Learn about Fragment using these simple examples.

Step 1: Dependencies

No external dependencies are needed for this project.

Step 2: Design Layouts

We will have 4 layouts. Two of them for Fragments, two for Activities:

(a). fragment_second.xml

Declare a TextView as the root and only element:

Kotlin Android Fragments Example

Kotlin Android Fragments Example
Kotlin Android Fragments Example
<?xml version="1.0" encoding="utf-8"?>

<TextView
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    android:background="@android:color/holo_red_light"
    android:text="@string/secondFragmentText"/>

(b). fragment_main.xml

Declare a TextView and a Button inside a RelativeLayout:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
                android:layout_width="match_parent"
                android:layout_height="match_parent">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"
        android:text="@string/mainFragmentText"/>

    <Button
        android:id="@+id/mainFragmentButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_margin="20dp"
        android:text="@string/mainFragmentButton"/>

</RelativeLayout>

(c). activity_second.xml

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout
    android:id="@+id/contentFrame"
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <!-- Do not add Views inside FrameLayout used for fragment transactions. This is only for a test purpose-->
    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:onClick="showFragment"
        android:text="@string/secondActivityButton"/>
</FrameLayout>

(d). activity_main.xml

Declare our main Fragment inside this layout:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
    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:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".activities.MainActivity">

    <fragment
        android:id="@+id/mainFragment"
        android:name="fr.esilv.fragmentexample.fragments.MainFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"/>
</RelativeLayout>

Step 3: Write Fragment Code

(a). SecondFragment.kt

package fr.esilv.fragmentexample.fragments

import android.os.Bundle
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import androidx.fragment.app.Fragment
import fr.esilv.fragmentexample.R

class SecondFragment : Fragment() {
    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?,
                              savedInstanceState: Bundle?): View? {
        return inflater.inflate(R.layout.fragment_second, container, false)
    }
}

(b). MainFragment.kt

package fr.esilv.fragmentexample.fragments

import android.content.Intent
import android.os.Bundle
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import androidx.fragment.app.Fragment
import fr.esilv.fragmentexample.R
import fr.esilv.fragmentexample.activities.SecondActivity
import kotlinx.android.synthetic.main.fragment_main.mainFragmentButton

class MainFragment : Fragment() {
    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?,
                              savedInstanceState: Bundle?): View? {
        return inflater.inflate(R.layout.fragment_main, container, false)
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        mainFragmentButton.setOnClickListener(OnMainFragmentButtonClickListener())
    }

    private inner class OnMainFragmentButtonClickListener : View.OnClickListener {
        override fun onClick(v: View) {
            startActivity(Intent(v.context, SecondActivity::class.java))
        }
    }
}

Step 4: Create Activities

Create Activities code as follows:

(a). SecondActivity.kt

package fr.esilv.fragmentexample.activities

import android.os.Bundle
import android.view.View
import androidx.appcompat.app.AppCompatActivity
import fr.esilv.fragmentexample.R
import fr.esilv.fragmentexample.fragments.SecondFragment

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

    //this method is called by the Button in activity_second.xml. the android:onClick attribute binds with this method.
    fun showFragment(view: View) {
        view.visibility = View.GONE
        supportFragmentManager.beginTransaction()
                .apply {
                    //the id passed as parameter is the id of the FrameLayout defined in activity_second.xml.
                    replace(R.id.contentFrame, SecondFragment())
                    //the transaction has to be committed for changes to happen.
                    commit()
                }

    }
}

(b). MainActivity.kt

package fr.esilv.fragmentexample.activities

import android.os.Bundle
import androidx.appcompat.app.AppCompatActivity
import fr.esilv.fragmentexample.R

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

Reference

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