How can fragments be implemented in an Activity…

Fragments in an Activity can be implemented declaratively in XML layout or programmatically using FragmentManager.

Declaratively (in XML):

<fragment
    android:id="@+id/my_fragment"
    android:name="com.example.MyFragment" // Full class name of the fragment
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

The fragment is added during Activity creation.
Easy to use for static layouts.
Less flexible for dynamic operations.

Programmatically (using FragmentManager):

// Inside an Activity method, e.g., onCreate or in response to an event
val fragmentManager = supportFragmentManager // For AppCompatActivity
val fragmentTransaction = fragmentManager.beginTransaction()

// Adding a fragment
val myFragment = MyFragment()
fragmentTransaction.add(R.id.fragment_container, myFragment)

// Replacing a fragment (removes existing in container and adds new)
// val newFragment = NewFragment()
// fragmentTransaction.replace(R.id.fragment_container, newFragment)

// Removing a fragment
// val fragmentToRemove = fragmentManager.findFragmentById(R.id.fragment_container)
// if (fragmentToRemove != null) {
//     fragmentTransaction.remove(fragmentToRemove)
// }

// Adding to Back Stack (allows user to return to previous fragment via "Back" button)
fragmentTransaction.addToBackStack(null) // Tag string for back stack (can be null)

// Committing the transaction
fragmentTransaction.commit() // Asynchronous execution
// or
// fragmentTransaction.commitNow() // Synchronous execution

FragmentManager is used to manage fragments.
FragmentTransaction is used to perform operations (add, remove, replace, etc.).
Provides great flexibility for dynamic UI changes at runtime.
Can manage Back Stack for navigation.

Using a container:

For programmatic addition or replacement of fragments, a container in the Activity's XML layout is usually used, e.g., FrameLayout or LinearLayout:

<FrameLayout
    android:id="@+id/fragment_container"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

Then, in the Activity code, the fragment is added or replaced inside this container.

Important aspects:

Fragment lifecycle is closely related to the Activity lifecycle.
Fragment Manager manages the lifecycle and state of fragments.
Fragment Transaction is atomic (either the whole operation or none).
When using Back Stack, call addToBackStack().
For working with fragments in AppCompatActivity, use supportFragmentManager.

How can fragments be implemented in an Activity?