iOS 按钮同时支持渐变、圆角、阴影

// 添加渐变layer到登录按钮最底层，避免遮住title
CAGradientLayer *subLayer = [CAGradientLayer layer];
subLayer.frame = _loginButton.bounds;
[_loginButton.layer insertSublayer:subLayer atIndex:0];

// 使用渐变色
subLayer.colors = @[(__bridge id)[UIColor colorWithRGB:0xff6c6c].CGColor,
                    (__bridge id)[UIColor colorWithRGB:0xff9451].CGColor];
// 开始点
subLayer.startPoint = CGPointMake(0, 0);
// 结束点
subLayer.endPoint = CGPointMake(1, 1);

// 使用圆角
subLayer.cornerRadius = _loginButton.height / 2;

// 使用阴影
subLayer.shadowColor = [UIColor colorWithRGB:0xff865b].CGColor;
subLayer.shadowOffset = CGSizeMake(0, 3);
subLayer.shadowOpacity = 0.6;