Organizing an application based on multiple activities may not always be optimal. The Android OS world is quite fragmented and consists of many devices. And if for mobile devices with small screens the interaction between different activities looks pretty good, then on large screens - tablets, TVs, activity windows would not look very good due to the large screen size. That's why the concept of fragments appeared. A fragment represents a piece of the application's visual interface that can be reused and reused. A fragment can have its layout file, fragments have their lifecycle. A fragment exists in the context of activity and has its life cycle; it cannot exist outside the activity in isolation. Each activity can have multiple fragments. Fragments cannot live on their own-they must be by activity or another fragment. The Fragment also has its life cycle: hosted Let’s create Fragment. To do this, we need to create layout files for them and classes with an ancestor androidx.fragment.app.Fragment : fragment1.xml <?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="#77ff0000"
    android:orientation="vertical">
    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Fragment 1">
    </TextView>
</LinearLayout> : Fragment1.kt class Fragment1: Fragment() {

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        // Inflate the layout for this fragment
        Log.e(TAG, "onCreateView: ")
        return inflater.inflate(R.layout.fragment1, container, false)
    }
} In the onCreateView method, the system asks us what to display inside the fragment. We tell the system that we want to see the contents of the corresponding layout file in the fragment. To do this, we ourselves create a View using inflater and give it to the system. Those. in meaning, this is an analog of the setContentView method, which we call in the Activity. Only here do we have to create the View ourselves, and not just pass the layout file ID. For interaction between fragments, the class is used - a special fragment manager. androidx.fragment.app.FragmentManager FragmentManager uses FragmentTransaction for adding, deleting, and other actions with a fragment to conduct a transaction Before starting a transaction, you need to get an instance of via the method. Next, various methods are called to manage the fragments. FragmentTransaction FragmentManager.beginTransaction() At the end of any transaction, which may consist of a chain of the above methods, the method should be called. commit() val fragmentManager = supportFragmentManager
fragmentManager.beginTransaction()
    .remove(fragment1)
    .add(R.id.fragment_container, fragment2)
    .show(fragment3)
    .hide(fragment4)
    .commit() FragmentTransaction Methods add() Adds a fragment to an activity Removes a fragment from an activity remove() Replaces one fragment with another replace() Hides the fragment (makes it invisible on the screen) hide() Displays a hidden fragment on the screen show() (API 13) Detaches the fragment from the GUI, but the class instance is retained detach() (API 13) Attaches a fragment that has been detached by the method attach() detach() Now we add the Fragment to Activity class MainActivity : AppCompatActivity() {


	override fun onCreate(savedInstanceState: Bundle?) {  
	    super.onCreate(savedInstanceState)  
	    setContentView(R.layout.activity_main)
		
		val fragmentManager = supportFragmentManager
		fragmentManager.beginTransaction()
			.add(R.id.fragment_container, Fragment1())
		    .commit()
	}

} A fragment must have only one empty constructor with no arguments. But you can create a static with arguments via the method. newInstance setArguments() class Fragment1 : Fragment() {

	...

	companion object {  
	  
	    private const val KEY = "VALUE_KEY"  
	  
	    fun newInstance(value: String) = Fragment1().apply {  
	        arguments = Bundle().apply {  
	            putString(KEY, value)  
	        }  
	    }  
	}
}

class MainActivity : AppCompatActivity() {


	override fun onCreate(savedInstanceState: Bundle?) {  
	    super.onCreate(savedInstanceState)  
	    setContentView(R.layout.activity_main)
		
		val fragmentManager = supportFragmentManager
		fragmentManager.beginTransaction()
			.replace(R.id.fragment_container, Fragment1.newInstance("some text "))
		    .commit()
	}

} The arguments can be accessed in the method of the fragment: onCreate() override fun onCreate(savedInstanceState: Bundle?) {  
    super.onCreate(savedInstanceState)  
    
	val text = requireArguments().getString(KEY, "")
	
} Fragment stack management Fragments, like activities, can be controlled by the button. You can add several fragments, and then use the button to return to the first fragment. If no fragments remain on the stack, then the next button press will close the activity. Back Back To add a transaction to the stack, call the method before committing the transaction. The string argument is an optional name to identify the stack, or . The class has a method that returns the previous stack state by that name. FragmentTransaction.addToBackStack(String) null FragmentManager popBackStack() val fragmentManager = supportFragmentManager
		fragmentManager.beginTransaction()
			.replace(R.id.fragment_container, Fragment1.newInstance("some text "))
			.addToBackStack(null)
		    .commit() If you call the method when removing or replacing a fragment, then the fragment methods will be called. addToBackStack() onPause(), onStop(), onDestroyView() When the user presses the back button, the fragment methods , , and are called. onCreateView() onActivityCreated() onStart() onResume() Let's consider an example of reacting to the button in a fragment without using the stack. The activity has an method that responds to the button being pressed. We can refer to the desired fragment in this method and call it the fragment method. Back onBackPressed() Summary The Fragment is a very interesting Android component and is quite easy to use and fast. I hope the article helped to understand the main points and how to use Fragment. In the first part of the article, I briefly talked about the main points on Fragments, in the next part of the article we will talk about the new Fragment API and its methods.

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Understanding Fragments in Android: Part 1

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