Android 使用 2 向绑定的微调器设置选择

Question

当配置了 2 向数据绑定时，我正在努力获得一些与 Android 微调器一起工作的功能。我想通过 android:selectedItemPosition 上的双向数据绑定设置微调器的初始值。微调器条目由 ViewModel 初始化并正确填充，因此数据绑定似乎正常工作。

问题出在 selectedItemPosition 的双向绑定上。该变量由 ViewModel 初始化为 5，但微调器的选定项保持为 0（第一项）。调试时，ObservableInt 的值似乎最初为 5（设置），但在 executeBindings 的第二阶段重置为零。

如有任何帮助，我们将不胜感激。

test_spinner_activity.xml

<layout xmlns:tools="http://schemas.android.com/tools"
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto">

    <data>
        <variable name="viewModel"
                  type="com.aapp.viewmodel.TestSpinnerViewModel"/>
    </data>
    <LinearLayout android:layout_width="match_parent"
                  android:layout_height="wrap_content">
       <android.support.v7.widget.AppCompatSpinner
            android:layout_width="wrap_content"
            android:layout_height="match_parent"
            android:id="@+id/sTimeHourSpinner"
            android:selectedItemPosition="@={viewModel.startHourIdx}"
            android:entries="@{viewModel.startTimeHourSelections}"/>
    </LinearLayout>
</layout>

TestSpinnerViewModel.java

public class TestSpinnerViewModel {
    public final ObservableArrayList<String> startTimeHourSelections = new ObservableArrayList<>();
    public final ObservableInt startHourIdx = new ObservableInt();

    public TestSpinnerViewModel(Context context) {
        this.mContext = context;

        for (int i=0; i < 24; i++) {
            int hour = i;
            startTimeHourSelections.add(df.format(hour));
        }
        startHourIdx.set(5);
    }
}

TestSpinnerActivity.java

public class TestSpinnerActivity extends AppCompatActivity {
    private TestSpinnerActivityBinding binding;
    private TestSpinnerViewModel mTestSpinnerViewModel;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        binding = DataBindingUtil.bind(findViewById(R.id.test_spinner));
        mTestSpinnerViewModel = new TestSpinnerViewModel(this);
        binding.setViewModel(mTestSpinnerViewModel);
    }

我正在使用 Android Studio 2.2.2 并且启用了数据绑定。

Answer 1

我最近在 GitHub 上创建了一个演示应用程序，以展示如何利用 bindingAdapter 和 InverseBindingAdapter 机制在微调器上实现双向数据绑定。

在这个应用程序中，我没有绑定 "android:selectedItemPosition" 属性，而是绑定了微调器的所选项目本身（利用 ObservableField class），如下面的代码片段所示。因为它是双向绑定，通过在微调器适配器设置期间为绑定的 ObservableField（即选定项）分配初始值，以及微调器的 bindingAdapter 中的特殊处理，可以实现微调器初始选择。

随时查看演示应用程序 here 了解更多详情。

acivity_main.xml

<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"
        xmlns:bind="http://schemas.android.com/apk/res-auto">

    <data>
        <variable
            name="bindingPlanet"
            type="au.com.chrisli.spinnertwowaydatabindingdemo.BindingPlanet"/>
        <variable
            name="spinAdapterPlanet"
            type="android.widget.ArrayAdapter"/>
    </data>

    <RelativeLayout
        android:id="@+id/activity_main"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        ...>

        <android.support.v7.widget.AppCompatSpinner
            android:id="@+id/spin"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_centerInParent="true"
            style="@style/Base.Widget.AppCompat.Spinner.Underlined"
            bind:selectedPlanet="@={bindingPlanet.obvSelectedPlanet_}"
            app:adapter="@{spinAdapterPlanet}"/>

