android - 嵌套 fragment 通信

Question

我目前在嵌套 fragment 时遇到问题，我创建了一个带有两个选项卡的 SlidingTabLayout，每个选项卡都有相应的 fragment ，在一个选项卡中我向父 fragment 添加了 5 个子 fragment FragmentPagerAdapter 返回的 fragment ，但我在处理父 fragment 和它的 fragment 子 fragment 之间的通信时遇到了问题，

@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);

            FragmentTransaction childManager=getChildFragmentManager().beginTransaction();
            childManager.add(R.id.linear08,new YearFragment08(),"year08");
            childManager.commit();
            YearFragment08 year08=(YearFragment08)getChildFragmentManager().findFragmentByTag("year08");
            year08.setinfo("This is fantastic");


}

`

public class YearFragment08 extends Fragment {
TextView yearInfo;
@Nullable
@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    View v=inflater.inflate(R.layout.year_frag08,container,false);
    yearInfo=(TextView)v.findViewById(R.id.txt_yearInfo_08);
    return v;
}
 public void setinfo(String data){
    yearInfo.setText(data);
}

这就是结果。

02-09 21:45:48.078    2177-2177/com.compassasu.compass E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.compassasu.compass, PID: 2177
java.lang.NullPointerException: Attempt to invoke virtual method 'void com.compassasu.compass.Fragments.YearFragment08.setinfo(java.lang.String)' on a null object reference
        at com.compassasu.compass.Fragments.HistoryFragment.onActivityCreated(HistoryFragment.java:33)
        at android.support.v4.app.Fragment.performActivityCreated(Fragment.java:1970)
        at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1051)
        at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1207)
        at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:738)
        at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1572)
        at android.support.v4.app.FragmentManagerImpl.executePendingTransactions(FragmentManager.java:545)
        at android.support.v4.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:141)
        at android.support.v4.view.ViewPager.populate(ViewPager.java:1106)
        at android.support.v4.view.ViewPager.populate(ViewPager.java:952)
        at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1474)
        at android.view.View.measure(View.java:18788)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
        at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1465)
        at android.widget.LinearLayout.measureVertical(LinearLayout.java:748)
        at android.widget.LinearLayout.onMeasure(LinearLayout.java:630)
        at android.view.View.measure(View.java:18788)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
        at android.support.v7.internal.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:124)
        at android.view.View.measure(View.java:18788)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
        at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1465)
        at android.widget.LinearLayout.measureVertical(LinearLayout.java:748)
        at android.widget.LinearLayout.onMeasure(LinearLayout.java:630)
        at android.view.View.measure(View.java:18788)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
        at android.view.View.measure(View.java:18788)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
        at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1465)
        at android.widget.LinearLayout.measureVertical(LinearLayout.java:748)
        at android.widget.LinearLayout.onMeasure(LinearLayout.java:630)
        at android.view.View.measure(View.java:18788)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
        at com.android.internal.policy.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2643)
        at android.view.View.measure(View.java:18788)
        at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2100)
        at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1216)
        at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1452)
        at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1107)
        at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6013)
        at android.view.Choreographer$CallbackRecord.run(Choreographer.java:858)
        at android.view.Choreographer.doCallbacks(Choreographer.java:670)
        at android.view.Choreographer.doFrame(Choreographer.java:606)
        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:844)
        at android.os.Handler.handleCallback(Handler.java:739)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:148)
        at android.app.ActivityThread.main(ActivityThread.java:5417)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

我只想知道子 fragment 如何从父 fragment 接收数据。

Answer 1

问题是您试图在实际添加 fragment 之前通过其标签找到该 fragment 。注意 documentation对于 FragmentTransaction.commit() :

Schedules a commit of this transaction. The commit does not happen immediately; it will be scheduled as work on the main thread to be done the next time that thread is ready.

如您所见， fragment 并未立即添加，这就是 null 的原因当您尝试获取 Fragment 时返回.

要解决此问题，您可以使用 FragmentManager.executePendingTransactions()提交事务后立即添加 fragment :

getChildFragmentManager().beginTransaction()
        .add(R.id.linear08,new YearFragment08(),"year08")
        .commit();

getChildFragmentManager().executePendingTransactions();
YearFragment08 year08=(YearFragment08)getChildFragmentManager().findFragmentByTag("year08");
year08.setinfo("This is fantastic");

---

另请注意，更好的方法是创建 YearFragment08首先 fragment 并使用 Fragment传递要在 Fragment 中显示的信息的参数.那么您就不需要使用上述解决方法，因为您不需要调用 yearInfo.setText("Hi")当Fragment首次创建。您可以找到创建 Fragment 的示例带参数 here .

更新

尽管第一种方法允许 getChildFragmentManager().findFragmentByTag("year08")，但正如冗长的评论中所述要返回 fragment ，调用 Fragment 上的任何方法仍然不是一个好主意。在 onViewStateRestored(...) 之前访问其 View 或在被调用之后。

这是因为在使用 FragmentManager 时将父 fragment 添加到子 fragment 的 onCreateView调用executePendingTransactions()后不会立即调用方法因此子 fragment 的引用 View 为空。如果父Fragment直接在布局中使用这不是问题 onCreateView子 fragment 的一部分被称为 executePendingTransactions() 的一部分并且它的 View 不会为空。

在这种情况下，最好的办法是使用前面提到的使用 Fragment 的其他方法。如果您需要为 fragment 中的 View 设置初始值，请使用参数。