swift - 带有可选参数的类 Init - 不可能？

Question

有人可以帮助我理解为什么我无法将 middleName 参数设置为一个值吗？我已将其声明为可选，但每当我尝试初始化它时，我总是在 getFullName() 函数中通过 guard 语句运行 nil。

var firstName: String
var middleName: String?
var lastName: String

init(firstName: String, middleName: String, lastName: String)
{
    self.firstName = firstName
    self.middleName? = middleName
    self.lastName = lastName
}

func getFullName() -> String
{        
    guard let midName = self.middleName else
    {
        return "\(firstName) \(lastName)"
    }

    return "\(firstName) \(midName) \(lastName)"
}

let person = Person(firstName: "Foo", middleName: "Bar", lastName: "FooBar")

打印时的返回值为“Foo Foobar”。这是为什么？

Answer 1

您不需要(或不想)在分配属性时解包该属性。直接引用即可；

init(firstName: String, middleName: String, lastName: String)
{
    self.firstName = firstName
    self.middleName = middleName
    self.lastName = lastName
}

当 middleName 为 nil(最初是这样)时，条件解包 self.middleName? 失败，因此不执行赋值.

有条件地展开 self.middleName 是一种有效的简写方式

if self.middleName != nil {
    self.middleName = middleName
}

由于 middleName 是可选的，那么您可能希望将初始化参数也设为可选:

init(firstName: String, middleName: String?=nil, lastName: String)
{
    self.firstName = firstName
    self.middleName = middleName
    self.lastName = lastName
}