What Is A Fragment
Simply put, a Fragment is a way to encapsulate a piece of your applications UI and UI interactions into a reusable set of resources and code. A Fragment is not tied to any particular Activity, but instead can be used within many Activities.
One example of where I've used this modularity in my applications is with Lists. Lists are common in mobile applications and often only differ by what they show. Using Fragments it would be easy to encapsulate creating, displaying, and interaction with a list into a set of reusable code and resources. You could then display multiple lists in a variety of places throughout your app using the same code and resources.
Fragment Life-cycle
The Fragment life-cycle is very similar to the Activity life-cycle we've already gone through in this series. You still have creating, starting, resuming, pausing, stopping, and destroying life-cycle events. But in addition to those you have life-cycle events that are associated with creating your Fragments view and attaching/detaching from an Activity. I'm not going to go through every Fragment life-cycle method but instead will call out two key life-cycle differences from Activities.
The first big difference is in creating the Fragments view. In an Activity this is done via the onCreate method. In a Fragment this is done in the onCreateView method. The onCreateView method is expected to return the view to use with this Fragment. Creating this view is pretty simple, just inflate your Fragments layout using the LayoutInflator passes into this method.
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
{
return inflater.inflate(R.layout.my_first_fragment_layout, container, false);
}
The next difference comes with the onActivityCreated method. The main purpose of this method is to allow the Fragment to restore state. There are two places that state can be stored. The first is internally within the Fragment via a Bundle. The second is in any arguments that were passed into the Fragment by it's composing Activity.
The internal saved state is passed into the onActivityCreated method in the form of a Bundle. The state that was passed in via arguments can be retrieved via a call to the getArguments method. It's important to restore Fragment state using the correct source of information. For instance, if you get the initial state from the Arguments but then update that state and save it in your Fragments Bundle then it's important to have some logic that determines the correct place to get the saved state from.
@Override
public void onActivityCreated(Bundle savedInstanceState)
{
super.onActivityCreated(savedInstanceState);
Bundle arguments = this.getArguments();
if(arguments != null && arguments.size() > 0)
{
// set any state that was passed in
// via the arguements bundle
this.someVariable = arguments.getString("SomeVariable");
}
}
Creating Fragments Options Menu
Creating menu items for your Fragment is a four step process. The fist step is declaring your Fragments options menu in an XML file. The second step is declaring that your Fragment has an options menu. The third step is inflating your options menu layout. The last step is handling users selecting an option in your Fragments menu.
First, create res/menu/my_first_fragment_menu.xml. The important thing to call out here is that each menu item needs to have an id. This id is important because there is one method that is called when the user selects a menu item. So we need a way to differentiate the desired action the user wishes to perform.
Declaring that your Fragment has an options menu is done via a call to setHasOptionsMenu. This call should be made in the Fragments default constructor.
public MyFirstAndroidFragment()
{
setHasOptionsMenu(true);
}
Inflating your Fragments options menu is done by overriding the onCreateOptionsMenu method. This is done by passing the MenuInflater's inflate method the id of the menu XML file you created. If you want to use an ActionProvider in your Fragment, like the ShareActionProvider, this is the right time to set that provider up.
@Override
public void onCreateOptionsMenu(Menu menu, MenuInflater inflater)
{
inflater.inflate(R.menu.my_first_fragment_menu, menu);
// this is a good time to setup a share action provider
}
Finally, handling the selection of a menu option is done by overriding the onOptionsItemSelected method. This method is called when any menu item is selected. It's a good idea to encapsulate the menu item action into it's own method and just call that method when the item has been selected. It's important to remember to return true in the onOptionsItemSelected method if you did handle the menu item selection.
@Override
public boolean onOptionsItemSelected(MenuItem item)
{
if (item.getItemId() == R.id.my_menu_item)
{
this.handleMyMenuItem();
return true;
}
return false;
}
Fragment Tips
Retaining
One thing that often causes people to stumble is putting a video, web browser, or any other stateful object inside a Fragment. The reason is that when the device changes orientation the Activity (and it's child Fragments) are torn down and recreated. This causes problems when the user isn't expecting it. One way to solve this problem is to tell Android to retain the Fragment instance across Activity re-creation. This is done via a call to setRetainInstance. This call should be made in the Fragments default constructor.
public MyFirstAndroidFragment()
{
setRetainInstance(true);
}
Cross Fragment Coordination
Cross Fragment coordination is done by declaring an interface in your Fragment for any events you want to allow others Fragments to respond to. The Activities that compose your fragment will implement your Fragments interface and can then dispatch messages to other Fragments that it is composing. This allows you to keep your concerns separated correctly by NOT tightly coupling your Fragment with any other Fragment. It's okay to tightly couple your Activity with your Fragment because your Activity is composing that Fragment.
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