Prerequisite: You are already aware of the basics of building a HelloWorld in Android and know how to use the APIs provided in the support library.
The code example is available on github.
Ever wanted a code snippet from an Activity to be available to other activities? Perhaps a Button or a ListView, maybe a Layout or any View/ViewGroup for that matter? Fragments let us do just that.
Necessity is the mother of invention.
Before understanding what Fragments are and how they work, we must first realize their existence in the first place.
FirstActivity contains two Views, a
TextView (textView) and a
Button (button1); and button1 has an
onClick() callback that
Toast‘s a simple message “Button pressed”.
SecondActivity contains both the Views present in FirstActivity and a
Now we want to utilize the two layout components(Views) of FirstActivity in SecondActivity, we can go about this with two approaches:
- Copy and Paste the xml elements of the two Views.
- Create a separate layout for common Views and reuse it using
<include />layout element.More about this here.
Electing the second approach makes sense cause it enables us to make reusable layouts. Everything seems great till now. We are able to make reusable layouts and use them as many times as we want.
Now recollect that we have an
onClick() callback assigned to button1. How do we reuse the same callback functionality of button1 across multiple Activities?
<include /> lets us reuse layouts and not the Activity source.
This is where Fragments come into play.
Fragments encompass both layout resource and Java source. Hence, unlike
<include />, they allow us to reuse the View components along with their functionality, if needed.
Fragments were first introduced in Honeycomb(API 11), living under the
Note: API 11 implies that Fragments have no support for devices less than Honeycomb and, for the record, as of writing this post, more than 50% of Android devices worldwide run versions of Android below Honeycomb. Developer dissapointed? You don’t have to be, cause google has been cautious enough to add the Fragment APIs to the support library. Yay!
In the support library Fragment APIs sit in the
android.support.v4.app package. This post assumes that your
minSdk support is below API 11. Hence we concentrate on the Fragment APIs of the support library.
Diving into code
Performing code reuse with Fragments involves three major steps:
- Creating reusable View components – Creating a layout for the fragment.
- Creating reusable Java source – Writing the layout’s corresponding Fragment class.
- Employing the reusable components in Activity – Making an Activity to host this Fragment.
1. Creating reusable View components
Creating a layout for the Fragment
This is done precisely as we do it for our activity layouts. The layout contains a root element (ViewGroup) defining the layout, For instance in our example we use a LinearLayout and its child elements(the reusable Views) that we want to have in our fragment.
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:orientation="vertical" android:layout_width="fill_parent" android:layout_height="fill_parent">
2. Creating reusable Java source
Writing the layout’s corresponding Fragment class
This class will inherit
Fragment class and must override the
In this method we inflate the fragment layout using the following line of code.
View view = inflater.inflate(R.layout.fragment_common, container, false);
container is the parent ViewGroup that the fragment’s UI should be attached to.
Once inflation is done, we can perform various operations on the component views of the fragment.
Accessing the view elements from the layout is done exactly as we do in an Activity (using
findViewById()) except that we use the
View for the fragment’s UI or an instance of the host Activity.
In Activity we access Views(a Button for our example) as follows
Button button1 = (Button) findViewById(R.id.button1);
In Fragment, we need to use the inflated view(if in
onCreateView()) or get the instance of the host activity and access the views through this instance(when in a lifecycle callback after
onCreateView(), I generally do it in
Button button1 = (Button) view.findViewById(R.id.button1);
Button button1 = (Button) getActivity().findViewById(R.id.button1);
getActivity() returns the instance of the Activity that is hosting this Fragment.
onCreateView() we must return the View for the fragment’s UI.
3. Employing the reusable components in Activity
Making an Activity to host this Fragment
This is done in two ways, statically by adding
<fragment /> elements into the Activity layout or dynamically, at run time, by using
Note: First thing we need to ensure is that our Activity extends
FragmentActivity class instead of the regular
minSdk is API 11 or higher, then we can leave our inheritance to
Activity class and not bother about
a. Static approach to hosting the Fragments
<fragment /> element in activity layout
Inorder to statically add a Fragment into your Activity, just add
<fragment /> element with the necessary layout attributes and the
android:name attribute set to the fully qualified class name of the corresponding Fragment.
b. Dynamic approach
Can be done using
FragmentTransaction classes. We call
add(), in our FragmentActivity implementation, on an instance of
FragmentTransaction to add a fragment to the host Activity. But that is not enough to show the fragment on the screen, i.e. the FragmentTransaction is not complete. We must call
commit() to finish the transaction.
CommonFragment fragment = new CommonFragment(); FragmentManager manager = getSupportFragmentManager(); FragmentTransaction transaction = manager.beginTransaction(); transaction.add(R.id.dynamicFragmentLayout, fragment); transaction.commit();
Similarly Fragments can be removed(
remove()) as well as replaced(
replace()) from the activity all at runtime.
Congratulations, now you can use Fragments to write reusable code and easily host them over multiple activities.
Till next time.