Fragment Lifecycle in Android

In Android, the fragment is the part of the Activity that represents a portion of the User Interface(UI) on the screen. It is the modular section of the Android activity that is very helpful in creating UI designs that are flexible in nature and auto-adjustable based on the device screen size. The UI flexibility on all devices improves the user experience and adaptability of the application. that can exist only inside an activity as its lifecycle is dependent on the lifecycle of the host activity. For example, if the host activity is paused, then all the methods and operations of the fragment related to that activity will stop functioning, the fragment is also termed a sub-activity. Fragments in Android can be added, removed, or replaced dynamically i.e., while the activity is running. 

<fragment> tag is used to insert the fragment in an android activity layout. By dividing the activity's layout multiple fragments can be added in it.

Below is the pictorial representation of fragment interaction with the activity:

Types of Android Fragments

1. Single Fragment: Display only one single view on the device screen. This type of fragment in android is mostly used for mobile phones.
2. List Fragment: This Fragment is used to display a list-view from which the user can select the desired sub-activity. The menu drawer of apps like Gmail is the best example of this kind of android fragment.
3. Fragment Transaction: This kind of fragments in android supports the transition from one fragment in android to another at run time. Users can switch between multiple fragments like switching tabs.

ANdroid Fragment Lifecycle

Each fragment has it's own lifecycle but due to the connection with the Activity it belongs to, the android fragment lifecycle is influenced by the activity's lifecycle.

Methods of the Android Fragment

Android Fragment When to call Method

Consider Fragment-1 is A and Fragment-2 is B and A is attached to the Main Activity
              1. If you can replace B with A.
                        A's call back:
                              onDestroyView()
                              onDestroy()
                              onDetach()

                       B's call back:
                              onAttach()
                             onCreate()
                             onCreateView()
                            onViewCreated()
 

              2. If you can replace B with A without Losing resources.
                       A's call back:
                             onDestroy()
                             onDetach()

                      B's call back:
                             onAttach()
                            onCreate()
                            onCreateView()
                           onViewCreated()
 

Example of Android Fragment

Fragments in android are always embedded in Activities i.e., they are added to the layout of activity in which they reside. Multiple fragments can be added to one activity. This task can be carried out in 2 ways:

1. Statically: Explicitly mention the fragment in the XML file of the activity. This type of fragment can not be replaced during the run time.
2. Dynamically: FragmentManager is used to embed fragments with activities that enable the addition, deletion, or replacement of fragments at run time.

Almost all android app uses dynamic addition of fragments as it improves the user experience. Below is the step-by-step implementation of adding 2 fragments in one activity. A default fragment will be visible when the activity appears on the screen and the user can switch between the 2 fragments at the run time. 

Note: Following steps are performed on Android Studio version 4.0

Step 1: Create a new project

1. Click on File, then New => New Project.
2. Choose Empty activity
3. Select language as Java
4. Select the minimum SDK as per your need.

Step 2: Modify strings.xml file

All the strings which are used in the activity are listed in this file

<resources>
    <string name="app_name">GfG | Fragment in Android</string>
    <string name="heading">Two Fragments in One Activity</string>
    <string name="fragment1_button">Display First Fragment</string>
    <string name="fragment2_button">Display Second Fragment</string>
    <string name="fragment1_text1">Displaying contents of the First Fragment</string>
    <string name="fragment2_text1">Displaying contents of the Second Fragment</string>
</resources>

Step 3: Working with the activity_main.xml file

Open the activity_main.xml file and add 2 buttons to it which will be used to switch between the 2 fragments. Further, add the fragment element in the activity layout. It is the area in which the fragments in android will be displayed.

