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这是我的问题。我必须将三个不同LiveData对象的数据生成到一个对象中。其中两个取决于其中之一。仅当收到所有三个对象数据时才应触发观察者。

这是一个例子：

生成的对象包含产品信息和该产品的特定评论评级。结果对象如下所示：

public class RatedProduct {
    private String productKey; // database key to the product
    private String ratingKey; // database key to the specific rating
    private Product product; // the product information
    private Rating rating; // the specifiv rating information
    private HashMap<String, Boolean> customTags; // list of tags
...
}

源分布在不同的数据库位置。

第一个对象UserRating包含附加数据 ( customTags) 和只有两个键，引用产品和评级信息。

public class UserRating {
    private String ratingKey;
    private String productKey;
    private HashMap<String, Boolean> customTags;
...
}

Rating和Product持有特定信息的对象。

public class Rating {
    private float value; // value of the rating
    private String productKey; // database key to the product
    private String ratingComment;
...
}

public class Product {
    private String name; // name of the product
    private HashMap<String, Boolean> ratings; // list of all ratings
...
}

任务是将所有数据从 和 收集UserRating到一个Rating，但隐藏必要的步骤。ProductRatedProduct

我认为最好的方法是使用三个不同的LiveData对象并在MediatorLiveData

但我不确定如何以正确的方式做到这一点。我有点卡住了。我知道如何将UserRatingLiveDataas 源添加到，MediatorLiveData但我不知道如何添加剩余的两个，调用它们RatingLiveData以及ProductLiveData关于UserRatingLiveData.

第二个问题是，MediatorLiveData<RatedProduct>只有在检索到所有数据而不是其中一个时才应该更新。

这就是我所拥有的（或多或少没有）。LiveData<RatedProduct> getRatedProductLiveData() 我猜有趣的部分是：

public class UserViewModel extends ViewModel {
    ...
    private UserRatingsLiveData mUserRatingsLiveDate;

    public UserViewModel() {

        //
        // Getting current user
        FirebaseUser mUser = FirebaseAuth.getInstance().getCurrentUser();

        if (mUser != null) {
            this.mUserRatingsLiveDate =
                    new UserRatingsLiveData(
                            mUsersDatabaseReference.child(mUser.getUid() + "/" + DB_PATH_RATINGS));
        } else {
            Log.d(LOG_TAG, "mUser is NULL");
        }
    }

    @NonNull
    public LiveData<RatedProduct> getRatedProductLiveData() {
        final MediatorLiveData<RatedProduct>  liveDataMerger = new MediatorLiveData<> ();
        final UserRating receivedUserRating = new UserRating();


        liveDataMerger.addSource(this.mUserRatingsLiveDate, new Observer<UserRating>() {
            public void onChanged(UserRating userRatings) {

            }
        });
        return liveDataMerger;
    }
}

作为附加提示：数据库是 Firebase RealTimeDatabase，如果这有帮助的话。

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1 回答 1

2

试试这样：


class MyViewModel {
    val mainLiveData: LiveData<CustomClass> get() = _mainLiveData

    private val firstLiveData = MutableLiveData<String>()
    private val secondLiveData : MutableLiveData<Int>? = null
    private val thirdLiveData : MutableLiveData<Long>? = null

    private val _mainLiveData = MediatorLiveData<CustomClass>().also { mergeLiveData ->
        mergeLiveData.addSource(firstLiveData) {
           if (secondLiveData != null ) mergeLiveData.removeSource(secondLiveData)
           if (thirdLiveData != null) mergeLiveData.removeSource(thirdLiveData)

           secondLiveData = getSecondLiveData(firstLiveData.value)
           thirdLiveData = getThirdLiveData(firstLiveData.value)

           mergeLiveData.addSource(secondLiveData) {
               updateMainLiveData(mergeLiveData)
           }
           
           mergeLiveData.addSource(thirdLiveData) {
               updateMainLiveData(mergeLiveData)
           }
        }
    }

    private fun updateMainLiveData(mainLiveData: MediatorLiveData<CustomClass>) {
        val first = firstLiveData.value ?: return
        val second = secondLiveData.value ?: return
        val third = thirdLiveData.value ?: return

        mainLiveData.value = CustomClass(first, second, third)
    }

    data class CustomClass(
        val first: String,
        val second: Int,
        val third: Long
    )
}
于 2020-10-18T15:31:55.917 回答