:colorView];
CAGradientLayer *gradient = [CAGradientLayer layer];
gradient.frame = colorView.bounds;
gradient.colors = @[(id)[UIColor colorWithRed:200 /255.0 green:192 /255.0 blue:192 /255.0 alpha:1].CGColor,(id)[UIColor colorWithRed:199 /255.0 green:192 /255.0 blue:192 /255.0 alpha:1].CGColor,(id)[UIColor colorWithRed:185 /255.0 green:187 /255.0 blue:192 /255.0 alpha:1].CGColor];
//渐变从左下角开始
gradient.startPoint = CGPointMake(0, 1);
//渐变到右上角结束
gradient.endPoint = CGPointMake(1, 0);
// gradient.locations = @[@(0.5f), @(1.0f)];
[colorView.layer addSublayer:gradient];
