iOS 使用CAGradientLayer给View添加渐变色

创建CAGradientLayer，添加到view.layer即可。

    CAGradientLayer *gradient = [CAGradientLayer layer];
    gradient.frame = _colorView.bounds;
    gradient.colors = [NSArray arrayWithObjects:
                       (id)[UIColor colorWithHexStr:@"#007AFF"].CGColor,
                       (id)[UIColor colorWithHexStr:@"#53A5FF"].CGColor,
                       nil];
    [_colorView.layer addSublayer:gradient];