android - MutableLiveData 的 postValue 方法返回 null

Question

我正在实现 Mvvm 模型并尝试使用 MutableLiveData 来监听变化。
有两种方法

setValue() - 设置值。如果有活跃的观察者，值将被分派(dispatch)给他们。这个方法必须从主线程调用。

postValue() - 当您需要从后台线程设置值时使用 postValue。

这是我正在尝试的代码，当我使用 postValue 并且如果我使用 setValue 它工作正常时，我会得到空值。

View 模型

private MutableLiveData<List<Model>> modelsLiveData = new MutableLiveData<>();

public MainActivityViewModel() {
    modelsLiveData = new MutableLiveData<>();
}

public MutableLiveData<List<Model>> getModelsLiveData() {
    return modelsLiveData;
}

public void setModelsLiveData(final List<Model> modelsList) {

    new Thread(new Runnable() {
        @Override
        public void run() {
            try {
                Log.i("LIVE_DATA_TEST", Thread.currentThread().toString());
                Thread.sleep(1000);
                modelsLiveData.postValue(modelsList);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

        }
    }).start();


}

主 Activity

 ActivityMainBinding activityBinding = DataBindingUtil
            .setContentView(this, R.layout.activity_main, new AppDataBindingComponent<>(this));
    MainActivityViewModel viewModel = new MainActivityViewModel();
    activityBinding.setViewModel(viewModel);

    Log.i("LIVE_DATA_TEST", "onCreate: "+ Thread.currentThread());

    List<Model> modelsList = new ArrayList<>();
    modelsList.add(new Model("Suzuki Swift", 2761111));
    modelsList.add(new Model("Renoult Megan", 3679060));
    modelsList.add(new Model("Fiat Panda", 4638764));
    modelsList.add(new Model("Fiat 500", 1234567));
    modelsList.add(new Model("Toyota Land Crouser", 9876543));

    viewModel.setModelsLiveData(modelsList);

布局

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

<data>
    <variable
        name="viewModel"
        type="com.matanmarciano.mvvmlivedata.viewmodels.MainActivityViewModel" />
</data>

<LinearLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <android.support.v7.widget.RecyclerView
        android:id="@+id/models_list"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:items="@{viewModel.modelsLiveData}" />
</LinearLayout>

数据绑定(bind)适配器

@BindingAdapter({"items"})
public void items(final RecyclerView recyclerView, List<Model> models) {
    Log.i("LIVE_DATA_TEST", "Let's see if this method is reached" + models);

    // TODO: update recyclerView adapter...
}

日志
这就是我得到的

I/LIVE_DATA_TEST: onCreate: Thread[main,5,main]
I/LIVE_DATA_TEST: Thread[Thread-4,5,main]
I/LIVE_DATA_TEST: Let's see if this method is reached null // null I am getting here when using postValue.

请帮助我在哪里做错了。

Answer 1

您必须添加 setLifecycleOwner 才能从数据绑定(bind)订阅 liveData。

ActivityMainBinding activityBinding = DataBindingUtil
            .setContentView(this, R.layout.activity_main, new AppDataBindingComponent<>(this));
activityBinding.setLifecycleOwner(this);

https://developer.android.com/topic/libraries/data-binding/architecture#livedata