ActionBar选项卡中的片段无法访问活动(Fragment in ActionBar Tab can't access activity)

 我在developer.android.com/guide/topics/ui/action.bar.html上按照指南在操作栏中实现了标签。 
 
 在我的主要活动中，我添加了三个标签： 
 
public class ToDoActivity extends Activity {
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        ActionBar actionBar = getActionBar();
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
        actionBar.setDisplayShowTitleEnabled(false);

        Tab tab = actionBar.newTab().setText("Task List")
            .setTabListener(new TabListener<TaskListFragment>(
                    this, "task list", TaskListFragment.class));
        actionBar.addTab(tab);

        tab = actionBar.newTab().setText("Pie Chart")
            .setTabListener(new TabListener<TaskPieChartFragment>(
                    this, "pie chart", TaskPieChartFragment.class));
        actionBar.addTab(tab);

        tab = actionBar.newTab().setText("Task Map")
            .setTabListener(new TabListener<TaskMapFragment>(
                    this, "task map", TaskMapFragment.class));
        actionBar.addTab(tab);
    }
}
 
 第一个标签的片段执行得很好。 它可以查找视图并访问活动。 单击导航中的第二个选项卡可显示片段（按预期），但不执行该片段的生命周期功能（onActivityCreated（），onStart（），onResume（）等）。 此外，每次从该片段调用getActivity（）都会返回一个null对象。 
 
public class TaskPieChartFragment extends Fragment {

    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        return inflater.inflate(R.layout.fragment_task_pie_chart, container, false);
    }

    public void updatePieChart() {
        TextView textView = (TextView) getActivity().findViewById(R.id.some_text);
        textView.setText("something something");
    }
}
 
 布局： 
 
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >
    <TextView
        android:id="@+id/some_text"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />
</LinearLayout>
 
 从TabListener中调用updatePieChart（）函数： 
 
public class TabListener<T extends Fragment> implements ActionBar.TabListener {

    public void onTabSelected(Tab tab, FragmentTransaction ft) {
        if (mFragment == null) {
            mFragment = Fragment.instantiate(mActivity,  mClass.getName());
            ft.add(android.R.id.content, mFragment, mTag);
        } else {
            ft.attach(mFragment);
        }
        try {
            TaskPieChartFragment pieChartFragment = (TaskPieChartFragment) mFragment;
            pieChartFragment.updatePieChart();
        } catch (ClassCastException e) {
        // not the fragment we were looking for
        }
    }

}
 
 每当我选择带有TaskPieChartFragment的选项卡时，我都会在updatePieChart（）的第一行得到一个NullPointerException。 为什么第二个片段与第一个片段的行为完全不同，我似乎无法获得对活动的引用？ 
 
 PS 
 
 我也试过传递TabListener中保存的活动。 然后NullPointerException不是源自updatePieChart（）中的第一行，而是第二行，因此活动无法找到该视图。 

I have followed the guide at developer.android.com/guide/topics/ui/action.bar.html on implementing tabs in the action bar.
 
In my main activity I add three tabs:
 
public class ToDoActivity extends Activity {
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        ActionBar actionBar = getActionBar();
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
        actionBar.setDisplayShowTitleEnabled(false);

        Tab tab = actionBar.newTab().setText("Task List")
            .setTabListener(new TabListener<TaskListFragment>(
                    this, "task list", TaskListFragment.class));
        actionBar.addTab(tab);

        tab = actionBar.newTab().setText("Pie Chart")
            .setTabListener(new TabListener<TaskPieChartFragment>(
                    this, "pie chart", TaskPieChartFragment.class));
        actionBar.addTab(tab);

        tab = actionBar.newTab().setText("Task Map")
            .setTabListener(new TabListener<TaskMapFragment>(
                    this, "task map", TaskMapFragment.class));
        actionBar.addTab(tab);
    }
}
 
The fragment of the first tab executes just fine. It can find views and access the activity. Clicking on the second tab in the navigation shows the fragment (as intended), but no life cycle functions of that fragment are executed (onActivityCreated(), onStart(), onResume(), etc.). Also every call to getActivity() from that fragment returns a null object.
 
public class TaskPieChartFragment extends Fragment {

    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        return inflater.inflate(R.layout.fragment_task_pie_chart, container, false);
    }

    public void updatePieChart() {
        TextView textView = (TextView) getActivity().findViewById(R.id.some_text);
        textView.setText("something something");
    }
}
 
The layout:
 
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >
    <TextView
        android:id="@+id/some_text"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />
</LinearLayout>
 
The updatePieChart() function is called from within the TabListener:
 
public class TabListener<T extends Fragment> implements ActionBar.TabListener {

    public void onTabSelected(Tab tab, FragmentTransaction ft) {
        if (mFragment == null) {
            mFragment = Fragment.instantiate(mActivity,  mClass.getName());
            ft.add(android.R.id.content, mFragment, mTag);
        } else {
            ft.attach(mFragment);
        }
        try {
            TaskPieChartFragment pieChartFragment = (TaskPieChartFragment) mFragment;
            pieChartFragment.updatePieChart();
        } catch (ClassCastException e) {
        // not the fragment we were looking for
        }
    }

}
 
Whenever I select the tab with the TaskPieChartFragment I get a NullPointerException originating at the first line in updatePieChart(). Why is it that this second fragment behaves completely different from the first one and I don't seem to be able to get a reference to the activity?
 
PS
 
I also tried passing on the activity saved in TabListener. Then the NullPointerException does not originate on the first line in updatePieChart(), but the second, so the activity can't find the view.

原文：https://stackoverflow.com/questions/22156195