When designing your application to support a wide range of screen sizes, you can reuse your fragments in different layout configurations to optimize the user experience based on the available screen space.
For example, on a handset device it might be appropriate to display just one fragment at a time for a single-pane user interface. Conversely, you may want to set fragments side-by-side on a tablet which has a wider screen size to display more information to the user.
Figure 1. Two fragments, displayed in different configurations for the same activity on different screen sizes. On a large screen, both fragments fit side by side, but on a handset device, only one fragment fits at a time so the fragments must replace each other as the user navigates.
The {@link android.support.v4.app.FragmentManager} class provides methods that allow you to add, remove, and replace fragments to an activity at runtime in order to create a dynamic experience.
Rather than defining the fragments for an activity in the layout file—as shown in the
previous lesson with the {@code
To perform a transaction such as add or remove a fragment, you must use the {@link android.support.v4.app.FragmentManager} to create a {@link android.support.v4.app.FragmentTransaction}, which provides APIs to add, remove, replace, and perform other fragment transactions.
If your activity allows the fragments to be removed and replaced, you should add the initial fragment(s) to the activity during the activity's {@link android.app.Activity#onCreate onCreate()} method.
An important rule when dealing with fragments—especially when adding fragments at runtime—is that your activity layout must include a container {@link android.view.View} in which you can insert the fragment.
The following layout is an alternative to the layout shown in the previous lesson that shows only one fragment at a time. In order to replace one fragment with another, the activity's layout includes an empty {@link android.widget.FrameLayout} that acts as the fragment container.
Notice that the filename is the same as the layout file in the previous lesson, but the layout
directory does not have the large qualifier, so this layout is used when the
device screen is smaller than large because the screen does not fit both fragments at
the same time.
res/layout/news_articles.xml:
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/fragment_container"
android:layout_width="match_parent"
android:layout_height="match_parent" />
Inside your activity, call {@link android.support.v4.app.FragmentActivity#getSupportFragmentManager()} to get a {@link android.support.v4.app.FragmentManager} using the Support Library APIs. Then call {@link android.support.v4.app.FragmentManager#beginTransaction} to create a {@link android.support.v4.app.FragmentTransaction} and call {@link android.support.v4.app.FragmentTransaction#add add()} to add a fragment.
You can perform multiple fragment transaction for the activity using the same {@link android.support.v4.app.FragmentTransaction}. When you're ready to make the changes, you must call {@link android.support.v4.app.FragmentTransaction#commit()}.
For example, here's how to add a fragment to the previous layout:
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
public class MainActivity extends FragmentActivity {
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.news_articles);
// Check that the activity is using the layout version with
// the fragment_container FrameLayout
if (findViewById(R.id.fragment_container) != null) {
// However, if we're being restored from a previous state,
// then we don't need to do anything and should return or else
// we could end up with overlapping fragments.
if (savedInstanceState != null) {
return;
}
// Create a new Fragment to be placed in the activity layout
HeadlinesFragment firstFragment = new HeadlinesFragment();
// In case this activity was started with special instructions from an
// Intent, pass the Intent's extras to the fragment as arguments
firstFragment.setArguments(getIntent().getExtras());
// Add the fragment to the 'fragment_container' FrameLayout
getSupportFragmentManager().beginTransaction()
.add(R.id.fragment_container, firstFragment).commit();
}
}
}
Because the fragment has been added to the {@link android.widget.FrameLayout} container at
runtime—instead of defining it in the activity's layout with a {@code
The procedure to replace a fragment is similar to adding one, but requires the {@link android.support.v4.app.FragmentTransaction#replace replace()} method instead of {@link android.support.v4.app.FragmentTransaction#add add()}.
Keep in mind that when you perform fragment transactions, such as replace or remove one, it's often appropriate to allow the user to navigate backward and "undo" the change. To allow the user to navigate backward through the fragment transactions, you must call {@link android.support.v4.app.FragmentTransaction#addToBackStack addToBackStack()} before you commit the {@link android.support.v4.app.FragmentTransaction}.
Note: When you remove or replace a fragment and add the transaction to the back stack, the fragment that is removed is stopped (not destroyed). If the user navigates back to restore the fragment, it restarts. If you do not add the transaction to the back stack, then the fragment is destroyed when removed or replaced.
Example of replacing one fragment with another:
// Create fragment and give it an argument specifying the article it should show ArticleFragment newFragment = new ArticleFragment(); Bundle args = new Bundle(); args.putInt(ArticleFragment.ARG_POSITION, position); newFragment.setArguments(args); FragmentTransaction transaction = getSupportFragmentManager().beginTransaction(); // Replace whatever is in the fragment_container view with this fragment, // and add the transaction to the back stack so the user can navigate back transaction.replace(R.id.fragment_container, newFragment); transaction.addToBackStack(null); // Commit the transaction transaction.commit();
The {@link android.support.v4.app.FragmentTransaction#addToBackStack addToBackStack()} method takes an optional string parameter that specifies a unique name for the transaction. The name isn't needed unless you plan to perform advanced fragment operations using the {@link android.support.v4.app.FragmentManager.BackStackEntry} APIs.