如何为接收 LiveData 作为输入的微调器编写绑定适配器？

Question

我有一个像这样在 xml 中定义的微调器

<Spinner
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:id="@+id/expense_category"
        app:sourceData="@{()->createExpenseViewModel.getAllSourceItems(1)}"
        app:layout_constraintStart_toStartOf="@+id/textView"
        android:layout_marginTop="20dp"
        app:layout_constraintTop_toBottomOf="@+id/textView" app:layout_constraintWidth_percent="0.7"
/>

createExpenseViewModel.getAllSourceItems(1) 这个方法返回 LiveData <List<Source>>，所以我为这种情况编写了一个绑定适配器

@BindingAdapter("app:sourceData")
fun setSourceData(spinner: Spinner, sourceList: List<Source>) {

    val categoryItems = ArrayList<String>()
    categoryItems.addAll(sourceList.map { it.sourceName })
    val spinnerAdapter =
        ArrayAdapter<String>(spinner.context, R.layout.simple_spinner_dropdown_item, categoryItems)
    spinner.adapter = spinnerAdapter


}

构建应用程序时，我收到以下错误， ****/ data binding error ****msg:Cannot find the proper callback class for app:sourceData. Tried java.util.List but it has 25 abstract methods, should have 1 abstract methods. file:/home/naveen/Desktop/project-expense/app/src/main/res/layout/activity_create_expense.xml loc:94:34 - 94:80 ****\ data binding error ****

这个错误实际上是什么意思，如何解决这个错误？

编辑：

我打算做的是获取实时数据返回的列表并转换为 ArrayList 类型，一旦 livedata 返回列表，我需要触发我的绑定适配器，但是如果我使用这个应用程序：sourceData="@{createExpenseViewModel.getAllSourceItems( 1)}" 并设置绑定适配器，适配器只获取空列表

Answer 1

您正在将一个方法绑定到app:sourceData，但您希望在绑定适配器中为它提供一个变量。那行不通。我猜您想将列表填充到微调器中。为此，我将在您的 viewModel 中创建一个属性并将此属性绑定到 xml 中。我在一个应用程序中做到了这一点，在该应用程序中，我有一个要在 Spinner 中显示的项目列表。这是包含 InverseBindingAdapter 的代码，用于自动将所选项目保存在 ViewModel 的另一个变量中。

视图模型：

// getProjects() returns the LiveData
val projects = metaDataRepository.getProjects() 

// use _selectedProject only within ViewModel. Do not expose MediatorLiveData to UI.
// in UI observe selectedProject
private val _selectedProject = MediatorLiveData<Project>()
val selectedProject: LiveData<Project>
    get() = _selectedProject 

布局 XML :

<Spinner
    android:id="@+id/spProjects"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    app:projects="@{viewModel.projects}"
    app:selectedProject="@={viewModel.selectedProject}" />

BindingAdapter（将数据从 viewModel 填充到 UI）：

/**
 * fill the Spinner with all available projects.
 * Set the Spinner selection to selectedProject.
 * If the selection changes, call the InverseBindingAdapter
 */
@BindingAdapter(value = ["projects", "selectedProject", "selectedProjectAttrChanged"], requireAll = false)
fun setProjects(spinner: Spinner, projects: List<Project>?, selectedProject: Project, listener: InverseBindingListener) {
    if (projects == null) return
    spinner.adapter = ProjectAdapter(spinner.context, android.R.layout.simple_spinner_dropdown_item, projects)
    setCurrentSelection(spinner, selectedProject)
    setSpinnerListener(spinner, listener)
}

BindingAdapter 的辅助方法：

private fun setSpinnerListener(spinner: Spinner, listener: InverseBindingListener) {
    spinner.onItemSelectedListener = object : AdapterView.OnItemSelectedListener {
        override fun onItemSelected(parent: AdapterView<*>?, view: View?, position: Int, id: Long) = listener.onChange()
        override fun onNothingSelected(adapterView: AdapterView<*>) = listener.onChange()
    }
}

private fun setCurrentSelection(spinner: Spinner, selectedItem: Project?): Boolean {
    if (selectedItem == null) {
        return false
    }

    for (index in 0 until spinner.adapter.count) {
        val currentItem = spinner.getItemAtPosition(index) as Project
        if (currentItem.name == selectedItem.name) {
            spinner.setSelection(index)
            return true
        }
    }

    return false
}

适合您的 Spinner 的简单适配器。将此更改为您的需要：

/**
 * Adapter for displaying the name-field of an Project in a Spinner
 */
class ProjectAdapter(context: Context, textViewResourceId: Int, private val values: List<Project>) : ArrayAdapter<Project>(context, textViewResourceId, values) {

    override fun getCount() = values.size
    override fun getItem(position: Int) = values[position]
    override fun getItemId(position: Int) = position.toLong()

    override fun getView(position: Int, convertView: View?, parent: ViewGroup): View {
        val label = super.getView(position, convertView, parent) as TextView
        label.text = values[position].name
        return label
    }

    override fun getDropDownView(position: Int, convertView: View?, parent: ViewGroup): View {
        val label = super.getDropDownView(position, convertView, parent) as TextView
        label.text = values[position].name
        return label
    }
}

InverseBindingAdapter（将选定的 Spinner 项存储在 viewModel 中）

/**
 * get the selected projectName and use it to return a
 * Project which is then used to set appEntry.value.project
 */
@InverseBindingAdapter(attribute = "selectedProject")
fun getSelectedProject(spinner: Spinner): Project {
    return spinner.selectedItem as Project
}