Injectable

A new approach to Dependency Injection for Swift and SwiftUI.

Why do something new?

Resolver was my first Dependency Injection system. While quite powerful and still in use in many of my applications, Resolver suffers from a few drawbacks.

1. Resolver requires pre-registration of all service factories up front.
2. Resolver uses type inference to dynamically find and return registered services in a container.

The first issue can lead to a performance hit on application launch. That said, the registration process is usually quick and not normally noticable. No, it’s the second issue that’s somewhat more problematic.

Failure to find a matching type could lead to an application crash if we attempt to resolve a given type and if a matching registration is not found. In practice we’ve found that this isn’t really a problem as it tends to be noticed and fixed rather quickly the very first time you run a unit test or when you run the application to see if your newest feature works.

But… could we do better? That question lead me on a quest for compile-time type safety. Several other systems have attempted to solve this, but I didn’t want to have to add a source code scanning and generation step to my build process, nor did I want to give up a lot of the control and flexibility inherent in a run-time-based system.

Could I have my cake and eat it too?

Injectable

Injectable is strongly influenced by SwiftUI, and in my opinion is highly suited for use in that environment. Injectable is…

• Safe: Injectable is compile-time safe; a dependency for a given type must exist or the code simply will not compile.
• Flexible: It’s easy to override dependencies at runtime and for use in SwiftUI Previews. And, like Resolver, Injectable supports application, cached, shared, and custom scopes.
• Lightweight: Injectable is slim and trim, coming in just under a mere 200 lines of code.
• Performant: Little to no setup time is needed for the vast majority of your servies, resolutions are extremely fast, and no compile-time scripts or build phases are needed.
• Concise: Defining a given registration usually takes but a single line of code.

Sound too good to be true? Let’s take a look.

An example

Most container-based dependency injection systems require you to define in some way that a given service type is injectable and many reqire some sort of factory or mechanism that will provide a new instance of the service when needed.

Injectable is no exception. Here’s a simple registraion and its associated factory.

extension Injections {
    var myService: MySimpleService { MySimpleService() }
}

Unlike Resolver which requires a plethora of registration functions, in Injectable you simple define a computed variable on Injections that returns an instance of your service. That’s it.

Injecting and using the service where needed is equally straightforward.

class ContentViewModel: ObservableObject {
    @Injectable(\.myService) var service
    ...
}

Here our view model uses an @Injectable property wrapper to request the desired dependency. Like @EnvironmentObject in SwiftUI, you simply provide the property wrapper with a keypath to the desired type and it handles the rest.

And that’s the core mechanism. In order to use the property wrapper you must provide the keypath. That keypath points to an factory that must return the desired type. Fail to do either one and the code will simply not compile. It’s compile-time safe.

GitHub

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