DelayedInjection.md

Delayed Injection

There are cases when - for various reasons - you don't want dependencies to be created immediately when the object is resolved from the Container, but only when they are needed. Swinject provides two ways of delayed injection: Lazy and Provider:

• Lazy dependency behaves similarly to Swift's lazy variables: it is created first time it is accessed, and remembers that same value for all subsequent calls.
• Provider dependency behaves similarly to Swift's computed variables: new instance is created every time it is accessed.

Differences between lazy, provider and direct injection can be demonstrated with an example. Consider this registration for Int:

var value = 0;
container.register(Int.self) { _ in 
    value += 1;
    print("creating Int")
    return value;
}

Direct injection

struct DirectCounter {
    let integer: Int

    func print() {
        print("printing")
        print(integer)
        print(integer)
        print(integer)
    }
}

container.register(DirectCounter.self) { DirectCounter(integer: $0.resolve(Int.self)!) }

let counter = container.resolve(DirectCounter.self)!
counter.print()

Output of the above code would be

creating Int
printing
1
1
1

Lazy Injection

struct LazyCounter {
    let integer: Lazy<Int>

    func print() {
        print("printing")
        print(integer)
        print(integer)
        print(integer)
    }
}

container.register(LazyCounter.self) { LazyCounter(integer: $0.resolve(Lazy<Int>.self)!) }

let counter = container.resolve(LazyCounter.self)!
counter.print()

Output of the above code would be

printing
creating Int
1
1
1

Provider Injection

struct ProviderCounter {
    let integer: Provider<Int>

    func print() {
        print("printing")
        print(integer)
        print(integer)
        print(integer)
    }
}

container.register(ProviderCounter.self) { ProviderCounter(integer: $0.resolve(Provider<Int>.self)!) }

let counter = container.resolve(ProviderCounter.self)!
counter.print()

Output of the above code would be

printing
creating Int
1
creating Int
2
creating Int
3