无法将 LiveData 列表绑定到 Spinner 条目答案

【问题标题】：Unable to bind LiveData List to Spinner entries无法将 LiveData 列表绑定到 Spinner 条目
【发布时间】：2018-12-31 11:45:36
【问题描述】：

我目前有一个片段MyFragment，它有一个微调器my_spinner。为了测试我的应用程序，我最初通过观察 AndroidViewModel MyViewModel 中的属性 myLiveDataList 手动填充了 my_spinner 的内容，如下所示：

my_fragment.xml

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:tools="http://schemas.android.com/tools"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".ui.fragments.MyFragment">

    <Spinner
            android:id="@+id/my_spinner"
            android:layout_width="match_parent"
            android:layout_height="100dp" />
</FrameLayout>

MyFragment.kt

import androidx.lifecycle.ViewModelProviders
import android.os.Bundle
import androidx.fragment.app.Fragment
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.ArrayAdapter
import androidx.lifecycle.Observer

import com.example.app.R
import com.example.app.data.room.entities.MyEntity
import com.example.app.ui.viewmodels.MyViewModel
import kotlinx.android.synthetic.main.my_fragment.*

class MyFragment : Fragment() {

    companion object {
        fun newInstance() = MyFragment()
    }

    private lateinit var viewModel: MyViewModel

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        return inflater.inflate(R.layout.my_fragment, container, false)
    }

    override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)
        viewModel = ViewModelProviders.of(this).get(MyViewModel::class.java)

        val myAdapter = ArrayAdapter<MyEntity>(this.context!!, android.R.layout.simple_spinner_item)

        // This is where I populate my_spinner
        viewModel.myLiveDataList.observe(this, Observer<List<MyEntity>> { data ->
            data?.forEach {
                myAdapter.add(it)
            }
        })

        my_spinner.adapter = myAdapter
    }
}

MyViewModel.kt

import android.app.Application
import androidx.lifecycle.AndroidViewModel
import androidx.lifecycle.LiveData
import androidx.lifecycle.toLiveData
import com.example.app.data.repositories.MyRepository
import com.example.app.data.room.entities.MyEntity

class MyViewModel(application: Application) : AndroidViewModel(application) {
    private val myRepository = MyRepository(application)

    val myLiveDataList: LiveData<List<MyEntity>>
        get() = myRepository.getAllData().toLiveData()
}

当我导航到MyFragment 时，这会成功填充my_spinner：

由于它按预期填充，我继续对my_fragment.xml 进行以下更改：

<?xml version="1.0" encoding="utf-8"?>
<layout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    xmlns:app="http://schemas.android.com/apk/res-auto">

    <data>
        <variable name="viewmodel"
                  type="com.example.app.ui.viewmodels.MyViewModel" />
    </data>

    <FrameLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            tools:context=".ui.fragments.MyFragment">

        <Spinner
                android:id="@+id/my_spinner"
                android:layout_width="match_parent"
                android:layout_height="100dp"
                app:entries="@{viewmodel.myLiveDataList}"/>
    </FrameLayout>
</layout>

我已在绑定适配器文件BindingAdapterUtil 中添加（以下代码是从this article 复制的）：

import android.R
import android.view.View
import android.widget.AdapterView
import android.widget.ArrayAdapter
import android.widget.Spinner
import androidx.databinding.BindingAdapter
import androidx.databinding.InverseBindingAdapter
import androidx.databinding.InverseBindingListener
import com.example.app.ui.adapter.SpinnerExtensions.getSpinnerValue
import com.example.app.ui.adapter.SpinnerExtensions.setSpinnerEntries
import com.example.app.ui.adapter.SpinnerExtensions.setSpinnerInverseBindingListener
import com.example.app.ui.adapter.SpinnerExtensions.setSpinnerItemSelectedListener
import com.example.app.ui.adapter.SpinnerExtensions.setSpinnerValue

@BindingAdapter("entries")
fun Spinner.setEntries(entries: List<Any>?) {
    setSpinnerEntries(entries)
}

@BindingAdapter("onItemSelected")
fun Spinner.setItemSelectedListener(itemSelectedListener: SpinnerExtensions.ItemSelectedListener?) {
    setSpinnerItemSelectedListener(itemSelectedListener)
}

@BindingAdapter("newValue")
fun Spinner.setNewValue(newValue: Any?) {
    setSpinnerValue(newValue)
}

@BindingAdapter("selectedValue")
fun Spinner.setSelectedValue(selectedValue: Any?) {
    setSpinnerValue(selectedValue)
}

@BindingAdapter("selectedValueAttrChanged")
fun Spinner.setInverseBindingListener(inverseBindingListener: InverseBindingListener?) {
    setSpinnerInverseBindingListener(inverseBindingListener)
}

