Fragments

A Fragment represents a reusable portion of your app's UI. A fragment defines and manages its own layout, has its own lifecycle, and can handle its own input events.

You can consider a fragment as lightweight version of an activity.

Typical example - on small screen app can show fragment with list. When the user clicks on a list item app will replace this fragment by fragment that show details. On larger screen app can show both fragments at the same time: list on the left and details of the current list item on the right.

Add dependency to include the AndroidX Fragment library.

val fragment_version = "1.3.4"

// Java language implementation
implementation("androidx.fragment:fragment:$fragment_version")
// Kotlin
implementation("androidx.fragment:fragment-ktx:$fragment_version")

To create a fragment, extend the AndroidX Fragment class, and override its methods to insert your app logic, similar to the way you would create an Activity class.

The arguments Bundle can then be retrieved from within your fragment by calling requireArguments(), and the appropriate Bundle getter methods can be used to retrieve each argument.

class ExampleFragment : Fragment(R.layout.example_fragment) {
    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        val someInt = requireArguments().getInt("some_int")
        // ...
    }
    
    // ...
}

Fragments are hosted by an activity or another fragment. The fragment’s view hierarchy becomes part of the host’s view hierarchy.

FragmentContainerView is extension of FrameLayout designed specifically for Fragments. Add it to the layout of activity or parent fragment in place where you want display fragment.

<!--  res/layout/example_activity.xml -->
<androidx.fragment.app.FragmentContainerView
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/fragment_container_view"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

FragmentManager class is responsible for performing actions on your app's fragments, such as adding, removing, or replacing them, and adding them to the fragment backstack (read more how to manage fragments)

class ExampleActivity : AppCompatActivity(R.layout.example_activity) {
 
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        // set our fragment as root fragment at first time  
        if (savedInstanceState == null) {

            val bundle = bundleOf("some_int" to 0)
            
            supportFragmentManager.commit {
                setReorderingAllowed(true) // You should always use it in transaction
                add<ExampleFragment>(R.id.fragment_container_view, args = bundle)
            }
        }
    }
}

Also you can add fragment in the layout.xml file via FragmentContainerView or <fragment> element.

<!-- res/layout/example_activity.xml -->
<androidx.fragment.app.FragmentContainerView
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/fragment_container_view"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:name="com.example.ExampleFragment" />
<!-- not recommended, but you can meet it in old code -->
<fragment
    android:id="@+id/cat_fragment"
    android:name="com.example.ExampleFragment"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

life cycle