Category: Swift

Especially on iOS it’s often necessary to create new objects like new view controllers or such things. This often cannot be avoided. This usually is solved by having the object that creates new objects also require all the dependencies of the objects it wants to create. This of course is not a great thing as changes in one class can ripple through the whole project. The protocol composition approach solves this by having the flow controllers take the whole AppDependency object so they can pass it on.

This solves the problem of having to change some classes when the dependencies of others changes. But it still is bad for testing as you can’t be sure which dependencies are actually needed, and you have no way to influence which class is instantiated. A common solution to this is injecting factory objects that hold all the necessary resources. This of course can result in rather verbose code if a whole new type is created. Luckily closures can also take the role of those factories.

While implementing this I realized that an extra object is not even necessary with the protocol composition DI. The AppDependency object itself can take the role of the factory:

protocol MakesSomething {
 func makeSomething() -> Something
}

extension AppDependency: MakesSomething {
 func makeSomething() -> Something {
 return Something(dependencies: self)
 }
}

That code is not so bad and we can avoid having dependencies on unneeded objects which is good for testing. Usage is like any other dependency:

class MyClass {
 typealias Dependencies = MakesSomething & HasOtherThing
 let dependencies: Dependencies
 
 init(dependencies: Dependencies) {
 self.dependencies = dependencies
 }
 
 func doWork() {
 let something = dependencies.makeSomething()
 something.doWhateverItIsSomethingDoes()
 }
}

 

I recently read the article “Using protocol compositon for dependency injection” by Krzysztof Zabłocki. I found this to be a rather interesting technique for doing DI, but when trying to implement it I found it somewhat limited:

• It doesn’t really handle dependencies that have dependencies themselves.
• No support for lazy instantiation, everything needs to be instantiated up front.

Lets look at simple and pretty common example of the object graph I wanted to create:

• A view controller at the leaf level, which needs
• An object that provides business logic (like the Interactor from the VIPER pattern), which needs
• the data store (a NSPersistentContainer in my case).

This can be translated pretty easily to this pattern:

protocol HasCoreData {
  var coreData: NSPersistentContainer { get }
}

protocol HasListInteractor {
  var listInteractor: ListInteractor { get }
}

struct AppDependencies: HasCoreData, HasListInteractor {
  let coreData: NSPersistentContainer
  let listInteractor: ListInteractor
}

final class ListViewController: UIViewController {
  typealias Dependencies = HasListInteractor
  let dependencies: Dependencies

  init(dependencies: Dependencies) {
    self.dependencies = dependencies)
    // ...
  }
}

final class ListInteractor {
  typealias Dependencies = HasCoreData
  let dependencies: Dependencies

  init(dependencies: Dependencies) {
    self.dependencies = dependencies)
    // ...
  }
}

This all works fine until I try to fill in my AppDependencies struct – to create it I need an instance of my ListInteractor, and to create that I need the AppDependencies struct. So this is not going to work.

Instead I could create a second struct that only implements HasCoreData and use that to initialise the ListInteractor. This approach will work, but will quickly become messy and as hard to refactor as passing every dependency separately. It also won’t address the point of avoiding eager initialisation.

But there is a simple solution that addresses both problems:

final class AppDependencies: HasCoreData, HasListInteractor {
  lazy var coreData: NSPersistentContainer = NSPersistentContainer(name: "Whatever")
  lazy var listInteractor: ListInteractor = ListInteractor(dependencies: self)
}

By making the AppDependencies a class instead of a struct we can use lazy properties to fulfil the protocol requirements. That way we can’t get into the catch-22 situation I described and we get lazy instantiation for free.

Of course there are some other drawbacks to consider:

• Cyclic dependencies will cause infinite recursion
• Objects are created lazily, so if you need eager instantiation of some class you need to manually call the getter