androidandroid-fragmentsandroid-jetpackback-stackandroid-jetpack-navigation

Back stack not working properly in Jetpack Navigation

I have created a simple demo project to test the default back stack feature of the Android Jetpack Navigation component.
I'm having one main activity and two fragments. The home fragment is being displayed when the app runs. The home fragment has a button. On click on that navigates to the other fragment.
But whenever I'm pressing the system back button my complete application is finished rather than the usual back stack behavior where I should see the home fragment.
Any help is appreciated. Thanks in advance.
The code is as follows :
MainActivity.kt

package com.callsamik.jetpacknavigationcomponentdemo

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
    }
}

activity_main.xml (layout for the main activity)

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <androidx.fragment.app.FragmentContainerView
        android:id="@+id/nave_host_fragment"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:navGraph="@navigation/nav_graph"
        />

</androidx.constraintlayout.widget.ConstraintLayout>

HomeFragment.kt

package com.callsamik.jetpacknavigationcomponentdemo

import android.os.Bundle
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import androidx.fragment.app.Fragment
import androidx.navigation.fragment.findNavController
import com.callsamik.jetpacknavigationcomponentdemo.databinding.FragmentHomeBinding

class HomeFragment: Fragment() {
    var binding: FragmentHomeBinding? = null
    override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        binding = FragmentHomeBinding.inflate(layoutInflater)
        return binding?.root
    }
    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        binding?.loginButton?.setOnClickListener{
            val action = HomeFragmentDirections.actionHomeFragmentToLoginFragment()
            findNavController().navigate(action)
        }
    }

    override fun onDestroyView() {
        super.onDestroyView()
        binding = null
    }
}

fragment_home.xml (layout for HomeFragment)

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    tools:context= ".HomeFragment">
    <androidx.appcompat.widget.AppCompatTextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="16dp"
        android:text="Home Screen"
        android:textAppearance="@style/TextAppearance.AppCompat.Large"/>

    <androidx.appcompat.widget.AppCompatButton
        android:id="@+id/loginButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Login"/>
</LinearLayout>

LoginFragment.kt (the 2nd fragment)

package com.callsamik.jetpacknavigationcomponentdemo

import androidx.fragment.app.Fragment

class LoginFragment: Fragment(R.layout.fragment_login) {
}

fragment_login.xml (2nd fragment layout)

<?xml version="1.0" encoding="utf-8"?>
<androidx.appcompat.widget.LinearLayoutCompat xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    android:padding="16dp"
    android:orientation="vertical"
    tools:context=".LoginFragment">

    <androidx.appcompat.widget.AppCompatEditText
        android:id="@+id/usernameEditText"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="Username"/>

    <androidx.appcompat.widget.AppCompatEditText
        android:id="@+id/passwordEditText"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="Password"/>

    <androidx.appcompat.widget.AppCompatButton
        android:id="@+id/confirmButton"
        android:text="Confirm"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"/>

</androidx.appcompat.widget.LinearLayoutCompat>

nav_graph.xml (navigation graph)

<?xml version="1.0" encoding="utf-8"?>
<navigation 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"
    android:id="@+id/nav_graph"
    app:startDestination="@id/homeFragment">

    <fragment
        android:id="@+id/homeFragment"
        android:name="com.callsamik.jetpacknavigationcomponentdemo.HomeFragment"
        android:label="fragment_home"
        tools:layout="@layout/fragment_home" >
        <action
            android:id="@+id/action_homeFragment_to_loginFragment"
            app:destination="@id/loginFragment"
            app:enterAnim="@anim/slide_in_right"
            app:exitAnim="@anim/slide_out_left"
            app:popEnterAnim="@anim/slide_in_left"
            app:popExitAnim="@anim/slide_out_right" />
    </fragment>
    <fragment
        android:id="@+id/loginFragment"
        android:name="com.callsamik.jetpacknavigationcomponentdemo.LoginFragment"
        android:label="fragment_login"
        tools:layout="@layout/fragment_login" />
</navigation>

Top-level build.gradle

// Top-level build file where you can add configuration options common to all sub-projects/modules.
buildscript {
    ext.kotlin_version = "1.5.31"
    ext.nav_version = "2.3.5"
    repositories {
        google()
        mavenCentral()
    }
    dependencies {
        classpath "com.android.tools.build:gradle:7.0.3"
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
        classpath "androidx.navigation:navigation-safe-args-gradle-plugin:$nav_version"

        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

task clean(type: Delete) {
    delete rootProject.buildDir
}

app module build.gradle

plugins {
    id 'com.android.application'
    id 'kotlin-android'
    id 'androidx.navigation.safeargs.kotlin'
}

android {
    compileSdk 31

    defaultConfig {
        applicationId "com.callsamik.jetpacknavigationcomponentdemo"
        minSdk 22
        targetSdk 31
        versionCode 1
        versionName "1.0"

        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
    }

    buildFeatures {
        viewBinding true
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }
    kotlinOptions {
        jvmTarget = '1.8'
    }
}

dependencies {

    implementation 'androidx.core:core-ktx:1.7.0'
    implementation 'androidx.appcompat:appcompat:1.3.1'
    implementation 'com.google.android.material:material:1.4.0'
    implementation 'androidx.constraintlayout:constraintlayout:2.1.1'
    implementation "androidx.navigation:navigation-fragment-ktx:$nav_version"
    implementation "androidx.navigation:navigation-ui-ktx:$nav_version"

    testImplementation 'junit:junit:4.+'
    androidTestImplementation 'androidx.test.ext:junit:1.1.3'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.4.0'
}
Solution

  • As per the documentation:

    • The app:defaultNavHost="true" attribute ensures that your NavHostFragment intercepts the system Back button. Note that only one NavHost can be the default. If you have multiple hosts in the same layout (two-pane layouts, for example), be sure to specify only one default NavHost.

    You are missing that attribute so Navigation is not handling the system back button.

    <androidx.fragment.app.FragmentContainerView
    android:id="@+id/nave_host_fragment"
    android:name="androidx.navigation.fragment.NavHostFragment"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    app:navGraph="@navigation/nav_graph"
    app:defaultNavHost="true"
    />