12月 252019
 

swift 的类代码里,init()是正常初始化类,但如果初始数据不满足要求,或需要返回nil时,怎么处理呢? init?()就是派这个用场地!

引用https://www.hackingwithswift.com/example-code/language/whats-the-difference-between-init-and-init的内容,下面说的很详细,也有例子举例,大家一看就明白:

It’s the job of a regular Swift initializer to create a fully fledged instance of a new type, however sometimes the data that has been provided is insufficient or incorrect, and creation can’t proceed.

For example, consider this code:

struct Person {
    var ssn: String

    init(socialSecurityNumber: String) {
        self.ssn = socialSecurityNumber
    }
}

let person = Person(socialSecurityNumber: "111-11-1111")
print(person)

That defines a Person struct that can be created using a nine-digit social security number, then creates an instance of that struct.

But what should happen here?

let person = Person(socialSecurityNumber: "FISH")

In that instance we’re passing an invalid social security number, so really we expect creating a Person to fail.

This is where failable initializers come in: they are written as init?(), and can return nil rather than a value if something goes wrong during creation. For example, we could write a quick check to make sure the social security number is more or less correct like this:

struct Person {
    var ssn: String

    init?(socialSecurityNumber: String) {
        if socialSecurityNumber.count < 11 {
            return nil
        } else {
            self.ssn = socialSecurityNumber
        }
    }
}

Notice the initializer is now called init?() to reflect that it returns an optional – the process might return nil if the creation fails. The logic is pretty simple: if there are 11 digits we assume it’s correct, otherwise we return nil. Note: if you really wanted to validate that number you’d need to use a regular expression.

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