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.
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.
Table of Contents
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.
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:
- Data Binding
- RXJava
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
Adding the Data Binding Library
Add the following code to your app’s build.gradle file:
android {
dataBinding {
enabled = true
}
}
This enables Data Binding in your Application.
Adding the Dependencies
Add the following dependencies in your build.gradle file :
implementation 'android.arch.lifecycle:extensions:1.1.0'
Model
The Model would hold the user’s email and password. The following User.java class does it:
package com.journaldev.androidmvvmbasics.model;
public class User {
private String email;
private String password;
public User(String email, String password) {
this.email = email;
this.password = password;
}
public void setEmail(String email) {
this.email = email;
}
public String getEmail() {
return email;
}
public void setPassword(String password) {
this.password = password;
}
public String getPassword() {
return password;
}
}
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:
<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="https://schemas.android.com/apk/res/android"
xmlns:bind="https://schemas.android.com/tools">
<data>
<variable
name="viewModel"
type="com.journaldev.androidmvvmbasics.viewmodels.LoginViewModel" />
</data>
<ScrollView
android:layout_width="match_parent"
android:layout_height="match_parent">
<LinearLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:layout_margin="8dp"
android:orientation="vertical">
<EditText
android:id="@+id/inEmail"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:hint="Email"
android:inputType="textEmailAddress"
android:padding="8dp"
android:text="@={viewModel.userEmail}" />
<EditText
android:id="@+id/inPassword"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:hint="Password"
android:inputType="textPassword"
android:padding="8dp"
android:text="@={viewModel.userPassword}" />
<Button
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginTop="8dp"
android:onClick="@{()-> viewModel.onLoginClicked()}"
android:text="LOGIN"
bind:toastMessage="@{viewModel.toastMessage}" />
</LinearLayout>
</ScrollView>
</layout>
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:
package com.journaldev.androidmvvmbasics.viewmodels;
import android.databinding.BaseObservable;
import android.databinding.Bindable;
import android.text.TextUtils;
import android.util.Patterns;
import com.android.databinding.library.baseAdapters.BR;
import com.journaldev.androidmvvmbasics.model.User;
public class LoginViewModel extends BaseObservable {
private User user;
private String successMessage = "Login was successful";
private String errorMessage = "Email or Password not valid";
@Bindable
private String toastMessage = null;
public String getToastMessage() {
return toastMessage;
}
private void setToastMessage(String toastMessage) {
this.toastMessage = toastMessage;
notifyPropertyChanged(BR.toastMessage);
}
public void setUserEmail(String email) {
user.setEmail(email);
notifyPropertyChanged(BR.userEmail);
}
@Bindable
public String getUserEmail() {
return user.getEmail();
}
@Bindable
public String getUserPassword() {
return user.getPassword();
}
public void setUserPassword(String password) {
user.setPassword(password);
notifyPropertyChanged(BR.userPassword);
}
public LoginViewModel() {
user = new User("","");
}
public void onLoginClicked() {
if (isInputDataValid())
setToastMessage(successMessage);
else
setToastMessage(errorMessage);
}
public boolean isInputDataValid() {
return !TextUtils.isEmpty(getUserEmail()) && Patterns.EMAIL_ADDRESS.matcher(getUserEmail()).matches() && getUserPassword().length() > 5;
}
}
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.
The code for the MainActivity.java class is given below:
package com.journaldev.androidmvvmbasics.views;
import android.databinding.BindingAdapter;
import android.databinding.DataBindingUtil;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Toast;
import com.journaldev.androidmvvmbasics.R;
import com.journaldev.androidmvvmbasics.databinding.ActivityMainBinding;
import com.journaldev.androidmvvmbasics.viewmodels.LoginViewModel;
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
ActivityMainBinding activityMainBinding = DataBindingUtil.setContentView(this, R.layout.activity_main);
activityMainBinding.setViewModel(new LoginViewModel());
activityMainBinding.executePendingBindings();
}
@BindingAdapter({"toastMessage"})
public static void runMe(View view, String message) {
if (message != null)
Toast.makeText(view.getContext(), message, Toast.LENGTH_SHORT).show();
}
}
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:
This brings an end to this tutorial on Android MVVM Using DataBinding. You can download the project from the link given below.
Horrible waste of time. All of the imports are outdated. Then, when that was finally fixed I get an error in the ActivityMainBinding build file. The project name is hardcoded in and the file cannot be edited. Unable to write the project on my own.
Hi Joe,
This article was written before the introduction of AndroidX. Many updates were released during this time. Gladly, updating the project to the latest SDK is straightforward. Just do a “Migrate to AndroidX” from the Refactor menu. Also, ActivityMainBinding file is auto-generated. So you don’t need to dig too much into it from a coding point of you. A simple clean build should do the job. I would appreciate it if you could read up a bit on Data Binding in Android and then come back with your concerns.
I’ve updated the project download file. It should be live in a few hours.
Thanks,
Anupam
import com.android.databinding.library.baseAdapters.BR; is showing unused import statement.
and in line notifyPropertyChanged(BR.toastMessage); BR > showing errror “Can not resolve symbol BR”
Please look after this.
when I google MVVM android, this web always displays the first one. It shows that your tutorial is good. Keep up making a good tutorial.
Hi,
Thanks for this tutorial.. I have implemented this and i got it.. But i have a doubt. How can i go to another activity on success?
