android - 在 AndroidViewModel 中初始化 LiveData

Question

我正在寻找一种方法来初始化 ViewModel 中的 LiveData 对象。当前仅在从 Activity 调用 setQuery 方法时才初始化项目。

public class MyListViewModel extends AndroidViewModel {

private final LiveData <List<Item>> items;
private final MutableLiveData<String> query = new MutableLiveData<>();


private MyDatabase db;

public MyListViewModel(Application application) {
    super(application);
    db  = MyDatabase.getInstance(application);
    items = Transformations.switchMap(query, (search)->{
        if (search == null || search.trim().length() == 0) {
            return db.itemDao().getAllItems();
        } else {
            return db.itemDao().findItemsBySearchTerm(search);
        }
    });

}

public LiveData<List<Item>> getItems() {
    return items;
}

public void setQuery(String queryText) {
   query.setValue(queryText);
}

Answer 1

您必须调用 setQuery(String queryText) 至少一次根据转换文档

The transformations aren't calculated unless an observer is observing the returned LiveData object. Because the transformations are calculated lazily, lifecycle-related behavior is implicitly passed down without requiring additional explicit calls or dependencies.

所以如果你不调用setQuery(String queryText)从 Activity 中它不会更新 MutableLiveData<String> query并且不会触发转换。

如果你想避免来自Activity的初始调用，你可以像这样在Transformation初始化下面调用它，

public MyListViewModel(Application application) {
    super(application);
    db  = MyDatabase.getInstance(application);
    items = Transformations.switchMap(query, (search)->{
        if (search == null || search.trim().length() == 0) {
            return db.itemDao().getAllItems();
        } else {
            return db.itemDao().findItemsBySearchTerm(search);
        }
    });
    setQuery("")
}

它将触发空搜索部分并返回所有项目。

您还需要观察您案例中返回的数据items .如果没有观察者在观察数据，那么事务将不会触发。

正如我从 Documentation 中提到的那样很少有博客对我有用