        ...(not relevant content omitted for simplicity)
    </RelativeLayout>

</layout>

BindingPlanet.java

绑定适配器内的特殊处理

public final ObservableField<Planet> obvSelectedPlanet_ = new ObservableField<>(); //for simplicity, we use a public variable here

private static class SpinPlanetOnItemSelectedListener implements AdapterView.OnItemSelectedListener {

    private Planet initialSelectedPlanet_;
    private InverseBindingListener inverseBindingListener_;

    public SpinPlanetOnItemSelectedListener(Planet initialSelectedPlanet, InverseBindingListener inverseBindingListener) {
        initialSelectedPlanet_ = initialSelectedPlanet;
        inverseBindingListener_ = inverseBindingListener;
    }

    @Override
    public void onItemSelected(AdapterView<?> adapterView, View view, int i, long l) {
        if (initialSelectedPlanet_ != null) {
            //Adapter is not ready yet but there is already a bound data,
            //hence we need to set a flag so we can implement a special handling inside the OnItemSelectedListener
            //for the initial selected item
            Integer positionInAdapter = getPlanetPositionInAdapter((ArrayAdapter<Planet>) adapterView.getAdapter(), initialSelectedPlanet_);
            if (positionInAdapter != null) {
                adapterView.setSelection(positionInAdapter); //set spinner selection as there is a match
            }
            initialSelectedPlanet_ = null; //set to null as the initialization is done
        } else {
            if (inverseBindingListener_ != null) {
                inverseBindingListener_.onChange();
            }
        }
    }

    @Override
    public void onNothingSelected(AdapterView<?> adapterView) {}
}

@BindingAdapter(value = {"bind:selectedPlanet", "bind:selectedPlanetAttrChanged"}, requireAll = false)
public static void bindPlanetSelected(final AppCompatSpinner spinner, Planet planetSetByViewModel,
                                      final InverseBindingListener inverseBindingListener) {

    Planet initialSelectedPlanet = null;
    if (spinner.getAdapter() == null && planetSetByViewModel != null) {
        //Adapter is not ready yet but there is already a bound data,
        //hence we need to set a flag in order to implement a special handling inside the OnItemSelectedListener
        //for the initial selected item, otherwise the first item will be selected by the framework
        initialSelectedPlanet = planetSetByViewModel;
    }

    spinner.setOnItemSelectedListener(new SpinPlanetOnItemSelectedListener(initialSelectedPlanet, inverseBindingListener));

    //only proceed further if the newly selected planet is not equal to the already selected item in the spinner
    if (planetSetByViewModel != null && !planetSetByViewModel.equals(spinner.getSelectedItem())) {
        //find the item in the adapter
        Integer positionInAdapter = getPlanetPositionInAdapter((ArrayAdapter<Planet>) spinner.getAdapter(), planetSetByViewModel);
        if (positionInAdapter != null) {
            spinner.setSelection(positionInAdapter); //set spinner selection as there is a match
        }
    }
}

@InverseBindingAdapter(attribute = "bind:selectedPlanet", event = "bind:selectedPlanetAttrChanged")
public static Planet captureSelectedPlanet(AppCompatSpinner spinner) {
    return (Planet) spinner.getSelectedItem();
}

Answer 2

感谢您的建议。但我找到了我自己问题的答案。事实证明，android:selectedItemPosition=@={viewModel.startHourIdx} 变量从初始值 5 重置为 0 的原因是因为 selectedItemPosition 和 entries 属性的声明顺序。在我的示例中，它们是按特定顺序声明的，自动生成的绑定代码以相同顺序生成初始化。

因此，即使 selectedItemPosition 设置正确，entries 的初始化也会导致 ArrayAdapter 实例化，从而将 selectedItemPosition 重置为 0。

因此，解决方法是交换布局文件中的两个属性声明。

<data>
    <variable name="viewModel"
              type="com.aapp.viewmodel.TestSpinnerViewModel"/>
</data>
<LinearLayout android:layout_width="match_parent"
              android:layout_height="wrap_content">
   <android.support.v7.widget.AppCompatSpinner
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:id="@+id/sTimeHourSpinner"
        android:entries="@{viewModel.startTimeHourSelections}"
        android:selectedItemPosition="@={viewModel.startHourIdx}"/>
</LinearLayout>

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