1 . 对于变量: 格式化数字字面量

通过使用下划线可以提高数字字面量的可读性

        let testNumA = 123_456
        let testNumB = 123.45_67
        let testNumC = 123.000_456
        print(testNumA)
        print(testNumB)
        print(testNumC)

打印结果

        123456
        123.4567
        123.000456

2 . 对于元组: 忽略元组的元素值

当我们使用元组时，如果有的元素不需要使用，这时可以使用下划线将相应的元素进行忽略

        let http404Error = (404, "Not Found")
        let (_, errorMessage) = http404Error
        print(http404Error)
        print(errorMessage)

打印结果

(404, "Not Found")
Not Found

3 . 对于数组: 忽略区间值

有时候我们并不关心区间内每一项的值，可以使用下划线来忽略这些值。

        var base = 1;
        let arr = [1,2,3,4,5,6,7,8,9,10]
        print(arr)
        
        for _ in 1...arr.count {
            base += base    
        }
        print(base)

打印结果

        1024

4 . 对于函数: 忽略外部参数名

• (1)忽略方法的默认外部参数名
• (2)忽略具有默认值的参数的外部参数名

func RGBA_1( r:CGFloat, g:CGFloat, b:CGFloat,  a:CGFloat) -> UIColor {
    return UIColor(red: (r)/255.0, green: (g)/255.0, blue: (b)/255.0, alpha: a)
}

func RGBA_2(_ r:CGFloat, g:CGFloat, b:CGFloat,  a:CGFloat) -> UIColor {
    return UIColor(red: (r)/255.0, green: (g)/255.0, blue: (b)/255.0, alpha: a)
}

func RGBA_3(_ r:CGFloat,_ g:CGFloat, b:CGFloat,  a:CGFloat) -> UIColor {
    return UIColor(red: (r)/255.0, green: (g)/255.0, blue: (b)/255.0, alpha: a)
}

func RGBA_4(_ r:CGFloat,_ g:CGFloat,_ b:CGFloat,  a:CGFloat) -> UIColor {
    return UIColor(red: (r)/255.0, green: (g)/255.0, blue: (b)/255.0, alpha: a)
}

func RGBA_5(_ r:CGFloat,_ g:CGFloat,_ b:CGFloat, _ a:CGFloat) -> UIColor {
    return UIColor(red: (r)/255.0, green: (g)/255.0, blue: (b)/255.0, alpha: a)
}

func RGBA_6(_ r:CGFloat,_ g:CGFloat,_ b:CGFloat, _ a:CGFloat = 255) -> UIColor {
    return UIColor(red: (r)/255.0, green: (g)/255.0, blue: (b)/255.0, alpha: a)
}

函数

方法调用

调用

PS :
不过为了提高代码的可读性，一般不进行忽略