java - findFragmentByTag 不起作用

Question

我有一个问题 findFragmentByTag 总是返回 null；所以我想解决这个问题。

我在支持 v4 上尝试了相同的操作，但这总是返回 null

主要 Activity :`

public class MainActivity extends Activity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    FragmentTransaction t = getFragmentManager().beginTransaction();
    ContentFragment temp = new ContentFragment();
    t.add(R.id.content, temp, "mine");

    // it does the same
    //t.add(R.id.content, new ContentFragment(), "mine");
    //t.commitAllowingStateLoss(); // from supprt v4
    t.commit();

    //
    ContentFragment fragment = (ContentFragment) getFragmentManager().findFragmentByTag("mine");
    if (fragment == null) Log.e("ALARM", "fragment == null");
    else fragment.callMe();
    }
}

内容 fragment :`

public class ContentFragment extends ListFragment {

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    ArrayAdapter<String> adapter = new ArrayAdapter<String>(
            inflater.getContext(),
            android.R.layout.simple_list_item_1,
            Environment.getRootDirectory().list());

    setListAdapter(adapter);

    return super.onCreateView(inflater, container, savedInstanceState);
}

public void callMe(){
    Toast.makeText(this.getActivity().getBaseContext(), this.getTag(), Toast.LENGTH_LONG).show();
    }
}`

main.xml

<FrameLayout
    android:id="@+id/content"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:android="http://schemas.android.com/apk/res/android"/>

跟踪

11-30 22:37:41.824  13491-13491/? E/Trace﹕ error opening trace file: No such file or directory (2)
11-30 22:37:55.271  13526-13526/? E/Trace﹕ error opening trace file: No such file or directory (2)
11-30 22:37:56.492  13569-13569/by.tq.filemanager.app E/Trace﹕ error opening trace file: No such file or directory (2)
11-30 22:37:58.361  13569-13569/by.tq.filemanager.app E/ERR﹕ java.lang.NullPointerException
        at by.tq.filemanager.app.MainActivity.onCreate(MainActivity.java:30)
        at android.app.Activity.performCreate(Activity.java:5122)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1146)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2315)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2403)
        at android.app.ActivityThread.access$600(ActivityThread.java:165)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1373)
        at android.os.Handler.dispatchMessage(Handler.java:107)
        at android.os.Looper.loop(Looper.java:194)
        at android.app.ActivityThread.main(ActivityThread.java:5370)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:525)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:833)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600)
        at dalvik.system.NativeStart.main(Native Method)

Answer 1

是的，拉里是对的—— fragment 不会立即添加。如果您在将 fragment 附加到 Activity 后还有工作要做，一个不错的选择是重写 onAttach:

public class ContentFragment extends ListFragment {
    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        Toast.makeText(this.getActivity().getBaseContext(), this.getTag(), Toast.LENGTH_LONG).show();
    }
}

编辑

您稍后可以使用 findFragmentByTag 从 Activity 中检索 fragment :

getFragmentManager().findFragmentByTag("mine");

主要问题是您无法在将 fragment 添加到 Activity 后立即获取 fragment 。何时访问 fragment 的问题实际上取决于您需要在生命周期中的何时执行该工作。如果您正在响应用户输入，那么您当然可以在那时调用 findFragmentByTag 。如果是fragment的初始化，可以使用onAttach或onCreateView。您甚至可以从 onAttach 回调您的 Activity，因此 fragment 告诉 Activity 它已准备好初始化。

这有道理吗？如果您告诉我更多细节，我可以为您提供更好的建议。