@InverseBindingAdapter(attribute = "selectedValue", event = "selectedValueAttrChanged")
fun Spinner.getSelectedValue(): Any? {
    return getSpinnerValue()
}

object SpinnerExtensions {

    fun Spinner.setSpinnerEntries(entries: List<Any>?) {
        if (entries != null) {
            val arrayAdapter = ArrayAdapter(context, R.layout.simple_spinner_item, entries)
            arrayAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item)
            adapter = arrayAdapter
        }
    }

    fun Spinner.setSpinnerItemSelectedListener(listener: ItemSelectedListener?) {
        if (listener == null) {
            onItemSelectedListener = null
        } else {
            onItemSelectedListener = object : AdapterView.OnItemSelectedListener {
                override fun onItemSelected(parent: AdapterView<*>, view: View, position: Int, id: Long) {
                    if (tag != position) {
                        listener.onItemSelected(parent.getItemAtPosition(position))
                    }
                }

                override fun onNothingSelected(parent: AdapterView<*>) {}
            }
        }
    }

    fun Spinner.setSpinnerInverseBindingListener(listener: InverseBindingListener?) {
        if (listener == null) {
            onItemSelectedListener = null
        } else {
            onItemSelectedListener = object : AdapterView.OnItemSelectedListener {
                override fun onItemSelected(parent: AdapterView<*>, view: View, position: Int, id: Long) {
                    if (tag != position) {
                        listener.onChange()
                    }
                }

                override fun onNothingSelected(parent: AdapterView<*>) {}
            }
        }
    }

    fun Spinner.setSpinnerValue(value: Any?) {
        if (adapter != null ) {
            val position = (adapter as ArrayAdapter<Any>).getPosition(value)
            setSelection(position, false)
            tag = position
        }
    }

    fun Spinner.getSpinnerValue(): Any? {
        return selectedItem
    }

    interface ItemSelectedListener {
        fun onItemSelected(item: Any)
    }
}

我已经修改了MyFragment 中的onActivityCreated，如下所示：

override fun onActivityCreated(savedInstanceState: Bundle?) {
    super.onActivityCreated(savedInstanceState)
    viewModel = ViewModelProviders.of(this).get(MyViewModel::class.java)

    DataBindingUtil.setContentView<MyFragmentBinding>(
        this.activity!!, R.layout.my_fragment
    ).apply {
        this.setLifecycleOwner(this@MyFragment)
        this.viewmodel = viewModel
    }
}

这样做的结果是my_spinner 不再填充MyViewModel.myLiveDataList 的内容。为了确定该属性是否有问题，我在MyViewModel 中创建了一个新属性，如下所示：

val myList: List<String>?
    get() = listOf("First", "Second", "Third")

我已经将这个属性绑定到my_spinner，就像上面的MyViewModel.myLiveDataList，这次成功了。

MyRepository.getAllData() 中的函数（myLiveDataList 返回）返回一个 Flowable<List<MyEntity>> (RxJava)，它调用 Room DAO 来获取数据。我在这里的假设是 myLiveDataList 在第一次尝试绑定值时没有任何服务，并且再也不会尝试。

我在尝试将 LiveData 数据源绑定到 Spinner 时是否遗漏了什么？

【问题讨论】：

标签： android kotlin android-databinding android-livedata

【解决方案1】：

阅读this answer后，我将my_fragment.xml修改为：

...
<data>
    <import type="java.util.List" />
    <import type="com.example.app.data.room.entities.MyEntity" />
    <import type="androidx.lifecycle.LiveData" />
    <variable name="viewmodel"
              type="com.example.app.ui.viewmodels.MyViewModel" />

    <variable name="myTestList"
              type="LiveData&lt;List&lt;MyEntity&gt;&gt;" />
</data>
...
<Spinner
    android:id="@+id/my_spinner"
    android:layout_width="match_parent"
    android:layout_height="100dp"
    app:entries="@{myTestList}"/>
...

我还删除了MyFragment.onActivityCreated的内容并修改了MyFragment.onCreateView如下：

override fun onCreateView(
    inflater: LayoutInflater, container: ViewGroup?,
    savedInstanceState: Bundle?
): View? {
    viewModel = ViewModelProviders.of(this).get(MyViewModel::class.java)
    val binding = MyFragmentBinding.inflate(inflater, container, false)
    binding.setLifecycleOwner(this)
    binding.viewmodel = viewModel
    binding.myTestList = viewModel.myLiveDataList

    return binding.root
}

不是一个完美的解决方案，我仍然不知道为什么我最初在这个问题上的回合没有产生预期的结果，但它会做到的。如果有更好的方式以这种方式将 Spinner 绑定到 LiveData，请告诉我。

【讨论】：