Circular Dependencies

Circular dependencies are dependencies of instances that depend on each other. To define circular dependencies in Swinject, one of the dependencies must be injected through a property.

Initializer/Property Dependencies

Assume that you have Parent and Child classes depending on each other. Parent depends on ChildProtocol through its initializer, and Child on ParentProtocol through a property. The back-reference from Child to ParentProtocol is a weak property to avoid a memory leak.

protocol ParentProtocol: AnyObject { }
protocol ChildProtocol: AnyObject { }

class Parent: ParentProtocol {
    let child: ChildProtocol?

    init(child: ChildProtocol?) {
        self.child = child
    }
}

class Child: ChildProtocol {
    weak var parent: ParentProtocol?
}

The circular dependencies are defined as below:

let container = Container()
container.register(ParentProtocol.self) { r in
    Parent(child: r.resolve(ChildProtocol.self)!)
}
container.register(ChildProtocol.self) { _ in Child() }
    .initCompleted { r, c in
        let child = c as! Child
        child.parent = r.resolve(ParentProtocol.self)
    }

Here the injection to the parent property of Child must be specified in the initCompleted callback to avoid infinite recursion.

Property/Property Dependencies

Similarly, assume that you have the following classes depending on each other, each via a property:

protocol ParentProtocol: AnyObject { }
protocol ChildProtocol: AnyObject { }

class Parent: ParentProtocol {
    var child: ChildProtocol?
}

class Child: ChildProtocol {
    weak var parent: ParentProtocol?
}

The circular dependencies are defined as below:

let container = Container()
container.register(ParentProtocol.self) { r in
    let parent = Parent()
    parent.child = r.resolve(ChildProtocol.self)!
    return parent
}
container.register(ChildProtocol.self) { _ in Child() }
    .initCompleted { r, c in
        let child = c as! Child
        child.parent = r.resolve(ParentProtocol.self)
    }

Here both or either of the depending properties must be specified in the initCompleted callback to avoid infinite recursion.

Initializer/Initializer Dependencies

Not supported. This type of dependency causes infinite recursion.

Remark

When resolving circular dependencies one of the factory methods (one containing resolution of circular dependency) might be invoked twice. Only one of the resulting instances will be used in the final object graph but in some cases this can be problematic - particularly when there are side effects of factory invocation, such as

• time consuming operations
• interactions with resolved dependencies

You can avoid duplicate invocation by resolving both parts of the dependency cycle inside initCompleted closures, for example refactoring

container.register(ParentProtocol.self) { r in
    let parent = Parent()
    parent.child = r.resolve(ChildProtocol.self)!
    return parent
}

to

container.register(ParentProtocol.self) { _ in Parent() } 
    .initCompleted { r, p in
        let parent = p as! Parent
        parent.child = r.resolve(ChildProtocol.self)!
    }

Next page: Object Scopes

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