MVVM example

In this example, we'll use Data Binding only.

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.

let's see a very basic example of a login page using the MVVM pattern...

Project structure:

Enable data-binding:

have to enable data-binding in Gradle file:

note: do it in the app-level build file.

android{

   dataBinding{

               enabled = true; 

        }

}

Adding dependencies:

implementation 'android.arch.lifecycle:extensions:1.1.0'

Model class:

The model would hold the user details, like user name and password, here User is model class ,

package com.javaoneworld.mvvmsample.model;

public class User {

    private String email;

    private String password;

    public User(String email, String password) {

        this.email = email;

        this.password = password;

    }

    public String getEmail() {

        return email;

    }

    public void setEmail(String email) {

        this.email = email;

    }

    public String getPassword() {

        return password;

    }

    public void setPassword(String password) {

        this.password = password;

    }

}

Data Binding allow us to bind the data in XML layout, so there is no need of findViewById in our acticity class.

Syntax- @={variable}

So there is one more very important thing in data binding case, 

We have to wrap up our layout in <layout>.........</layout> tag,

means that our root tag must be <layout>.

and <layout> contains two thing...

Data

Layout(UI or View)

Data contains two thing,

1. name
2. type

Layout:

<?xml version="1.0" encoding="utf-8"?>

<layout xmlns:android="http://schemas.android.com/apk/res/android"

    xmlns:app="http://schemas.android.com/apk/res-auto" 

   xmlns:tools="http://schemas.android.com/tools">

    <data>

        

        <variable   

         name="viewModel"    

        type="com.javaoneworld.mvvmsample.view_model.LoginViewModel" />

    </data>

    <RelativeLayout     

                android:id="@+id/rootLayout"

                android:layout_width="match_parent"

                android:layout_height="match_parent"

                tools:context=".view.MainActivity">

<LinearLayout   

         android:layout_width="match_parent"

            android:layout_height="wrap_content"

            android:orientation="vertical">

           <EditText

                android:id="@+id/inEmail"

                android:layout_width="match_parent"

                android:layout_height="wrap_content"

                android:hint="Email"

                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:text="@={viewModel.userPassword}"/>

            <Button

                android:text="Login"

                android:layout_width="wrap_content"

                android:layout_height="wrap_content"

                android:layout_gravity="center"

                android:onClick="@{()-> viewModel.onLoginClicked()}"

                bind:toastMessage="@{viewModel.toastMessage}"/>

        </LinearLayout>

    </RelativeLayout>

</layout> 

And finally here is ViewModel

ViewModel:

package com.javaoneworld.mvvmsample.view_model;

import android.text.TextUtils;

import android.util.Patterns;

import com.javaoneworld.mvvmsample.model.User;

import androidx.databinding.BaseObservable;

import androidx.databinding.Bindable;

import androidx.databinding.library.baseAdapters.BR;

public class LoginViewModel extends BaseObservable {

    private User user;

    private String successMessage = "Login was successful";

    private String failedMessage = "Login failed";

    @Bindable    private String toastMessage = null;

    @Bindable    private String userEmail;

    @Bindable    private String userPassword;

    public String getToastMessage(){

        return  toastMessage;

    }

    public void setToastMessage(String toastMessage)

    {

        this.toastMessage = toastMessage;

        notifyPropertyChanged(BR.toastMessage);

    }

    public void setUserEmail(String email)

    {

        user.setEmail(email);

        notifyPropertyChanged(BR.userEmail);

    }

    public String getUserEmail()

    {

        return user.getEmail();

    }

    public void setUserPassword(String password)

    {

        user.setPassword(password);

        notifyPropertyChanged(BR.userPassword);

    }

    public String getUserPassword()

    {

        return user.getPassword();

    }

    public LoginViewModel()

    {

        user = new User("","");

    }

    public void onLoginClicked() {

        if (isInputDataValid())

            setToastMessage(successMessage);

        else            setToastMessage(failedMessage);

    }

    public boolean isInputDataValid() {

        return !TextUtils.isEmpty(getUserEmail()) && Patterns.EMAIL_ADDRESS.matcher(getUserEmail()).matches() && getUserPassword().length() > 5;

    }

}

Activity:

package com.javaoneworld.mvvmsample.view;

import androidx.appcompat.app.AppCompatActivity;

import androidx.databinding.BindingAdapter;

import androidx.databinding.DataBindingUtil;

import android.os.Bundle;

import android.view.View;

import android.widget.Toast;

import com.javaoneworld.mvvmsample.R;

import com.javaoneworld.mvvmsample.databinding.ActivityMainBinding;

import com.javaoneworld.mvvmsample.view_model.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();

    }

}