Android 为什么片段管理器在已经添加了片段的情况下返回null';你的观点是什么?
为什么Android 为什么片段管理器在已经添加了片段的情况下返回null';你的观点是什么?,android,android-fragments,Android,Android Fragments,为什么f1仍然为空,即使它现在应该包含FragmentA实例 MainActivity.java public class MainActivity extends AppCompatActivity implements FragmentA.Communicator { FragmentManager manager; FragmentA f1; @Override protected void onCreate(Bundl
f1
仍然为空,即使它现在应该包含FragmentA
实例
MainActivity.java
public class MainActivity extends AppCompatActivity implements FragmentA.Communicator {
FragmentManager manager;
FragmentA f1;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
manager = getSupportFragmentManager();
Fragment fragment = manager.findFragmentById(R.id.main_fragment_container);
if (fragment == null) {
fragment = new FragmentA();
manager.beginTransaction()
.add(R.id.main_fragment_container, fragment)
.commit();
}
f1 = (FragmentA) manager.findFragmentById(R.id.main_fragment_container);
f1.setCommunicator(this);
}
@Override
public void changeText(String text) {
FragmentB f2 = (FragmentB) manager.findFragmentById(R.id.details_fragment_container);
if (f2 != null && f2.isVisible()) {
f2.updateText(text);
} else {
Intent intent = new Intent(MainActivity.this, DetailsActivity.class);
intent.putExtra("text", text);
startActivity(intent);
}
activity_main.XML
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="match_parent"
android:layout_height="match_parent">
<FrameLayout
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="@+id/main_fragment_container" />
</LinearLayout>
FragmentA.Java
public class FragmentA extends Fragment {
Communicator communicator;
ListView listView;
public FragmentA() {
// Required empty public constructor
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
View view = inflater.inflate(R.layout.fragment_a, container, false);
listView = view.findViewById(R.id.listview);
return view;
}
public void setCommunicator(Communicator communicator) {
this.communicator = communicator;
}
@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
final ArrayList<String> arrayList = new ArrayList<>();
arrayList.add("first row");
arrayList.add("second row");
arrayList.add("third row");
ArrayAdapter<String> adapter = new ArrayAdapter<>(getActivity(),
android.R.layout.simple_list_item_1,
arrayList);
listView.setAdapter(adapter);
listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
communicator.changeText(arrayList.get(position));
}
});
}
public interface Communicator {
void changeText(String text);
}
公共类片段扩展片段{
沟通者;
列表视图列表视图;
公共碎片a(){
//必需的空公共构造函数
}
@凌驾
创建视图上的公共视图(布局、充气机、视图组容器、,
Bundle savedInstanceState){
//为该碎片膨胀布局
视图=充气机。充气(R.layout.fragment_a,容器,错误);
listView=view.findViewById(R.id.listView);
返回视图;
}
公共通讯器(通讯器){
this.communicator=通信器;
}
@凌驾
ActivityCreated上的公共无效(@Nullable Bundle savedinStateCState){
super.onActivityCreated(savedInstanceState);
最终ArrayList ArrayList=新ArrayList();
添加(“第一行”);
arrayList.add(“第二行”);
arrayList.add(“第三行”);
ArrayAdapter=新的ArrayAdapter(getActivity(),
android.R.layout.simple\u list\u item\u 1,
arrayList);
setAdapter(适配器);
setOnItemClickListener(新的AdapterView.OnItemClickListener(){
@凌驾
public void onItemClick(AdapterView父对象、视图、整型位置、长id){
communicator.changeText(arrayList.get(position));
}
});
}
公共接口通信器{
void changeText(字符串文本);
}
fragment_a.xml
<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=".FragmentA">
<ListView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="@+id/listview" />
</FrameLayout>
Log: `Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void
com.example.workout.FragmentA.setCommunicator(com.example.workout.FragmentA$Communicator)' on a
null object reference`
日志:`原因:java.lang.NullPointerException:尝试调用虚拟方法'void
com.example.workout.FragmentA.setCommunicator(com.example.workout.FragmentA$Communicator)“”在
空对象引用`
commit()
是异步的。如果希望通过findFragmentById
立即获得片段,则应使用commitNow()
当然,在您的情况下,您可以将片段
和f1
变量组合在一起:
f1 = manager.findFragmentById(R.id.main_fragment_container);
if (f1 == null) {
f1 = new FragmentA();
manager.beginTransaction()
.add(R.id.main_fragment_container, f1)
.commit();
}
// f1 is now never null
f1.setCommunicator(this);