将属性文本设置为UIButton

默认情况下,第一个参数没有显式名称,而其他名称则是必需的.

从official guide

Methods in Swift are very similar to their counterparts in Objective-C. As in Objective-C,the name of a method in Swift typically refers to the method’s first parameter using a preposition such as with,for,or by,as seen in the incrementBy method from the preceding Counter class example. The use of a preposition enables the method to be read as a sentence when it is called. Swift makes this established method naming convention easy to write by using a different default approach for method parameters than it uses for function parameters.

Specifically,Swift gives the first parameter name in a method a local parameter name by default,and gives the second and subsequent parameter names both local and external parameter names by default. This convention matches the typical naming and calling convention you will be familiar with from writing Objective-C methods,and makes for expressive method calls without the need to qualify your parameter names.

下划线表示：此参数没有外部参数名称.对于方法的第一个参数,这是多余的,但您可以使用它来更改上面讨论的默认行为.

例如

class Foo {
    func bar(a: String,b: Int) {}
    func baz(a: String,_ b: Int) {}
}

将导致以下正确使用方法调用：

var f = Foo()
f.bar("hello",b: 1)
f.baz("hello",1)

添加_具有使b成为本地名称的效果,因此您无法做到

f.baz("hello",b: 1)