I have implemented the same code using Kotlin. I am not understanding why the toast message is shown initially while opening the app. I want to show the toast only on the click of the login button. How to achieve this?
You need to assign null to the “private String toastMessage = null;” like this “var b: String? = null” as the toast message is having some value it is showing up via binding adapter.
Does BR.userEmail inside setUserEmail(String email) method generated for you? Becuse there is no such property called userEmail defined inside LoginViewModel. Can you please explain?
Hellow, u can rebuild your program.So BR will auto-generated
Hi Thanks for you tutorial,
How to get the email id and password in activity while pressing the button, because in your case you just showing the toast message like success or failure in view model, but there is no function to show the input details in activity.
in your entire code you missed one annotation , due to which BR will never generate. The annotation would be @Bindable above the two getters inside the model class.
Thank you for great explanation. Only one question-
How to move to other activity in case of login success? Where and how the correct point to do that?
why we should use bindingadapter for toast message? why do email and password not use binding adapter?
Because we are just observing the changes in email and password, Both email and password is bound to the XML via ViewModel so that when there values are updated our ViewModel is notified and we do not have to make a call to getText again and again, But we are not doing any UI update or any complex transactions. Here we are updating UI(showing and updating toast message string based on the validity of login credentials) hence we have bound it with a adapter so as soon as the toast message is update, the UI will be updated as well.
If i want to start a new activity on click of Login button then how to implement setOnClickListener() inside MainActivity ??
Your tutorial is good but you have incorrect information
you said you using Two way binding but in reality you still using one way binding with illusion of two way binding.
in two way binding we don’t need listeners to listen for changes, that’s the whole point of the Two way binding
maybe you set notifyPropertyChanged and Bindable but both are useless remove them and this code still work because it’s not two way binding
in two way binding we directly observe for changes using notifyPropertyChanged and Bindable and the change appear in our setter method without any listener
and most impotent thing we have to use @={ } Notation to define TWO WAY binding not @{ } that you used.
here just starting few lines will tell you..checkout this and update post.
https://developer.android.com/topic/libraries/data-binding/two-way
Thanks, Ashwini for noticing that. I’ve updated the tutorial.
please let me know how i can set two or multiple different validation message for email and password
Very neat and simple to understand this tutorial. I though MVVM is very hard to learn, but with your support and tutorial it makes me learner. Thanks lot man!!
But i am obstruction of below error… Please help to find this error
Found data binding errors.
****/ data binding error ****msg:cannot find method afterEmailTextChanged(android.text.Editable) in class com.eyeraise.kot.model.login.LoginModel file:/Users/bala/Documents/Muthu/Source/eyeraise/kot/app/src/main/res/layout/activity_login.xml loc:27:64 – 27:104 ****\ data binding error ****
Found my issue in layout I wrongly mentioned type model
Thanks. Great that you figured out the issue yourself.
ViewModel: It acts as a link between the Model and the ViewModel.
I think it should be “It acts as a link between the Model and the View”
Thanks for noticing the typo, I have fixed it.
Hi Anupam,
From past few days i have been searching for mvvm in android, then i have found yours code simple and understandable, but as the comments section says this is not pure mvvm, So, please can u provide the pure mvvm understandable example for simple login page. Please help ASAP, I am in extreme need of it. Thank you in advance.
Brother can you give me same example of mvvb and mvp to understand the patterns better ?
Found data binding errors.
****/ data binding error ****msg:Cannot find the setter for attribute ‘bind:toastMessage’ with parameter type java.lang.String on android.widget.Button
Hi, If I don’t use Databinding/Livedata, then will my app fall in MVVM pattern?
You can use RxJava.
hi,i do this step by step but it showed me a “duplicate class found in activity_main.xml” error!
please help
Hello,
Is there a way i can subscribe to you, so that I would be notified whenever a new article related to Android/Java is posted. Your articles are great.
Thanks
Aakash
Hi Aakash,
You can always enable push notifications in your web browser.
Or you can download our Android Application from Play Store.
We send notifications whenever any new article is published.
This is not pure MVVM pattern, It is mixture of MVVM and MVP.
In MVVM ViewModel doesn’t know about View, but here ViewModel has reference of View(Activity) through LoginResultCallback .
Exactly, this is MVP with some kind of ViewModel smell and data binding (which doesn’t make it MVVM). This MVVM is not correctly implemented. Learn what LiveData is and how to implement it or use RxJava.
Hi Glenn,
MVVM can be implemented with DataBinding or RxJava.
This tutorial demonstrates MVVM with Data Binding only. We are not obliged to use LiveData with MVVM always.
To prevent this tutorial from becoming too long we have omitted LiveData.
Stay tuned for the tutorials in which we’ll use LiveData and also the one with RxJava and MVVM.
Thanks
Hi Shubham,
MVVM states that the ViewModel must not have a direct reference of the View
For that we can use interface callbacks as I did earlier OR Two-way data binding.
Check the updated post!
Thanks
Hi Anupam,
I did all of the steps in your article. and don’t see that the ViewModel refers to Activity. I think the issue that Shubham mentioned had been resolved by your, right?
Thanks for your post.
Yes indeed. Glad it helped you.
Great tutorial.
But i’m facing a little problem. When I rotate the phone, the data remains there…
when I “rerotate” de phone, the data remains there, but the cursor comes back to initial point….
Do you have some thoughts about it?
Hi Victor,
Add the following line in your activity onCreate below setViewModel()
activityMainBinding.executePendingBindings();