What is the Repository Pattern in iOS?

The Repository pattern is a design principle, which separates business logic with a data access layer transparently. Great use in iOS development when you are dealing with multiple data sources. This pattern lets you scale your app much better, makes it easier to maintain and is even more testable than making use of the standard MVC approach.

1. Separation of Concerns

2. Centralized Data Access

3. Abstract Interface

4. Multiple Data Sources  

5. Testability

6. Flexibility and Extensibility

1. View:

This would be your iOS app’s UIViewController or SwiftUI view. The data sources themselves aren’t touched by the view. Instead, it has a lovely conversation with the ViewModel.

2. ViewModel:

The repository fetches, transforms or updates the data interacted with the ViewModel`. The ViewModel is independent from which interface it gets the data from — it simply needs to request data from the repository. It hides what the data is from and where.

3. Repository:

It connects your view model and data sources. It might represent the logic necessary to fetch or save data from multiple sources API, databases, or even a cache. Data from many sources and business logic that might comprise of caching or reloading outdated data may be encompassed.

4. Data Sources (API, Database, Cache):

The actual sources of storing or getting data are these. The repository abstracts the access to these sources and exposes the logic we want the ViewModel of the business layer to use without having it interact with the sources directly.

You built a movie app that will render a list of movies. You might even fly the list of movies from a remote API and desire to store the data locally for offline performance reasons (Core Data).

Without the Repository pattern, you’d see API calls everywhere in your ViewControllers, and it would be hard to manage how the data is flowing throughout your application. If you think later on, you will want local caching, you've got a refactor of 80% of your codebase coming at you once the API and Core Data have gotta be doing the caching in all different places.

With the Repository pattern, You then define a MovieRepository protocol which states what your application needs to do to fetch or save movie data. Your class above is: MovieRepositoryImpl which implements the protocol and therefore fulfills everything regarding the network implications as well as background processing of Core Data operations.

Consequently, your ViewModel does not know how to get movies, it just knows how to ask your repository for movies, and then the repository makes the decision to go out to an API or Core Data or both.

The Repository pattern in iOS helps to separate the data access, and to make applications have increased scale-ability, test-ability, and maintainability. It encapsulates data gatherers, API, databases and cache for better management of multiple data and so on, but provides clean model code.

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