Question

我在一个由活动IssueActivity

控制的视图寻呼机中有两个片段

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

    viewPager = findViewById(R.id.view_pager);
    tabLayout = findViewById(R.id.tab);
    PagerAdapter adapter = new PagerAdapter(getSupportFragmentManager());
    layout = findViewById(R.id.layout);
    progressBar = new ProgressBar(this, null, android.R.attr.progressBarStyleLarge);
    // progressBar.
    RelativeLayout.LayoutParams params = new RelativeLayout.LayoutParams(100, 100);
    params.addRule(RelativeLayout.CENTER_IN_PARENT);
    layout.addView(progressBar, params);
   // getProjects();
    fragment = new issuesFragment();

    viewPager.setAdapter(adapter);
    tabLayout.setupWithViewPager(viewPager);
    tabLayout.getTabAt(0).setText("ISSUES");
    tabLayout.getTabAt(1).setText("PROJECT");

}

这是使用接口将项目列表发送到问题或项目片段的代码

private void lookForProjects() {
    showProgressBar();
    db = FirebaseFirestore.getInstance();
    projects = new ArrayList<>();
    CollectionReference projectReference = db.collection(getString(R.string.collection_project));

    projectReference.get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
        @Override
        public void onComplete(@NonNull Task<QuerySnapshot> task) {
            //  progressBar.setVisibility(View.INVISIBLE);
                hideProgressBar();
            if (task.isSuccessful()) {

                for (QueryDocumentSnapshot snapshot : task.getResult()) {
                    Project project = snapshot.toObject(Project.class);
                    //projectNames[i] = project.getName();
                    projects.add(project);
                    //  i++;

                    Log.d(TAG, "onComplete: " + project.getName());
                }


                // setUpAdapterForAutoCompleteTextView(projectNames);
            } else {
               //if (progressBar != null) {
                //    hideProgressBar();
                //}
                //Toast.makeText(IssueActivity.this, "Cannot Load Projects", Toast.LENGTH_LONG).show();
            }
            sendToFragments(projects);


        }
    });

}

private void sendToFragments(ArrayList<Project> projects) {
   // issuesFragment issueFragment = new issuesFragment();
    fragment.addProjects(projects);
  //  ProjectFragment projectFragment = new ProjectFragment();
  //  projectFragment.addProjects(projects);
}

但是问题片段

中存在的问题是这个

fragment.addProjects(projects);

在连接时被称为

 @Override
public void onAttach(Context context) {
    super.onAttach(context);
    try{
        issuesInterface = (issuesInterface)getActivity();

      //  Log.d(TAG, "addProjects: to know if fragment is added + " + isAdded());\
        getProjects();

    }
    catch (ClassCastException e){
        e.printStackTrace();
    }

}

getActivity（）返回一个值，但是当我想用它来创建一个ArrayAdapter时，它是一个嵌套在getProject方法中的方法

ArrayAdapter<String> issueSpinnerAdapter = new ArrayAdapter<String>(getActivity(),R.layout.support_simple_spinner_dropdown_item,stuffs);
        issueSpinner.setAdapter(issueSpinnerAdapter);

它是null;并且应用程序崩溃时出现此错误

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference
    at android.view.LayoutInflater.from(LayoutInflater.java:233)
    at android.widget.ArrayAdapter.<init>(ArrayAdapter.java:181)
    at android.widget.ArrayAdapter.<init>(ArrayAdapter.java:139)
    at com.example.samuel.firestore.issuesFragment.initSpinner(issuesFragment.java:143)
    at com.example.samuel.firestore.issuesFragment.addProjects(issuesFragment.java:128)
    at com.example.samuel.firestore.Activities.IssueActivity.sendToFragments(IssueActivity.java:122)
    at com.example.samuel.firestore.Activities.IssueActivity.access$000(IssueActivity.java:31)
    at com.example.samuel.firestore.Activities.IssueActivity$1.onComplete(IssueActivity.java:112)
    at com.google.android.gms.tasks.zzj.run(Unknown Source)
    at android.os.Handler.handleCallback(Handler.java:751)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:154)
    at android.app.ActivityThread.main(ActivityThread.java:6776)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1496)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1386)

但在一个场景中，没有从控制活动发送到片段的任何内容，我使用一个虚拟字符串数组来构建片段中的适配器，在同一方法中，我没有得到任何错误，适配器工作正好。任何人都可以帮助解释为什么当我尝试从Controlling活动获取详细信息然后尝试构造一个结果得到的数组适配器时，getActivity返回null？

Answer 1

我发现了问题，我调用方法sendToFragment的片段对象不是我传递给视图分页器适配器的片段对象...一旦更改了它...并使用了传递给适配器的相同对象在..上调用我的方法开始工作..

Answer 2

在Fragment中调用onAcitivtyCreated（）回调方法之前，

getActivity（）不会返回非null对象。在您的情况下，如果您想与Activity通信，则必须在onAttach（）中执行此操作。见下文：

    @Override
public void onAttach(Context context) {
    super.onAttach(context);
    try{
        issuesInterface = (issuesInterface) context;
        getProjects();
    }
    catch (ClassCastException e){
        e.printStackTrace();
    }

}

或者，如果您将此片段附加到另一个片段，您可以使用此：

    @Override
public void onAttach(Context context) {
    super.onAttach(context);
    try{
        Fragment parent = getParentFragment();
        issuesInterface = (issuesInterface) parent;
        getProjects();
    }
    catch (ClassCastException e){
        e.printStackTrace();
    }

}

getActivity（）在视图分页器片段

2 个答案: