MobAppDev

Fragments & ViewGroups, Fragment Management, Fragment Creation, Activity vs Fragment Lifecycles

Vladimir Kulyukin

www.vkedco.blogspot.com

Outline

● Fragments & ViewGroups● Fragment Management● Fragment Creation● Activity vs Fragment Lifecycle

Fragments & ViewGroups

Fragment Lifecycle

source image at http://developer.android.com/guide/components/fragments.html

Fragments & ViewGroups

● When a fragment is added to an activity layout, it becomes a component in a ViewGroup (a container that may contain other views called children) inside the activity's view hierarchy

● A fragment may define its own view layout● A fragment can be inserted into the activity's layout file

via the tags <fragment></fragment>

Example: ListFragment Inside LinearLayout

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

android:orientation="vertical"

android:layout_width="match_parent"

android:layout_height="match_parent">

<fragment class="org.vkedco.mobappdev.rumi_quatrain_fragments.QuatrainNumberListFragment"

android:id="@+id/quatrain_numbers"

android:layout_width="match_parent"

android:layout_height="match_parent" />

</LinearLayout>

Fragments & ViewGroups

● Fragments are not required to be a part of the activity layout; there may be fragments w/o UIs as background workers for activities

● A Fragment should be designed as a modular and reusable activity component

● Why? Since fragments may define their own layouts and have their own lifecycle callbacks, one fragment may be included in multiple activities

● Direct manipulation of one fragment from another fragment should be avoided: breaks modularity and leads to cycle-related errors

Fragments on Tablets

Here is a useful link to the design and implementation practices of how to design fragments for reuse if you want to develop for multiple screen sizes:

http://developer.android.com/guide/practices/tablets-and-handsets.html

Fragment Management

Managing Fragments● Fragments are managed with the FragmentManager class

● FragmentManager object can be obtained via Activity. getFragmentManager()

● FragmentManager is used to:

– Find fragments in an activity either with findFragmentById() (for fragments that have IDs) or findFragmentByTag() (for fragments that do not have Uis)

– Pop fragments off the back stack, with popBackStack() (this is the same as the user's pressing Back button)

– Register a listener for changes to the back stack, with addOnBackStackChangedListener()

– open FragmentTransaction objects to add and remove fragments

Fragment Transactions

● FragmentTransactions are used to add, remove, replace, and perform other actions with them, in response to UI gestures

● A transaction is a set of changes that must be committed with respect to the activity

● It is also possible to save each transaction to a back stack managed by the activity

● The user can navigate navigate back through the fragment changes

Beginning & Ending Transactions

FragmentManager fragmentManager = getFragmentManager();

FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();

// some code

// each transaction must be committed to take effect

fragmentTransaction.commit();

A Typical Transaction Sequence

// Create new fragment and transaction

Fragment newFrgmnt = new SomeFragment();

FragmentTransaction trans = getFragmentManager().beginTransaction();

// 1) replace the contents of the view container with the newly created fragment,

// 2) add the transaction to the back stack

trans.replace(R.id.view_container, newFrgmnt);

trans.addToBackStack(null);

// Commit the transaction

transaction.commit();

Fragment Creation

Fragment Creation

● To create a fragment, the developer must either create a subclass of Fragment or a subclass of an existing subclass of Fragment

● The Fragment class has code that looks a lot like an Activity in that it contains callbacks such as onCreate(), onStart(), onPause(), & onStop()

● In many cases, converting existing activities to fragments is as simple as moving code from the activities' callback methods into the corresponding methods of fragments

Fragment Layouts

● To provide a layout for a fragment, the onCreateView() callback must be implemented

● Android system calls this method when the fragment is ready to draw its layout

● The implementation of this method must return the View root of the fragment's layout

● Fragment typically implement UI parts to the host activity

Fragment Layouts

● To provide a layout for a fragment, the onCreateView() callback must be implemented

● Android system calls this method when the fragment is ready to draw its layout

● The implementation of this method must return the View root of the fragment's layout

● Fragment typically implement UI parts to the host activity

Programmatic Addition of Fragments

// getFragmentManager() is a method of the Activity class

FragmentManager fragmentManager = getFragmentManager();

FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();

// Create a fragment and use the transaction to add the fragment to the view container

SomeFragment frgmnt = new SomeFragment();

fragmentTransaction.add(R.id.view_container, frgmnt);

fragmentTransaction.commit();

