How does viewpager and fragmentpageradapter work together ?

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Android

Android4x

I know Viewpager is kind of not used much since Viewpager 2, but I was as a noob recently learning Tablayout with Fragments .

I followed few blogs and tutorial, and honestly saying I understood the 50% and rest 50% I have no idea about.

I can understand other parts of the code like the layout and the MainACtivity, but I really dont understand how the Adapter and the Viepager workout in a CYCLE.

There are a 2/3 methods in the Adapter which sync up with the Viewpager and stuff.

I am posting my code below.

(I am not posting my fragments , which are to be shown in the viewpager)



App Contains :

MainActivity.java
activity_main.xml
PageAdapter.java

(Code not given for fragments)
3 fragments named :
tab1_fragment , tab2_fragment & tab3_fragment.



--------------------------------------------------------------------------------------

MainActivity.java :

package com.deepesh.tabapp;
import androidx.appcompat.app.AppCompatActivity;
import androidx.viewpager.widget.ViewPager;

import android.os.Bundle;

import com.google.android.material.tabs.TabItem;
import com.google.android.material.tabs.TabLayout;

public class MainActivity extends AppCompatActivity {

    TabLayout tabLayout1;
    ViewPager viewPager1;
    TabItem tab1,tab2,tab3;
    PageAdapter pageAdapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);


        tabLayout1 = findViewById(R.id.tablayout1);
        tab1 = findViewById(R.id.calltab1);
        tab2=findViewById(R.id.mailtab2);
        tab3=findViewById(R.id.smstab3);
        viewPager1 = findViewById(R.id.Viewpager1);

        pageAdapter=new PageAdapter(getSupportFragmentManager(),tabLayout1.getTabCount());
        viewPager1.setAdapter(pageAdapter);

        tabLayout1.setOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
            @Override
            public void onTabSelected(TabLayout.Tab tab) {

                viewPager1.setCurrentItem(tab.getPosition());

                if (tab.getPosition()==0 || tab.getPosition()==1 || tab.getPosition()==2 ){
                    pageAdapter.notifyDataSetChanged();
                }
            }

            @Override
            public void onTabUnselected(TabLayout.Tab tab) {

            }

            @Override
            public void onTabReselected(TabLayout.Tab tab) {

            }
        });

        viewPager1.addOnPageChangeListener(new TabLayout.TabLayoutOnPageChangeListener(tabLayout1));

    }
}

Activity_main.xml :

<?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"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:orientation="vertical"
    android:layout_height="match_parent"
    tools:context=".MainActivity">


    <com.google.android.material.tabs.TabLayout
        android:id="@+id/tablayout1"
        android:layout_width="409dp"
        android:background="@color/colorPrimary"
        android:layout_height="wrap_content"
        app:tabSelectedTextColor="#ffff"
        app:tabIndicatorColor="#ffff"
        tools:layout_editor_absoluteX="1dp"
        tools:layout_editor_absoluteY="287dp">

        <com.google.android.material.tabs.TabItem
            android:layout_width="wrap_content"
            android:text="Call"
            android:id="@+id/calltab1"
            android:layout_height="wrap_content"/>

        <com.google.android.material.tabs.TabItem
            android:layout_width="wrap_content"
            android:text="MAIL"
            android:id="@+id/mailtab2"
            android:layout_height="wrap_content"/>

        <com.google.android.material.tabs.TabItem
            android:layout_width="wrap_content"
            android:text="SMS"
            android:id="@+id/smstab3"
            android:layout_height="wrap_content"/>
    </com.google.android.material.tabs.TabLayout>

    <androidx.viewpager.widget.ViewPager
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/Viewpager1"
        />
</LinearLayout>

PageAdapter.java :


package com.deepesh.tabapp;
import androidx.annotation.NonNull;
import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentManager;
import androidx.fragment.app.FragmentPagerAdapter;

public class PageAdapter extends FragmentPagerAdapter {

int tabcount;

public PageAdapter(@NonNull FragmentManager fm, int behavior) {
super(fm, behavior);

tabcount=behavior;
}

@NonNull
@Override
public Fragment getItem(int position) {
switch (position){

case 0: return new tab1_fragment();
case 1: return new tab2_fragment();
case 2: return new tab3_fragment();
default: return null;

}
}

@Override
public int getCount() {
return tabcount;
}
}

What I have tried:

I searched alott on the web , but dindt find what I was looking for.