在Swift中，如果optional是nil，则初始化非可选var的最佳方法

Question

I would like to initialize a variable obj by taking it from UserDefaults , which returns a String? , and if it's nil build the value and assign it.

The following code works, but, at the end, my obj is a String? while I want it to be a String (since it can't be nil at this stage).

var obj = UserDefaults.standard.string(forKey: "my_key")// Here, obj is a String?
if obj == nil {
    obj =  ProcessInfo.processInfo.globallyUniqueString// Returns, a String
    defaults.set(obj, forKey: "my_key")
    defaults.synchronize()
}
// Here, obj is still a String?

Is there a good pattern / best practice for this kind of situation ?

Answer 1

You can use the nil-coalescing operator ?? with an "immediately evaluated closure":

let obj = UserDefaults.standard.string(forKey: "my_key") ?? {
    let obj = ProcessInfo.processInfo.globallyUniqueString
    UserDefaults.standard.set(obj, forKey: "my_key")
    return obj
}()

print(obj) // Type is `String`

If the user default is not set, the closure is executed. The closure creates and sets the user default (using a local obj variable) and returns it to the caller, so that it is assigned to the outer obj variable.

Answer 2

Optional Binding. You can read up on it here

let obj: String

if let string = UserDefaults.standard.string(forKey: "my_key") {
    obj = string
} else {
    obj = ProcessInfo.processInfo.globallyUniqueString
    UserDefaults.standard.set(obj, forKey: "my_key")
}

print(obj)

Answer 3

Use either guard let or if let .

1) guard let (not common for your case, though)

guard let obj = UserDefaults.standard.string(forKey: "my_key") else { 
    // guard failed - obj is nil; perform something and return
    obj =  ProcessInfo.processInfo.globallyUniqueString
    defaults.set(obj, forKey: "my_key")
    return 
}

// obj is not nil, you have it at your disposal
print(obj)

2) if let

if let obj = UserDefaults.standard.string(forKey: "my_key") {
   // obj exists
} else {
    let obj =  ProcessInfo.processInfo.globallyUniqueString
    defaults.set(obj, forKey: "my_key")
    print(obj)
} 

(!) Also, there really is no more need to call defaults.synchronize() :

Waits for any pending asynchronous updates to the defaults database and returns; this method is unnecessary and shouldn't be used.

_{https://developer.apple.com/documentation/foundation/nsuserdefaults/1414005-synchronize}

Answer 4

You can also use an implicitly unwrapped optional, like this:

var obj: String! = UserDefaults.standard.string(forKey: "my_key")// Here, obj is a (possibly `nil`) `String!`
if obj == nil {
    obj =  ProcessInfo.processInfo.globallyUniqueString // Returns, a String
    defaults.set(obj, forKey: "my_key")
    defaults.synchronize()
}

// Here, obj is a non-`nil` `String!`, which will work the same as a `String`