Question

我正在研究房间livedata数据绑定概念。我正在使用自定义绑定适配器将列表设置为微调框。但是我第一次从房间检索时，得到的实时数据为空。

代码：

     @Query("select * from Student")
        LiveData<List<Student>> getAllItems();


        @BindingAdapter({"bind:entries"})
            public static void customSpinnerBinding(AppCompatSpinner appCompatSpinner,LiveData<List<Student>> listLiveData){
                if(listLiveData != null && listLiveData.getValue().size() > 0){
                    ArrayAdapter<Student> spinnerArrayAdapter = new ArrayAdapter<Student>
                            (appCompatSpinner.getContext(), android.R.layout.simple_spinner_item,
                                    listLiveData.getValue()); //selected item will look like a spinner set from XML
                    spinnerArrayAdapter.setDropDownViewResource(android.R.layout
                            .simple_spinner_dropdown_item);
                    appCompatSpinner.setAdapter(spinnerArrayAdapter);
                }
            }

        public LiveData<List<Student>> getListLiveData() {
            if(listLiveData == null){
               listLiveData = studentDB.studentDao().getAllItems();
            }
            return listLiveData;
        }

        <android.support.v7.widget.AppCompatSpinner
                    android:id="@+id/spinner"
                    android:layout_width="0dp"
                    bind:entries="@{viewmodel.listLiveData}"
                    android:layout_height="wrap_content"
                    android:layout_marginStart="16dp"
                    android:layout_marginLeft="16dp"
                    android:layout_marginTop="16dp"
                    android:layout_marginEnd="16dp"
                    android:layout_marginRight="16dp"
                    android:layout_marginBottom="16dp"
                    app:layout_constraintBottom_toBottomOf="parent"
                    app:layout_constraintEnd_toEndOf="parent"
                    app:layout_constraintStart_toStartOf="parent"
                    app:layout_constraintTop_toTopOf="parent" />

but if i observe livedata in activity this is working

  mainViewModel.getListLiveData().observe(this, new Observer<List<Student>>() {
           @Override
           public void onChanged(@Nullable List<Student> students) {
               if(students.size() > 0){
                   ArrayAdapter<Student> spinnerArrayAdapter = new ArrayAdapter<Student>
                    (MainActivity.this, android.R.layout.simple_spinner_item,
                            students); //selected item will look like a spinner set from XML
            spinnerArrayAdapter.setDropDownViewResource(android.R.layout
                    .simple_spinner_dropdown_item);
            activityMainBinding.appbar.contentid.spinner.setAdapter(spinnerArrayAdapter);
               }
           }
       });

Answer 1

尝试

listLiveData = StudentDB.getAppDatabase(application).studentDao().getAllItems();
return listLiveData;

Answer 2

检查以下两项：

将您的@for $i from 1 through 20 { // the hash forces SCSS to create a string .item:nth-child(#{$i}){ // you need to use $i for BOTH since its the only defined increment animation-delay: $i * $timeOffset, $i * $aniLength; // delay #1 = iteration * 0.5s, delay #2 = iteration * 2s } }更改为接受.customSpinnerBinding而不是List<Student>。

LiveData<List<Student>>接受BindingAdapter内部的内容，而不接受LiveData本身。 LiveData不会改变。仅LiveData通知的内容会更改。

BindingAdapter示例：

LiveData

请勿延迟加载@BindingAdapter({"bind:entries"}) public static void customSpinnerBinding(AppCompatSpinner appCompatSpinner, List<Student> students) { ArrayAdapter<Student> spinnerArrayAdapter = new ArrayAdapter<Student> (appCompatSpinner.getContext(), android.R.layout.simple_spinner_item, students); //selected item will look like a spinner set from XML spinnerArrayAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item); appCompatSpinner.setAdapter(spinnerArrayAdapter); }

您的视图似乎仍然需要它，延迟加载的目的是什么？通过使您的视图模型更有状态，这只会使您的代码更复杂且更难调试。只是热切地加载学生名单，并将其声明为最终名单，以使其变得更好。

listLiveData

如果必须延迟加载public final LiveData listLiveData = studentDB.studentDao().getAllItems();，请确保它已初始化。在您的.xml中：

listLiveData

微调框条目绑定适配器房间实时数据不起作用

2 个答案: