Interactor: contains the business logic as specified by a use case. View: displays what it is told to by the Presenter and relays user input back to the Presenter. By separating the logic that organizes that data into a use case, we are able to keep the user interface code clean and easily wrap the use case in tests to make sure it continues to work the way we expect it to. One of the use cases or requirements for our to-do list app was to group the to-dos in different ways based on a user's selection. Components act like plugins to the use cases, and VIPER is a way of describing what the role of each of these components is and how they can interact with one another. Additionally, it's important to consider how the use case fits together with other core components of an application, such as networking and data persistence. The use case also affects the user interface. Application logic is a major part of implementing a use case, but it's not the only part. Deciding how to break down a complex app into smaller use cases is challenging and requires practice, but it's a helpful way to limit the scope of each problem you are solving and each class that you are writing.īuilding an app with VIPER involves implementing a set of components to fulfill each use case. They should also be small and well-defined. Use cases should be independent from the user interface implementation of them. A use case is the layer of an application that is responsible for business logic. Maybe a list needs to be sortable by date, type, or name. Use cases are also known as acceptance criteria, or behaviors, and describe what an app is meant to do. You'll also find that the code in the view controllers and all of the other classes is easy to understand, easier to test, and as a result, also easier to maintain.Īpps are often implemented as a set of use cases. With VIPER applied, you'll notice that the view controllers in our to-do list example are lean, mean, view controlling machines. VIPER's distinct layers help deal with this challenge by providing clear locations for application logic and navigation-related code. Slimming down these massive view controllers is not the only challenge faced by iOS developers seeking to improve the quality of their code, but it is a great place to start. This leads to an issue known as a Massive View Controller, where the view controllers end up doing too much. Since much of the application logic does not belong in a model or view, it usually ends up in the controller. Using MVC as an application architecture can guide you to thinking every class is either a model, a view, or a controller. Most iOS apps are architected using MVC (model–view–controller). your database) and to test the interactions at the boundaries between layers: This makes it easier to isolate dependencies (e.g. Clean Architecture divides an app’s logical structure into distinct layers of responsibility. The word VIPER is a backronym for View, Interactor, Presenter, Entity, and Routing. VIPER is an application of Clean Architecture to iOS apps. We decided that if we were going to improve the way we test our software, we would first need to come up with a better way to architect our apps. As we embarked on a quest to improve our testing practices at Mutual Mobile, we found that writing tests for iOS apps was difficult. Testing was not always a major part of building iOS apps. You can follow along with the example project here on GitHub: VIPER has been used to build many large projects, but for the purposes of this article we will be showing you VIPER by building a to-do list app. In this article, we will introduce an approach to iOS application architecture called VIPER. Good architecture is not what makes a product successful, but it does make a product maintainable and helps preserve the sanity of the people maintaining it! This is what we call software architecture. It's important to design our code so that each piece is easily identifiable, has a specific and obvious purpose, and fits together with other pieces in a logical fashion. As all programmers eventually learn, this applies just as well to building software. It's well known in the field of architecture that we shape our buildings, and afterward our buildings shape us.
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