10Building a Flexible UI

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.

Add a Fragment to an Activity at Runtime

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Rather than defining the fragments for an activity in the layout file—as shown in the previous lesson with the<fragment> element—you can add a fragment to the activity during the activity runtime. This is necessary if you plan to change fragments during the life of the activity.

To perform a transaction such as add or remove a fragment, you must use the FragmentManager to create aFragmentTransaction, 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 onCreate() method.

An important rule when dealing with fragments—especially when adding fragments at runtime—is that your activity layout must include a container 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 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 nothave 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" />

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();        }    }}

Replace One Fragment with Another

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The procedure to replace a fragment is similar to adding one, but requires the replace() method instead ofadd().

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 addToBackStack() before you commit theFragmentTransaction.

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 showArticleFragment 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 backtransaction.replace(R.id.fragment_container, newFragment);transaction.addToBackStack(null);// Commit the transactiontransaction.commit();

The 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 theFragmentManager.BackStackEntry APIs.