Re-initialize a lazy initialized variable in Swift

Question

I have a variable that initialized as:

lazy var aClient:Clinet = {     var _aClient = Clinet(ClinetSession.shared())     _aClient.delegate = self     return _aClient }() 

The problem is, at some point, I need to reset this aClient variable so it can initialize again when the ClinetSession.shared() changed. But if I set the class to optional Clinet?, LLVM will give me an error when I try to set it to nil. If I just reset it somewhere in the code using aClient = Clinet(ClinetSession.shared()), it will end up with EXEC_BAD_ACCESS.

Is there a way that can use lazy and being allowed to reset itself?

Answer 1

lazy is explicitly for one-time only initialization. The model you want to adopt is probably just an initialize-on-demand model:

var aClient:Client {     if(_aClient == nil) {         _aClient = Client(ClientSession.shared())     }     return _aClient! }  var _aClient:Client? 

Now whenever _aClient is nil, it will be initialized and returned. It can be reinitialized by setting _aClient = nil

Answer 2

Because the behavior of lazy changed in Swift 4, I wrote a few structs that give very specific behavior, which should never change between language versions. I put these on GitHub, under the BH-1-PD license: https://github.com/RougeWare/Swift-Lazy-Patterns

ResettableLazy

Here is the one relevant to this question, which gives you a way to lazily-initialize a value, cache that value, and destroy it so it can be lazily-reinitialized later.

Note that this requires Swift 5.1! For the Swift 4 version, see version 1.1.1 of that repo.

The simple usage of this is very straightforward:

@ResettableLazy var myLazyString = "Hello, lazy!"  print(myLazyString) // Initializes, caches, and returns the value "Hello, lazy!" print(myLazyString) // Just returns the value "Hello, lazy!"  _myLazyString.clear() print(myLazyString) // Initializes, caches, and returns the value "Hello, lazy!" print(myLazyString) // Just returns the value "Hello, lazy!"  myLazyString = "Overwritten" print(myLazyString) // Just returns the value "Overwritten" _myLazyString.clear() print(myLazyString.wrappedValue) // Initializes, caches, and returns the value  "Hello, lazy!" 

This will print:

Hello, lazy! Hello, lazy! Hello, lazy! Hello, lazy! Overwritten Hello, lazy! 

If you have complex initializer logic, you can pass that to the property wrapper:

func makeLazyString() -> String {     print("Initializer side-effect")     return "Hello, lazy!" }  @ResettableLazy(initializer: makeLazyString) var myLazyString: String  print(myLazyString) // Initializes, caches, and returns the value "Hello, lazy!" print(myLazyString) // Just returns the value "Hello, lazy!"  _myLazyString.clear() print(myLazyString) // Initializes, caches, and returns the value "Hello, lazy!" print(myLazyString) // Just returns the value "Hello, lazy!"  myLazyString = "Overwritten" print(myLazyString) // Just returns the value "Overwritten" _myLazyString.clear() print(myLazyString.wrappedValue) // Initializes, caches, and returns the value  "Hello, lazy!" 

You can also use it directly (instaed of as a property wrapper):

var myLazyString = ResettableLazy<String>() {     print("Initializer side-effect")     return "Hello, lazy!" }  print(myLazyString.wrappedValue) // Initializes, caches, and returns the value "Hello, lazy!" print(myLazyString.wrappedValue) // Just returns the value "Hello, lazy!"  myLazyString.clear() print(myLazyString.wrappedValue) // Initializes, caches, and returns the value "Hello, lazy!" print(myLazyString.wrappedValue) // Just returns the value "Hello, lazy!"  myLazyString.wrappedValue = "Overwritten" print(myLazyString.wrappedValue) // Just returns the value "Overwritten" _myLazyString.clear() print(myLazyString.wrappedValue) // Initializes, caches, and returns the value  "Hello, lazy!" 

These will both print:

Initializer side-effect Hello, lazy! Hello, lazy! Initializer side-effect Hello, lazy! Hello, lazy! Overwritten Initializer side-effect Hello, lazy! 

---

This answer has been updated; its original solution no longer works in Swift 4 and newer.

Instead, I recommend you use one of the solutions listed above, or @PBosman's solution

Previously, this answer hinged on behavior which was a bug. Both that old version of this answer, its behavior, and why it's a bug are described in the text and comments of Swift bug SR-5172 (which has been resolved as of 2017-07-14 with PR #10,911), and it's clear that this behavior was never intentional.

That solution is in that Swift bug's text, and also in the history of this answer, but because it's a bug exploit that doesn't work in Swift 3.2+ I recommend you do not do that.