Fragments in android 

By Michael Sadgun Rao Kona,

Fragment

• A Fragment represents a reusable portion of  app's UI. 
• A fragment defines and manages its own layout.
• Fragment has its own lifecycle
• Fragment can handle its own input events. 
• Fragments cannot live on their own--they must be hosted by an activity or another fragment. 
• The fragment’s view hierarchy becomes part of, or attaches to, the host’s view hierarchy.

Source:- https://www.tutorialspoint.com/android/android_fragments.htm

modularity

• Divide the UI into discrete chunks ( FRAGMENTS)
• Dividing UI into fragments makes it easier to modify your activity's appearance at runtime.
• Fragments can be activated and deactivated during runtime.
• Fragments are introduced in HoneyComb API 11.
• Activity - ideal place to put global elements around app's UI.
• Ex:-navigation drawer.
• Fragments - better suited to define and manage the UI of a single screen or portion of a screen.

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• While the activity is in the STARTED lifecycle state or higher, fragments can be added, replaced, or removed. 
• Keep a record of these changes in a back stack (that is managed by the activity), allowing the changes to be reversed.
• Multiple instances of the same fragment class can be used
• within the same activity,
• in multiple activities,
• or even as a child of another fragment. 
• Note:-
• Provide a fragment with the logic necessary to manage its own UI. 
• Avoid depending on or manipulating one fragment from another.

Example - app that responds to various screen sizes.

Requirement:

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 On larger screens, the app should display a static navigation drawer and a list in a grid layout.

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On smaller screens, the app should display a bottom navigation bar and a list in a linear layout.

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Solution:

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The activity is then responsible for displaying the correct navigation UI while the fragment displays the list with the proper layout.

CREATE FRAGMENT

• To create a fragment,
• extend the AndroidX Fragment class, and
• override its methods to insert your app logic
• To create a minimal fragment that defines its own layout, provide your fragment's layout resource to the base constructor

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Different fragment classes available

• DialogFragment: Displays a floating dialog
• ListFragment: Displays a list of items that are managed by an adapter.
• PreferenceFragment: Displays a hierarchy of Preference objects as a list, useful when creating a "settings" Activity for the app.

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Using a fragment

• The general steps to use a Fragment:
• Create a subclass of Fragment.
• Create a layout for the Fragment.
• Add the Fragment to a host Activity
• either statically (XML)

           or 

• Dynamically (Java)

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ADD a fragment via xml

Source:-https://developer.android.com/guide/fragments/create#java

ADD a fragment via xml

• The android:name attribute - class name of the Fragment
• When the activity's layout is inflated, the specified fragment is instantiated
• onInflate() is called on the newly instantiated fragment, 

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• FragmentTransaction is created to add the fragment to the FragmentManager.

Source:-https://developer.android.com/guide/fragments/create#java

Add a fragment programmatically

• no specific fragment is automatically instantiated.
• Instead, a FragmentTransaction is used to instantiate a fragment and add it to the activity's layout.
• While the activity is running, fragment transactions such as adding, removing, or replacing a fragment can be performed.

Source:-https://developer.android.com/guide/fragments/create#java

Add a fragment programmatically

• In FragmentActivity, get an instance of the FragmentManager, which can be used to create a FragmentTransaction. 
• Then instantiate fragment within activity's onCreate() method using FragmentTransaction.add()
• Arguments are 
• ViewGroup ID of the container in layout and
• fragment class
• then commit the transaction,

Source:-https://developer.android.com/guide/fragments/create#java

XML Code  ( with no "name" attribute)

Source:-https://developer.android.com/guide/fragments/create#java

Java code to add fragment into an activity

Source:-https://developer.android.com/guide/fragments/create#java

Fragment transactions

• Possible transactions with a fragment are:
• Add a Fragment using add().
• Remove a Fragment using remove().
• Replace a Fragment with another Fragment using replace().
• Hide and show a Fragment using hide() and show().

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Source:- https://google-developer-training.github.io/android-developer-advanced-course-concepts/unit-1-expand-the-user-experience/lesson-1-fragments/1-1-c-fragments/1-1-c-fragments.html#creating

if (savedInstanceState == null)

• To ensure that the fragment is added only once, when the activity is first created.
• When a configuration change occurs and the activity is recreated, savedInstanceState is no longer null, and the fragment does not need to be added a second time.
• fragment is automatically restored from the savedInstanceState.

Passing arguments to fragment through Bundle

ViewGroup ID of the fragment

Name of the Fragment class

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To pass arguments or input to fragment

To read arguments

Fragment life cycle

• The Fragment is added by an Activity (which acts as the host of the Fragment). 
• Once added, the Fragment goes through three states:
• Active (or resumed)
• Paused
• Stopped

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Fragment life cycle states

• Fragment is always hosted by an Activity
• Fragment lifecycle is directly affected by the host Activity lifecycle. 
• For example, 
• when the Activity is paused, so are all Fragments in it, and
• when the Activity is destroyed, so are all Fragments.

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Fragment life cycle states

• Each lifecycle callback for the Activity results in a similar callback for each Fragment, as shown in the following table.

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Fragment lifecycle callbacks

Fragment Lifecycle states and their relation both the fragment's lifecycle callbacks and the fragment's view Lifecycle.

Fragment call backs

• onCreate():
• Initialize essential components and variables of the Fragment 
• Anything initialized in onCreate() is preserved if the Fragment is paused and resumed.
• onCreateView():
• Inflate the XML layout for the Fragment in this callback.
• The system calls this method to draw the Fragment UI for the first time.
• As a result, the Fragment is visible in the Activity.
• To draw a UI for your Fragment, you must return the root View of your Fragment layout. Return null if the Fragment does not have a UI.

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Fragment call backs

• onPause(): 
• Save any data and states that need to survive beyond the destruction of the Fragment.
• The system calls this method if any of the following occurs:
• The user navigates backward.
• The Fragment is replaced or removed, or another operation is modifying the Fragment.
• The host Activity is paused.

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Fragment call backs

• onResume(): Called by the Activity to resume a Fragment that is visible to the user and actively running.
• onAttach(): Called when a Fragment is first attached to a host Activity. 
• onDestroyView(): Called when the View previously created by onCreateView() has been detached from the Fragment. 

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