In the initial stages of Android development, learners do write codes in such a manner that eventually creates a MainActivity class which contains all the implementation logic(real-world business logic) of the application. This approach of app development leads to Android activity gets closely coupled to both UI and the application data processing mechanism. Further, it causes difficulties in the maintenance and scaling of such mobile applications. To avoid such problems in maintainability, readability, scalability, and refactoring of applications, developers prefer to define well-separated layers of code. By applying software architecture patterns, one can organize the code of the application to separate the concerns. MVP (Model — View — Presenter) architecture is one of the most popular architecture patterns and is valid in organizing the project.
MVP (Model — View — Presenter) comes into the picture as an alternative to the traditional MVC (Model — View — Controller) architecture pattern. Using MVC as the software architecture, developers end up with the following difficulties:
- Most of the core business logic resides in Controller. During the lifetime of an application, this file grows bigger and it becomes difficult to maintain the code.
- Because of tightly-coupled UI and data access mechanisms, both Controller and View layer falls in the same activity or fragment. This cause problem in making changes in the features of the application.
- It becomes hard to carry out Unit testing of the different layer as most of the part which are under testing needs Android SDK components.
MVP pattern overcomes these challenges of MVC and provides an easy way to structure the project codes. The reason why MVP is widely accepted is that it provides modularity, testability, and a more clean and maintainable codebase. It is composed of the following three components:
- Model: Layer for storing data. It is responsible for handling the domain logic(real-world business rules) and communication with the database and network layers.
- View: UI(User Interface) layer. It provides the visualization of the data and keep a track of the user’s action in order to notify the Presenter.
- Presenter: Fetch the data from the model and applies the UI logic to decide what to display. It manages the state of the View and takes actions according to the user’s input notification from the View.
Key Points of MVP Architecture
- Communication between View-Presenter and Presenter-Model happens via an interface(also called Contract).
- One Presenter class manages one View at a time i.e., there is a one-to-one relationship between Presenter and View.
- Model and View class doesn’t have knowledge about each other’s existence.
Example of MVP Architecture
To show the implementation of the MVP architecture pattern on projects, here is an example of a single activity android application. The application will display some strings on the View(Activity) by doing a random selection from the Model. The role of the Presenter class is to keep the business logic of the application away from the activity. Below is the complete step-by-step implementation of this android application. Note that we are going to implement the project using both Java and Kotlin language.
Note: Following steps are performed on Android Studio version 4.0
Step 1: Create a new project
- Click on File, then New => New Project.
- Choose Empty activity
- Select language as Java/Kotlin
- Select the minimum SDK as per your need.
Step 2: Modify String.xml file
All the strings which are used in the activity are listed in this file.
Step 3: Working with the activity_main.xml file
Open the activity_main.xml file and add a Button, a TextView to display the string, and a Progress Bar to give a dynamic feel to the application. Below is the code for designing a proper activity layout.
Step 4: Defining the Contract Interface file for the Model, View, and Presenter
To establish communication between View-Presenter and Presenter-Model, an interface is needed. This interface class will contain all abstract methods which will be defined later in the View, Model, and Presenter class.
Step 5: Creating the Model class
Create a new class named Model to separate all string data and the methods to fetch those data. This class will not know the existence of View Class.
Step 6: Creating the Presenter class
The methods of this class contain core business logic which will decide what to display and how to display. It triggers the View class to make the necessary changes to the UI.
Step 7: Define functionalities of View in the MainActivity file
The View class is responsible for updating the UI according to the changes triggered by the Presenter layer. The data provided by the Model will be used by View and the appropriate changes will be made in the activity.
Advantages of MVP Architecture
- No conceptual relationship in android components
- Easy code maintenance and testing as the application’s model, view, and presenter layer are separated.
Disadvantages of MVP Architecture
- If the developer does not follow the single responsibility principle to break the code then the Presenter layer tends to expand to a huge all-knowing class.