我正在尝试实现PreferenceFragmentCompatSharedPreferences.OnSharedPreferenceChangeListener

我的应用包含主要活动和片段。 home片段有一个带有标题的URL列表,我想添加一个设置以将URL添加到此列表中。到目前为止,这是我尝试过的:

这是SettingsFragment.java

@Override
public void onSharedPreferenceChanged(SharedPreferences sharedPreferences, String key) {
    Preference preference = findPreference(key);
    if (preference instanceof EditTextPreference) {
        EditTextPreference editTextPreference = (EditTextPreference) preference;
        String value = editTextPreference.getText();
        new HomeFragment().addLink(value);
    } else {
        assert preference != null;
        preference.setSummary(sharedPreferences.getString(key, ""));

    }
}


HomeFragment.java

private ArrayList<LinkItem> urls = new ArrayList<>(Arrays.asList(
        new LinkItem("LifeHacker RSS Feed", "https://lifehacker.com/rss"),
        new LinkItem("Google News Feed", "https://news.google.com/news/rss");
private LinkItemAdapter itemAdapter;
private ListView listView;
@Override
public View onCreateView(@NonNull LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.fragment_home, container, false);
    listView = view.findViewById(R.id.postListView);
    itemAdapter = new LinkItemAdapter(getActivity(), R.layout.link_item, urls);
    listView.setAdapter(itemAdapter);
    listView.setOnItemClickListener(onItemClickListener);
    itemAdapter.notifyDataSetChanged();
    return view;
}

void addLink(String title) {
    urls.add(new LinkItem(title, "https://google.com"));
    itemAdapter.notifyDataSetChanged();
}

private AdapterView.OnItemClickListener onItemClickListener = new AdapterView.OnItemClickListener() {
    @Override
    public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
        HomeFragmentDirections.ActionHomeFragmentToRssFragment action =
                HomeFragmentDirections.actionHomeFragmentToRssFragment(urls.get(position).Link, urls.get(position).Title);
        NavHostFragment.findNavController(HomeFragment.this).navigate(action);
    }
};


如果我尝试这样做,则itemAdapter将为null,从而使应用程序崩溃,因此我不确定如何实现此目的。如果我尝试像在HomeFragment的onCreate方法中那样在addLink中重新创建它,则该活动最终为null。如果我尝试通过设置片段传递活动或上下文,则会发生相同的结果。

LinkItemAdapter适应以下对象:

public class LinkItem {
    public String Title;
    public String Link;
}


到目前为止,我的结果始终是相同的:由于指针为空,更改后在编辑文本字段上单击“确定”后立即崩溃。有人可以帮我这个忙吗?我是android新手。

堆栈跟踪:

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.myfragmentapp, PID: 5185
    java.lang.NullPointerException: Attempt to invoke virtual method 'void com.example.myfragmentapp.adapters.LinkItemAdapter.notifyDataSetChanged()' on a null object reference
        at com.example.myfragmentapp.screens.HomeFragment.addLink(HomeFragment.java:86)
        at com.example.myfragmentapp.screens.SettingsFragment.onSharedPreferenceChanged(SettingsFragment.java:42)
        at android.app.SharedPreferencesImpl$EditorImpl.notifyListeners(SharedPreferencesImpl.java:560)
        at android.app.SharedPreferencesImpl$EditorImpl.apply(SharedPreferencesImpl.java:443)
        at androidx.preference.Preference.tryCommit(Preference.java:1632)
        at androidx.preference.Preference.persistString(Preference.java:1663)
        at androidx.preference.EditTextPreference.setText(EditTextPreference.java:80)
        at androidx.preference.EditTextPreferenceDialogFragmentCompat.onDialogClosed(EditTextPreferenceDialogFragmentCompat.java:99)
        at androidx.preference.PreferenceDialogFragmentCompat.onDismiss(PreferenceDialogFragmentCompat.java:267)
        at android.app.Dialog$ListenersHandler.handleMessage(Dialog.java:1377)
        at android.os.Handler.dispatchMessage(Handler.java:105)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6709)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:769)

最佳答案

创建适配器后,应调用addLink()

 public View onCreateView(@NonNull LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
     View view = inflater.inflate(R.layout.fragment_home, container, false);
     listView = view.findViewById(R.id.postListView);
     itemAdapter = new LinkItemAdapter(getActivity(), R.layout.link_item, urls);
     listView.setAdapter(itemAdapter);
     listView.setOnItemClickListener(onItemClickListener);
     addLnk();
     return view;
 }


如果您尝试将一个片段中的值设置为另一个片段,则应使用回调或ViewModel,其中更简单的是回调:

定义一个回调接口:

interface OnSetPreferenceItem{
   void setPrefItemInList(String item);
}


在SettingsFragment中,定义一个变量:

 private OnSetPreferenceItem callback;


在同一片段中,在onAttach中填写变量:

 public void onAttach(Context context) {
     super.onAttach(context);

     callback = (OnSetPreferenceItem )context;
 }


现在,不调用new HomeFragment()。addLink(value);,而是

callback.setPrefItemInList(value);


让您的父级活动实现该接口并实现建议的方法:

public void setPrefItemInList(String item){
     homeFragment.addLink(item);
}


修改您的addLink方法以保护它:

void addLink(String title) {
   urls.add(new LinkItem(title, "https://google.com"));
    if(itemAdapter!=null){
       itemAdapter.notifyDataSetChanged();
    }
}

08-03 20:32