Order of Multiple Fragments

● A call to commit() must be last● When multiple fragments are added to the same

container, then the order in which they are added determines the order they appear in the view hierarchy

● If there is no call to addToBackStack() in a transaction removes a fragment, the removed fragment is destroyed after the transaction is committed and the user cannot navigate back to it

Finding Fragments w/in Activity

FragmentManager fm = getFragmentManager();

SomeFragment fragment =

(SomeFragment) fm.findFragmentById(R.id.some_fragment);

Finding Fragments w/in Activity

FragmentManager fm = getFragmentManager();

SomeFragment fragment =

(SomeFragment) fm.findFragmentById(R.id.some_fragment);

Three Important States

● Resumed: fragment is visible● Paused: another activity is in the foreground

and has focus.● Stopped: fragment is not visible

Activity vs Fragment Lifecycles

source image at http://developer.android.com/guide/components/fragments.html

Rumi's Quatrains with Fragments

source code is here

Application Class Contents

● RumiQuatrainMainActivity.java – main activity● QuatrainNumberListFragment.java – ListFragment

associated with main activity● QuatrainTextDisplayActivity.java – host activity for

QuatrainTextDisplayFragment.java● QuatrainTextDisplayFragment.java – fragment to

display quatrain texts

AndroidManifest.xml<application android:icon="@drawable/rumi" android:label="Rumi's Quatrains">

<activity android:name="org.vkedco.mobappdev.rumi_quatrain_fragments.RumiQuatrainMainActivity"

android:label="Rumi's Fragments">

<intent-filter>

<action android:name="android.intent.action.MAIN" />

<category android:name="android.intent.category.LAUNCHER" />

</intent-filter>

</activity>

<activity android:name="org.vkedco.mobappdev.rumi_quatrain_fragments.QuatrainTextDisplayActivity"

android:label="Quatrain's Text">

<intent-filter>

<action android:name="android.intent.action.VIEW" />

<category android:name="android.intent.category.DEFAULT" />

</intent-filter>

</activity>

</application>

XML Layout of the Main Activity: Main.xml

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

android:orientation="vertical"

android:layout_width="match_parent"

android:layout_height="match_parent">

<fragment class="org.vkedco.mobappdev.rumi_quatrain_fragments.QuatrainNumberListFragment"

android:id="@+id/quatrain_numbers"

android:layout_width="match_parent"

android:layout_height="match_parent" />

</LinearLayout>

XML Layout of the Secondary Activity: Main.xml

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

android:orientation="vertical"

android:layout_width="match_parent"

android:layout_height="match_parent">

<fragment class="org.vkedco.mobappdev.rumi_quatrain_fragments.QuatrainNumberListFragment"

android:id="@+id/quatrain_numbers"

android:layout_width="match_parent"

android:layout_height="match_parent" />

</LinearLayout>

XML Layout of the Secondary Activity: Main.xml

<LinearLayout>

<ScrollView android:id="@+id/scvQuatrainText"

android:layout_width="match_parent"

android:layout_height="match_parent">

<TextView

android:id="@+id/tvQuatrainText"

android:layout_width="match_parent"

android:layout_height="match_parent" />

</ScrollView>

</LinearLayout>

RumiQuatrainMainActivity.javapublic class RumiQuatrainMainActivity extends Activity {

static final String LOGTAG = RumiQuatrainMainActivity.class.getSimpleName() + "_TAG";

static Resources mRes = null;

static FragmentManager mFrgmntMngr = null;

static RumiQuatrainMainActivity mThisAct = null;

public void onCreate(Bundle savedInstanceState) {

Log.d(LOGTAG, "onCreate()");

super.onCreate(savedInstanceState);

FragmentManager.enableDebugLogging(true);

setContentView(R.layout.main);

mRes = getResources();

mFrgmntMngr = getFragmentManager();

mThisAct = this;

}

RumiQuatrainMainActivity.java

● The real workhorse of is displayQuatrainText(int quatrain_index)

● It finds a fragment by id and replaces it with a newly created fragment

● It also shows how to check the orientation● If the orientation is landscape, then two fragments are

displayed● If the orientation is portrait, then a new activity is

launched

References

● http://developer.android.com/guide/components/fragments.html

● S. Komatineni & D. MaClean. Pro Android 4. APRESS

● R. Meier. Pro Android 4 Application Development. WROX