How to get current selected Fragment in FragmentTabHost

Question

There are some questions related to this same issue. For example, this one. But it doesn't work.

Let me show what I did in my code.

activity_main.xml

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools" android:id="@+id/container"
    android:layout_width="match_parent" android:layout_height="match_parent"
    tools:context=".MainActivity" tools:ignore="MergeRootFrame" >

    <fragment
        android:id="@+id/my_fragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:name="com.example.zhouhao.test.MyFragment"
        tools:layout="@layout/fragment_my" />

</FrameLayout>

fragment_my.xml (my main fragment which include a FragmentTabHost)

<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.app.FragmentTabHost xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@android:id/tabhost"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:weightSum="100"
        android:orientation="horizontal">

        <FrameLayout
            android:id="@android:id/tabcontent"
            android:layout_width="0dp"
            android:layout_height="0dp"
            android:layout_weight="0" />

        <LinearLayout
            android:layout_width="0dp"
            android:layout_weight="95"
            android:layout_height="match_parent"
            android:orientation="vertical">

            <FrameLayout
                android:id="@+id/panel_content"
                android:layout_width="match_parent"
                android:layout_height="0dp"
                android:layout_weight="1" />
        </LinearLayout>
    </LinearLayout>
</android.support.v4.app.FragmentTabHost>

fragment_tab1.xml (for the fragment corresponding to different tab, they are similar so that I only show you one code)

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
    android:layout_height="match_parent" tools:context="com.example.zhouhao.test.TabFragment">

    <!-- TODO: Update blank fragment layout -->
    <TextView android:layout_width="match_parent" android:layout_height="match_parent"
        android:text="@string/fragment_tab1" />

</FrameLayout>

MainActivity.java

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    if (savedInstanceState == null) {
        FragmentManager fm = getSupportFragmentManager();
        mMyFragment = (MyFragment) fm.findFragmentById(R.id.my_fragment);
    }
}

public class MyFragment extends Fragment implements TabHost.OnTabChangeListener {

FragmentTabHost mFragmentTabHost;


public MyFragment() {
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    View rootView = inflater.inflate(R.layout.fragment_my, container, false);
    if (rootView != null) {
        mFragmentTabHost = (FragmentTabHost) rootView.findViewById(android.R.id.tabhost);
        mFragmentTabHost.setup(getActivity(), getActivity().getSupportFragmentManager(), R.id.panel_content);
        TabFragment1 tab1Fragment = new TabFragment1();
        TabFragment2 tab2Fragment = new TabFragment2();
        TabFragment3 tab3Fragment = new TabFragment3();
        TabHost.TabSpec spec1 = mFragmentTabHost.newTabSpec("1").setIndicator("");
        TabHost.TabSpec spec2 = mFragmentTabHost.newTabSpec("2").setIndicator("");
        TabHost.TabSpec spec3 = mFragmentTabHost.newTabSpec("3").setIndicator("");
        mFragmentTabHost.addTab(spec1,tab1Fragment.getClass(), null);
        mFragmentTabHost.addTab(spec2,tab2Fragment.getClass(), null);
        mFragmentTabHost.addTab(spec3,tab3Fragment.getClass(), null);
        mFragmentTabHost.setOnTabChangedListener(this);
        return rootView;
    } else {
        return super.onCreateView(inflater, container, savedInstanceState);
    }
}

@Override
public void onTabChanged(String tabId) {

    BaseFragment f = (BaseFragment)getActivity().getSupportFragmentManager().findFragmentByTag(tabId);
    Log.d("Log",tabId);
}

}

My problem is the BaseFragment is always null in my onTabChanged. Can anybody help? Thanks.

Answer 1

I think I get the right way to solve this problem, below is my answer.

1. Create a TabBean

private int imgId;    
private int textId;    
private Class fragment;   
public MainTabBean(int textId, int imgId, Class fragment) {    
    this.textId = textId;    
    this.imgId = imgId;    
    this.fragment = fragment;    
}

public int getTextId() {
    return textId;
}

public void setTextId(int textId) {
    this.textId = textId;
}

public int getImgId() {
    return imgId;
}

public void setImgId(int imgId) {
    this.imgId = imgId;
}

public Class getFragment() {
    return fragment;
}

public void setFragment(Class fragment) {
    this.fragment = fragment;
}

2. Then, get three TabBean instances with three fragments

List<MainTabBean> tabs = new ArrayList<>();
// three TabBean instances
TabBean homeTab = new MainTabBean(R.string.home, R.drawable.tab_home, HomepageFragment.class);
TabBean postcardTab = new MainTabBean(R.string.postcard, R.drawable.tab_postcard, PostcardFragment.class);
Bean notificationTab = new MainTabBean(R.string.notification, R.drawable.tab_notification, NotificationFragment.class);
// put tabs into a list
tabs.add(homeTab);
tabs.add(postcardTab);
tabs.add(notificationTab);

3. Third, just associate the TabBeans with FragmentTabHost as usual

// Obtain FragmentTabHost
tabHost = findViewById(android.R.id.tabhost);
// Associate fragmentManager with container
tabHost.setup(this, getSupportFragmentManager(), R.id.main_content);
// Create TabSpecs with TabBean and add it on TabHost
for(MainTabBean tab : tabs) {
    TabHost.TabSpec spec = tabHost.newTabSpec(String.valueOf(tab.getTextId()))// set a tag
                                .setIndicator(getTabIndicator(tab));// set an indicator
    tabHost.addTab(spec, tab.getFragment(), null);// add a tab
}

4. Forth, that's the most important step, I didn't get fragment instance with findFragmentByTag() like this

getSupportFragmentManager()
                .findFragmentByTag(String.valueOf(tabs.get(1).getTextId()));

Instead, my answer is exactly like this:

tabs.get(0).getFragment().newInstance();

and cast it.

And I finally got the fragment.

Answer 2

You cannot get the selected fragment immediately if the fragment has never been instantiated.

@Override
public void onTabChanged(final String tabId) {
    Fragment fg = getSupportFragmentManager().findFragmentByTag(tabId);
    Log.d(TAG, "onTabChanged(): " + tabId + ", fragment " + fg);

    if (fg == null) {
        new Handler().postDelayed(new Runnable() {            
            @Override
            public void run() {
                Fragment fg = getSupportFragmentManager().findFragmentByTag(tabId);
                Log.d(TAG, "onTabChanged() delay 50ms: " + tabId + ", fragment " + fg);              
            }
        }, 50);
    }
}

the output:

// cannot get the selected fragment immediately if the fragment has never been instantiated.
onTabChanged(): 1, fragment null
onTabChanged() delay 50ms: 1, fragment HistoryFragment{6f7a9d5 #2 id=0x7f09006e 1}
onTabChanged(): 2, fragment null
onTabChanged() delay 50ms: 2, fragment HistoryFragment{10c59e72 #3 id=0x7f09006e 2}

// can get the selected fragment immediately if the fragment already instantiated.
onTabChanged(): 1, fragment HistoryFragment{6f7a9d5 #2 id=0x7f09006e 1}
onTabChanged(): 2, fragment HistoryFragment{10c59e72 #3 id=0x7f09006e 2}