Re-initialize a lazy initialized variable in Swift

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

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.