Handling bundles in activities and fragments


Bundle is a useful data holder, which maps String values to various

Parcelable types. So basically it is a heterogenous key/value map. Bundles are used in

Intents,
Activities and
Fragments for transporting data. I would like to describe how I work with Bundles on Android and show you some good tips.

Activity

When you are creating a new instance of Activity via Intent, you can pass some extra data to the Activity. Data are stored in Bundle and can be retrieved by calling

getIntent().getExtras()

. It's a good practise to implement static method

newIntent()

which returns a new Intent that can be used to start the Activity. This way you have compile time checking for the arguments passed to the Activity. This pattern is suitable for Service and Broadcast as well.

Bundle is also used if the Activity is being re-initialized (e.g. because of configuration changes) for keeping the current state of the instance. You can supply some data in

onSaveInstanceState(Bundle)

and retrieve them back in

onCreate(Bundle)

method or

onRestoreInstanceState(Bundle)

. Main difference between these methods is that

onRestoreInstanceState(Bundle)

is called after

onStart()

. Sometimes it's convenient to restore data here after all of the initialization has been done. Another purpose could be allowing subclasses to decide whether to use your default
implementation.

See example code below. Note that

EXTRA_PRODUCT_ID

constant is public. That's because we could need it in a Fragment encapsulated in this Activity.

public class ExampleActivity extends Activity
{
public static final String EXTRA_PRODUCT_ID = "product_id";
public static final String EXTRA_PRODUCT_TITLE = "product_title";

private static final String SAVED_PAGER_POSITION = "pager_position";

public static Intent newIntent(Context context, String productId, String productTitle)
{
Intent intent = new Intent(context, ExampleActivity.class);

// extras
intent.putExtra(EXTRA_PRODUCT_ID, productId);
intent.putExtra(EXTRA_PRODUCT_TITLE, productTitle);

return intent;
}

@Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_example);

// restore saved state
if(savedInstanceState != null)
{
handleSavedInstanceState(savedInstanceState);
}

// handle intent extras
Bundle extras = getIntent().getExtras();
if(extras != null)
{
handleExtras(extras);
}
}

@Override
public void onSaveInstanceState(Bundle outState)
{
// save current instance state
super.onSaveInstanceState(outState);

// TODO
}

@Override
public void onRestoreInstanceState(Bundle savedInstanceState)
{
// restore saved state
super.onRestoreInstanceState(savedInstanceState);

if(savedInstanceState != null)
{
// TODO
}
}

private void handleSavedInstanceState(Bundle savedInstanceState)
{
// TODO
}

private void handleExtras(Bundle extras)
{
// TODO
}
}

Fragment

When you are creating a new instance of Fragment, you can pass arguments through

setArguments(Bundle)

method. Data can be retrieved with

getArguments()

method. It would be a mistake to supply initialization data through an overloaded constructor. Fragment instance can be re-created (e.g. because of configuration
changes) so you would lose data, because constructor with extra parameters is not called when re-initializing Fragment. Only empty constructor is called. Best way to solve this problem is implementing static creator method

newInstance()

which returns a new instance of Fragment and sets the arguments via

setArguments(Bundle)

.

Fragment state can be saved using

onSaveInstanceState(Bundle)

method. It is similar to the Activity. Data can be restored in

onCreate(Bundle)

,

onCreateView(LayoutInflater, ViewGroup, Bundle)

,

onActivityCreated(Bundle)

or

onViewStateRestored(Bundle)

methods.

Fragment has also access to the Intent extras which were passed during creating the Activity instance. You can get this extra data by calling

getActivity().getIntent().getExtras()

.

See example code below.

public class ExampleFragment extends Fragment
{
private static final String ARGUMENT_PRODUCT_ID = "product_id";

private static final String SAVED_LIST_POSITION = "list_position";

public static ExampleFragment newInstance(String productId)
{
ExampleFragment fragment = new ExampleFragment();

// arguments
Bundle arguments = new Bundle();
arguments.putString(ARGUMENT_PRODUCT_ID, productId);
fragment.setArguments(arguments);

return fragment;
}

public ExampleFragment() {}

@Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);

// handle fragment arguments
Bundle arguments = getArguments();
if(arguments != null)
{
handleArguments(arguments);
}

// restore saved state
if(savedInstanceState != null)
{
handleSavedInstanceState(savedInstanceState);
}

// handle intent extras
Bundle extras = getActivity().getIntent().getExtras();
if(extras != null)
{
handleExtras(extras);
}
}

@Override
public void onSaveInstanceState(Bundle outState)
{
// save current instance state
super.onSaveInstanceState(outState);

// TODO
}

private void handleArguments(Bundle arguments)
{
// TODO
}

private void handleSavedInstanceState(Bundle savedInstanceState)
{
// TODO
}

private void handleExtras(Bundle extras)
{
// TODO
}
}

You can find example code also on my GitHub in
Android Templates and Utilities repo. This blogpost was inspired by
Nick Butcher's post on Google Plus. Gotta some questions or ideas about Bundles? Follow me on
Twitter or
Google Plus.