11. Using Fragment in android development(Theory)

11. Using Fragment in android development(Theory)

Fragment is a part of an activity which enable more modular activity design. It will not be wrong if we say a fragment is a kind of sub-activity. It represents a behaviour or a portion of user interface in an Activity. We can combine multiple Fragments in Single Activity to build a multi panel UI and reuse a Fragment in multiple Activities. We always need to embed Fragment in an activity and the fragment lifecycle is directly affected by the host activity’s lifecycle.

Android introduced fragments in Android 3.0 (API level 11), primarily to support more dynamic and flexible UI designs on large screens, such as tablets. Because a tablet's screen is much larger than that of a handset, there's more room to combine and interchange UI components. Fragments allow such designs without the need for you to manage complex changes to the view hierarchy. By dividing the layout of an activity into fragments, you become able to modify the activity's appearance at runtime and preserve those changes in a back stack that's managed by the activity.

Figure 1. An example of how two UI modules defined by fragments can be combined into one activity for a tablet design, but separated for a handset design.

Fragment Lifecycle:

Declare the fragment inside the activity's layout file.

In this case, you can specify layout properties for the fragment as if it were a view. For example, here's the layout file for an activity with two fragments:

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

   android:orientation="horizontal"

   android:layout_width="match_parent"

   android:layout_height="match_parent">

   <fragment android:name="com.example.news.ArticleListFragment"

           android:id="@+id/list"

           android:layout_weight="1"

           android:layout_width="0dp"

           android:layout_height="match_parent" />

   <fragment android:name="com.example.news.ArticleReaderFragment"

           android:id="@+id/viewer"

           android:layout_weight="2"

           android:layout_width="0dp"

           android:layout_height="match_parent" />

</LinearLayout>

Or, programmatically add the fragment to an existing ViewGroup.

To make fragment transactions in your activity (such as add, remove, or replace a fragment), you must use APIs from FragmentTransaction. You can get an instance of FragmentTransaction from your Activity like this:

FragmentManager fragmentManager = getFragmentManager();

FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();

You can then add a fragment using the add() method, specifying the fragment to add and the view in which to insert it. For example:

ExampleFragment fragment = new ExampleFragment();

fragmentTransaction.add(R.id.fragment_container, fragment);

fragmentTransaction.commit();

Managing Fragments

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To manage the fragments in your activity, you need to use FragmentManager. To get it, call getFragmentManager() from your activity.

Some things that you can do with FragmentManager include:

• Get fragments that exist in the activity, with findFragmentById() (for fragments that provide a UI in the activity layout) or findFragmentByTag()(for fragments that do or don't provide a UI).
• Pop fragments off the back stack, with popBackStack() (simulating a Back command by the user).
• Register a listener for changes to the back stack, with addOnBackStackChangedListener().

For more information about these methods and others, refer to the FragmentManager class documentation.

As demonstrated in the previous section, you can also use FragmentManager to open a FragmentTransaction, which allows you to perform transactions, such as add and remove fragments.

Performing Fragment Transactions

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You can acquire an instance of FragmentTransaction from the FragmentManager like this:

FragmentManager fragmentManager = getFragmentManager();

FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();

Each transaction is a set of changes that you want to perform at the same time. You can set up all the changes you want to perform for a given transaction using methods such as add(), remove(), and replace(). Then, to apply the transaction to the activity, you must call commit().

o retrieve fragments from the back stack, you must override onBackPressed() in the main activity class:

@Override

public void onBackPressed() {

   if (getFragmentManager().getBackStackEntryCount() > 0) {

       getFragmentManager().popBackStack();

   } else {

       super.onBackPressed();

   }

}

Thanks. Let's do some practical on fragment in next session.