<?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:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#168BC34A"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <!-- Heading of the activity -->
    <TextView
        android:id="@+id/textView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="20dp"
        android:layout_marginBottom="20dp"
        android:fontFamily="@font/roboto"
        android:text="@string/heading"
        android:textAlignment="center"
        android:textColor="@android:color/holo_green_light"
        android:textSize="24sp"
        android:textStyle="bold" />

    <!-- Button to display first fragment -->
    <Button
        android:id="@+id/button1"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_marginStart="20dp"
        android:layout_marginEnd="20dp"
        android:background="#4CAF50"
        android:fontFamily="@font/roboto"
        android:onClick="selectFragment"
        android:text="@string/fragment1_button"
        android:textColor="@android:color/background_light"
        android:textSize="18sp"
        android:textStyle="bold" />

    <!-- Button to display second fragment -->
    <Button
        android:id="@+id/button2"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_marginStart="20dp"
        android:layout_marginTop="20dp"
        android:layout_marginEnd="20dp"
        android:layout_marginBottom="20dp"
        android:background="#4CAF50"
        android:fontFamily="@font/roboto"
        android:onClick="selectFragment"
        android:text="@string/fragment2_button"
        android:textColor="@android:color/background_light"
        android:textSize="18sp"
        android:textStyle="bold" />

    <!-- Adding Fragment element in the activity -->
    <fragment
        android:id="@+id/fragment_section"
        android:name="com.example.fragments_backup.FragmentOne"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_marginStart="10dp"
        android:layout_marginEnd="10dp"
        android:layout_marginBottom="10dp"
        tools:layout="@layout/fragment_one" />

</LinearLayout>

The android:name tag under the <fragment> element is containing the file name of default fragment which is to be displayed when activity opens.

Step 4: Creating the two fragment class

These files contain only the onCreateView() method to inflate the UI of the fragment and returns the root of the fragment layout. If the fragment does not have any UI, it will return null.

1. First Fragment class:

import android.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

public class FragmentOne extends Fragment {
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

        // inflating the layout of the fragment
        // and returning the view component
        return inflater.inflate(R.layout.fragment_one, container, false);
    }
}

2. Second Fragment Class:

import android.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

public class FragmentTwo extends Fragment{
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

        // inflating the layout of the fragment
        // and returning the view component
        return inflater.inflate(R.layout.fragment_two, container, false);
    }
}

Step 5: Creating Layouts for both the fragments

Create two Layout Resource Files for both the fragments. Fragment displays a text on the screen and have a background color to differentiate their area in the Activity layout. Below is the code to implement this layout.

1. fragment_one.xml file:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#5C52CC57"
    android:orientation="vertical">

    <!-- Text to be displayed inside the Fragment -->
    <TextView
        android:id="@+id/textView1"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_weight="1"
        android:gravity="center"
        android:text="@string/fragment1_text1"
        android:textAlignment="center"
        android:textColor="@android:color/background_light"
        android:textSize="24sp"
        android:textStyle="bold" />

</LinearLayout>

2. fragment_two.xml file:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#5C3473A6"
    android:orientation="vertical">

    <!-- Text to be displayed inside the Fragment -->
    <TextView
        android:id="@+id/textView2"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:fontFamily="@font/roboto"
        android:gravity="center"
        android:text="@string/fragment2_text1"
        android:textAlignment="center"
        android:textColor="@android:color/background_light"
        android:textSize="24sp"
        android:textStyle="bold" />

</LinearLayout>

Step 6: Working with the MainActivity.java file

Now, the functionality of the button to perform operations on clicking will be defined in the MainActivity class. Moreover, the code for the replacement of fragments during run time is also mentioned in this file. Below is the code to implement this step.

import android.os.Bundle;
import android.view.View;
import android.app.Fragment;
import android.app.FragmentManager;
import android.app.FragmentTransaction;
import androidx.appcompat.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);
    }

    // method for displaying the appropriate
    // fragment according to the clicked button
    public void selectFragment(View view) {

        // creating object for Fragment
        Fragment fr;

        // displaying first fragment
        // if button1 is clicked
        if(view == findViewById(R.id.button1)) {
            fr = new FragmentOne();
        }

        // displaying second fragment
        // if button2 is clicked
        else {
            fr = new FragmentTwo();
        }

        FragmentManager fm = getFragmentManager();

        // fragment transaction to add or replace
        // fragments while activity is running
        FragmentTransaction fragmentTransaction = fm.beginTransaction();
        fragmentTransaction.replace(R.id.fragment_section, fr);

        // making a commit after the transaction
        // to assure that the change is effective
        fragmentTransaction.commit();
    }
}

Output: