Question

我当前有一个片段MyFragment，其中有一个旋转器my_spinner。为了测试我的应用，我最初是通过观察AndroidViewModel my_spinner中的属性myLiveDataList手动填充MyViewModel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my_spinner时，这将成功填充MyFragment：

由于它的填充符合预期，因此我继续对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>

我已经在绑定适配器文件BindingAdapterUtil中添加了以下代码（以下代码是从this article复制来的）：

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)
    }
}

我已经像这样修改onActivityCreated中的MyFragment：

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
    }
}

其结果是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")

这次，我成功地将此属性绑定到my_spinner上，就像上面的MyViewModel.myLiveDataList一样。

MyRepository.getAllData()（由myLiveDataList返回）中的函数返回Flowable<List<MyEntity>>（RxJava），该函数调用Room DAO来获取数据。我在这里的假设是，myLiveDataList在它第一次尝试绑定值并且永远不会再尝试时没有任何作用。

尝试将LiveData数据源绑定到Spinner时是否缺少某些内容？

Answer 1

阅读this answer之后，我将my_fragment.xml修改为以下内容：

...
<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，并按如下所示修改了MyFragment.onCreateView：

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
}

这不是一个完美的解决方案，而且我仍然不知道为什么我最初针对此问题的争吵没有产生预期的结果，但是确实可以。如果有一种更好的方式可以将Spinner绑定到LiveData，请告诉我。

无法将LiveData列表绑定到微调器条目

1 个答案: