java - 单击时微调器选定的项目不显示

Question

我添加了一个从房间数据库检索数据的微调器，当我单击微调器时，它会正确显示可用的项目，但是当我选择一个时，它不会出现在选定的微调器中，并且什么也不显示

我发现 CollegeSpinner.setOnItemSelectedListener即使我选择某些内容也不会触发

    ArrayAdapter<String> adapter;
    CollegeViewModel collegeViewModel;
    Spinner collegeSpinner;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_add_note);

        collegeSpinner = (Spinner) findViewById(R.id.spinner_college);
        final ArrayList<String> collegesList = new ArrayList<>();
        final CollegeAdapter collegeAdapter = new CollegeAdapter();
        collegeViewModel = ViewModelProviders.of(this).get(CollegeViewModel.class);
        collegeViewModel.getAllColleges().observe(this, new Observer<List<College>>() {
            @Override
            public void onChanged(List<College> colleges) {
                //this is what happens when when data changes // the code bellow should be related to changing the view
                collegeAdapter.setColleges(colleges);  // getting colleges from database and assigning it to collegeadapter
                for (int i = 0; i < collegeAdapter.getItemCount(); i++) { // accessing the adapter to add it's elements to collegeList
                    collegesList.add(collegeAdapter.getCollegeAt(i).getCollegeName());
                }
            }
        });
        adapter = new ArrayAdapter<>(getApplicationContext(), R.layout.small_view, collegesList);
        collegeSpinner.setAdapter(adapter);
        collegeSpinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
            @Override
            public void onItemSelected(AdapterView<?> adapterView, View view, int i, long l) {
                // this function doesn't work even when i select something
            }

            @Override
            public void onNothingSelected(AdapterView<?> adapterView) {
                // also doesn't work
            }
        });

    }

Answer 1

实际上我找到了一个解决方案，但我不明白它为什么有效但这与 CollegeAdapter 被声明为最终版本有关，但我必须将其声明为最终版本，否则我会在 onChanged() 中收到错误

我创建了一个函数，将 CollegeAdapter 中的元素添加到 CollegeList 中因此该函数将 CollegeAdapter 的值分配给参数中的一个新的 CollegeAdapter，该参数不是最终的，并且工作原理如下

这是我如何实现的

collegeViewModel.getAllColleges().observe(this, new Observer<List<College>>() {
            @Override
            public void onChanged(List<College> colleges) {
                //this is what happens when when data changes // the code bellow should be related to changing the view 
                collegeAdapter.setColleges(colleges);  // getting colleges from database and assigning it to collegeadapter
                addElements(collegeAdapter, collegesList); 
            }
        });

这是函数

public void addElements(CollegeAdapter collegeAdapter, ArrayList<String> collegesList) {
        for (int i = 0; i < collegeAdapter.getItemCount(); i++) {
            collegesList.add(collegeAdapter.getCollegeAt(i).getCollegeName());
        }
        collegeSpinner.setAdapter(adapter);
    }