swift中的可选项

Question

我看了苹果所有的swift教程，但我有一个例子有问题:

class Person {
    var residence: Residence?
}
class Residence {
    var address: Address?
}
class Address {
    var buildingNumber: String? = "234"
    var streetName: String? = "Main St."
    var appartmentNumber: String?
}
let paul = Person()
var addressNumber: Int?
addressNumber = paul.residence?.address?.buildingNumber?.toInt()
if let number = addressNumber {
    "correct"
} else {
    "fault"
}

总是打印"fault"。我是不是错过了什么显而易见的痛苦?

Answer 1

如果您将代码更改为:

，那么您实际上永远不会创建有效的Residence或Address

class Person {
    // actually create a residence object
    var residence: Residence? = Residence()
}
class Residence {
    // actually create a address object
    var address: Address? = Address()
}
class Address {
    // ...
}
let paul = Person()
var addressNumber: Int?
// Before Paul was never assigned a valid residence, now one will be created
addressNumber = paul.residence?.address?.buildingNumber?.toInt()
if let number = addressNumber {
    "correct"
} else {
    "fault"
}
// gives you `correct`

那么它应该工作得很好!

Answer 2

let paul = Person()

您没有对paul做任何其他操作，包括设置他的可选驻留变量。因此，在下一行代码中，您将访问paul的住所，即nil。

addressNumber = paul.residence?.address?.buildingNumber?.toInt()
                      ^ failing right here, residence? returns nil

因此，使用可选链接，整个表达式返回nil，因此当与下一个if let语句进行比较时，它是假的。这就是执行else子句的原因。

Answer 3

仔细想想，一步一步来。特别是，你的可选链接:

addressNumber = paul.residence?.address?.buildingNumber?.toInt()

问问自己:

1. 什么是paul

2. paul的residence属性设置为什么?

3. paul的residence的address是什么

提示:你不应该超过第2步

Answer 4

除了Person()，你永远不会初始化任何东西，所以paul上的residence属性是nil。

那么，在下面的可选链中:

addressNumber = paul.residence?.address?.buildingNumber?.toInt()
                              ^ you get nil here and don't continue

表示addressNumber是nil