java - Android: fragment findViewById 内的选项卡返回 null？

Question

我有一个包含 fragment 的 Activity 。该 fragment 内部有一个按钮，单击该按钮时，该按钮应该会导致该 fragment 被内部带有选项卡(也包含 fragment )的 fragment 替换。

但是，当加载选项卡 fragment 并且我尝试通过 id 查找其 viewpager 时，findViewById() 返回 null。

这是我的代码:在带有按钮的 fragment 内:

@Override
public void onClick(View view) {

    switch (view.getId()) {
        case R.id.gewichtAktualiserenButton:
            //TODO implement tabs
            // replace current fragment with GewichtTabsFragment
            MainActivity mainActivity = (MainActivity)getActivity();
            GewichtTabsFragment gewichtTabsFragment = new GewichtTabsFragment();
            mainActivity.replaceFragment(gewichtTabsFragment, Constants.GEWICHT_TABS_FRAGMENT);
            break;
        case R.id.geburtsdatumEditText:
            showDatePickerDialog(view);
           break;
        case R.id.hundImageView:
            // display alert to choose from camera or gallery
            ImageSourceChooserDialog newFragment = new ImageSourceChooserDialog();
            newFragment.setHundFragment(this);
            newFragment.setWithDeleteItem(true);
            imgChooserDialog = newFragment;
            newFragment.show(getActivity().getSupportFragmentManager(), "image_source_chooser");
            break;
        case R.id.zusaetzeSwitch:
            // toggle enabled state of zusaetze EditText
            getZusaetzeEditText().setEnabled(!getZusaetzeEditText().isEnabled());
        default:
            break;
    }

GewichtTabsFragment.java:

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    View view = inflater.inflate(R.layout.fragment_gewicht_tabs, container, false);

    MainActivity mainActivity = (MainActivity)getActivity();
    // Get the ViewPager and set it's PagerAdapter so that it can display items
    // the following call returns null? Why?
    ViewPager viewPager = (ViewPager) view.findViewById(R.id.viewpager);
    viewPager.setAdapter(new GewichtFragmentPagerAdapter(mainActivity.getSupportFragmentManager()));

    // Give the PagerSlidingTabStrip the ViewPager
    PagerSlidingTabStrip tabsStrip = (PagerSlidingTabStrip) getActivity().findViewById(R.id.tabs);
    // Attach the view pager to the tab strip
    tabsStrip.setViewPager(viewPager);

    return view;
}

以及布局文件fragment_gewicht_tabs.xml:

<?xml version="1.0" encoding="utf-8"?>

<com.astuetz.PagerSlidingTabStrip
    android:id="@+id/tabs"
    app:pstsShouldExpand="true"
    app:pstsTextAllCaps="true"
    android:layout_width="match_parent"
    android:layout_height="48dp">
</com.astuetz.PagerSlidingTabStrip>

<android.support.v4.view.ViewPager
    android:id="@+id/viewpager"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:background="@android:color/white" />

</LinearLayout>

为什么findViewById返回null？

Answer 1

它将为空，因为您正在尝试获取尚未创建的 View 。因此请更改

ViewPager viewPager = (ViewPager) view.findViewById(R.id.viewpager);

至

ViewPager viewPager = (ViewPager) getActivity().findViewById(R.id.viewpager);