Dependency Pattern

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We are using this pattern when creating a new Uber-FX dependency, it makes it easy adding new external dependencies later when your application evolves. To explain this better, we will do this with a Notifier example.

Preparation

Define our Notifier interface:

type Notifier interface{
    Alert(ctx context.Context, msg string)
}

Create a dependency “container” for all future dependencies of the Notifier implementation and embed fx.In into it. This will mark it and will tell Uber-FX to inject all the requested dependencies:
```
// It's not public as you can see
type notifierDeps struct {
    fx.In
}
```

Create an implementation struct:

// Notice again that's not public
type notifier struct{
    deps notifierDeps
}

Create a Constructor function that will return a Notifier implementation as a type:

func CreateNotifier(deps notifierDeps) (Notifier,error) {
    return &notifier(deps:deps), nil
}

Finally, implement it:

func (n *notifier) Alert(ctx context.Context, msg string) {
    // alert someone
}

Usage

Now, suppose you want to log every time you alert someone. All you need to do is:

Add a log.Logger dependency to the notifierDeps struct:

type notifierDeps struct {
    fx.In

    Logger log.Logger
}

Use it:

func (n *notifier) Alert(ctx context.Context, msg string) {
    n.deps.Logger.WithField("msg", msg).Debug(ctx, "alerting")
    // alert someone
}

Tests

You are happily using Notifier in your application, but what about tests? You know, to test logic that has Notifier as a dependency. Notifier will probably call an external service, which is not available during tests. One way to do it, is to use gomock.

You can add a comment above the Notifier interface:

//go:generate mockgen -source=notifier.go -destination=mock/notifier_mock.go
type Notifier interface{
    Alert(ctx context.Context, msg string)
}

Execute go generate ./... and it will generate all the mocks in your application.
Use the generated mock in your tests.
Remember not to call a real Notifier Constructor

Mortar includes mocks of all of its interfaces in their respective directories, for example a Logger mock.

You can find several test examples here.

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