Android MVVM Design Pattern With Examples

In this tutorial, we’ll be discussing and implementing the Android MVVM Architectural Pattern in our Android Application. We’ve previously discussed the Android MVP Pattern.

Why do we need these patterns?
Adding everything in a Single Activity or Fragment would lead to problems in testing and refactoring the code. Hence, the use of separation of code and clean architecture is recommended.

Android MVVM

MVVM stands for Model, View, ViewModel.

  • Model: This holds the data of the application. It cannot directly talk to the View. Generally, it’s recommended to expose the data to the ViewModel through Observables.
  • View: It represents the UI of the application devoid of any Application Logic. It observes the ViewModel.
  • ViewModel: It acts as a link between the Model and the View. It’s responsible for transforming the data from the Model. It provides data streams to the View. It also uses hooks or callbacks to update the View. It’ll ask for the data from the Model.

The following flow illustrates the core MVVM Pattern.

android-mvvm-pattern (1)

How does this differ from MVP?

  • ViewModel replaces the Presenter in the Middle Layer.
  • The Presenter holds references to the View. The ViewModel doesn’t.
  • The Presenter updates the View using the classical way (triggering methods).
  • The ViewModel sends data streams.
  • The Presenter and View are in a 1 to 1 relationship.
  • The View and the ViewModel are in a 1 to many relationship.
  • The ViewModel does not know that the View is listening to it.

There are two ways to implement MVVM in Android:

In this tutorial, we’ll be using Data Binding only.
Data Binding Library was introduced by Google in order to bind data directly in the xml layout. For more info on Data Binding, refer this tutorial.

We’ll be creating a simple Login Page Example Application that asks for user inputs. We’ll see how the ViewModel notifies the View when to show a Toast Message without keeping a reference of the View.

How is it possible to notify some class without having a reference of it?

It can be done in three different ways:

  • Using Two Way Data Binding
  • Using Live Data
  • Using RxJava

Two Way Data Binding

Two-way Data Binding is a technique of binding your objects to your XML layouts such that the Object and the layout can both send data to each other.

In our case, the ViewModel can send data to the layout and also observe changes.

For this, we need a BindingAdapter and custom attribute defined in the XML.

The Binding Adapter would listen to changes in the attribute property.

We’ll learn more about Two-way Data Binding through our example below.

Android MVVM Example Project Structure

android-mvvm-pattern (1)

Adding the Data Binding Library

Add the following code to your app’s build.gradle file:

This enables Data Binding in your Application.

Adding the Dependencies

Add the following dependencies in your build.gradle file :

Model

The Model would hold the user’s email and password. The following User.java class does it:

Two-way Data Binding allows us to bind objects in the XML layouts such that the object can send data to the layout, and vice versa.

The Syntax for two way data binding is @={variable}

Layout

The code for the activity_main.xml is given below:

Data Binding requires us to set the layout tag at the top. Here our ViewModel binds the data to the View.
()-> viewModel.onLoginClicked() invokes the Button click listener lambda defined in our ViewModel.
The EditText updates the values in the Model (via View Model).

bind:toastMessage="@{viewModel.toastMessage}" is a custom attribute we’ve created for two-way data binding.
Based on changes in the toastMessage in the ViewModel the BindingAdapter would get triggered in the View.

ViewModel

The code for the LoginViewModel.java is given below:

The methods were called in the layout are implemented in the above code with the same signature.
If the XML counterpart of the method doesn’t exist, we need to change the attribute to app:.

The above class can also extend ViewModel. But we need BaseObservable since it converts the data into streams and notifies when the toastMessage property is changed.

We need to define the getter and setter for the toastMessage custom attribute defined in the XML.
Inside the setter, we notify the observer (which will be the View in our application) that the data has changed.
The View(Our activity) can define the appropriate action.

BR class is auto-generated from data binding when you rebuild the project

The code for the MainActivity.java class is given below:

Thanks to DataBinding, the ActivityMainBinding class is auto-generated from the layout.
The @BindingAdapter method gets triggered whenever toastMessage attribute defined on the Button is changed.
It must use the same attribute as defined in the XML and in the ViewModel.

So in the above application, the ViewModel updates the Model by listening to the changes in the View.
Also, the Model can update the view via the ViewModel using the notifyPropertyChanged

The output of the above application in action is given below:
android mvvm data binding demo

This brings an end to this tutorial on Android MVVM Using DataBinding. You can download the project from the link given below.

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