一个 LiveData 返回 null

Question

我一直在尝试将两个实时数据结合起来显示在一项活动中。 我有表 Item、ItemMoto 和 ItemFavourite。当从存储在 Item 或 ItemMoto 中的产品中将产品添加到收藏夹时，它会保存 id、itemi、...

我使用这个库 livedata-combineutil-java 来组合来自 Item 和 ItemMoto 的结果。我试图将它们从 View Module 组合起来，但我没有得到结果。这是我在 Fragment 中的做法。

        LiveData<Pair<Resource<List<Item>>, Resource<List<ItemMoto>>>> news =
     combine(favouriteViewModel.getItemFavouriteData(), favouriteViewModelMoto.getItemFavouriteData(),
            (favorite, favoriteMoto) -> { 
         return Pair.create(favorite, favoriteMoto);
            });

    if (news != null) {
        news.observe(this, listResource -> {
            if (listResource != null) {

                switch (listResource.first.status) {
                    case LOADING:
                        // Loading State
                        // Data are from Local DB
                        if (listResource.first.data != null) {
                            //fadeIn Animation
                            fadeIn(binding.get().getRoot());
                            // Update the data
                            replaceDataItems(listResource.first.data, listResource.second.data);
                        }
                        break;
                    case SUCCESS:
                        // Success State
                        // Data are from Server
                        if (listResource.first.data != null) {
                            // Update the data
                            replaceDataItems(listResource.first.data, listResource.second.data);
                        }
                        favouriteViewModel.setLoadingState(false);

                        break;
                    case ERROR:
                        // Error State
                        favouriteViewModel.setLoadingState(false);
                        break;
                    default:
                        // Default
                        break;
                }
            } else {

                // Init Object or Empty Data

                if (favouriteViewModel.offset > 1) {
                    // No more data for this list
                    // So, Block all future loading
                    favouriteViewModel.forceEndLoading = true;
                }

            }

        });
    }

当我检查时：

 Log.d("favourite moto live", favouriteViewModelMoto.getItemFavouriteData().toString());
 
 Log.d("favourite  live", favouriteViewModel.getItemFavouriteData().toString());

我得到了结果，但 listResource.second.data 返回 null。

这里是资源文件的链接：

https://pastebin.com/ZGtBzLV0

更新：

最初，Item 和 ItemMoto 具有自己的 Activity 和 Fragment 以及自己的 LiveData。 我正在使用 ROOM 和 PHP 从服务器检索数据。

对于项目：

  @Query("SELECT prd.* FROM Item prd, ItemFavourite fp WHERE prd.id = 
  fp.itemId order by fp.sorting ")
   LiveData<List<Item>> getAllFavouriteProducts();

对于 ItemMoto：

  @Query("SELECT prd.* FROM ItemMoto prd, ItemFavourite fp WHERE prd.id =  
  fp.itemId order by fp.sorting ")
   LiveData<List<ItemMoto>> getAllFavouriteProducts();

视图模型

package com.panaceasoft.psbuyandsell.viewmodel.item;

import android.util.Log;
import androidx.core.util.Pair;
import androidx.lifecycle.LiveData;
import androidx.lifecycle.MediatorLiveData;
import androidx.lifecycle.MutableLiveData;
import androidx.lifecycle.Transformations;

import com.panaceasoft.psbuyandsell.Config;
import com.panaceasoft.psbuyandsell.repository.item.ItemMotoRepository;
import com.panaceasoft.psbuyandsell.repository.item.ItemRepository;
import com.panaceasoft.psbuyandsell.utils.AbsentLiveData;
import com.panaceasoft.psbuyandsell.utils.Utils;
import com.panaceasoft.psbuyandsell.viewmodel.common.PSViewModel;
import com.panaceasoft.psbuyandsell.viewobject.Item;
import com.panaceasoft.psbuyandsell.viewobject.ItemMoto;
 import com.panaceasoft.psbuyandsell.viewobject.common.Resource;

import java.util.List;

import javax.inject.Inject;

import static   
com.zhuinden.livedatacombineutiljava.LiveDataCombineUtil.combine;

public class FavouriteViewModel extends PSViewModel {
private final LiveData<Resource<Boolean>> sendFavouritePostData;


private MutableLiveData<FavouriteViewModel.TmpDataHolder> 
sendFavouriteDataPostObj = new MutableLiveData<>();
private LiveData<Resource<List<Item>>> itemFavouriteData;
private LiveData<Resource<List<ItemMoto>>> itemFavouriteDataMoto;
private MutableLiveData<FavouriteViewModel.TmpDataHolder>  
itemFavouriteListObj = new MutableLiveData<>();
private MutableLiveData<FavouriteMotoViewModel.TmpDataHolder> 
itemFavouriteListObjMoto = new MutableLiveData<>();

private final LiveData<Resource<Boolean>> nextPageFavouriteLoadingData;
private MutableLiveData<FavouriteViewModel.TmpDataHolder> nextPageLoadingFavouriteObj = new MutableLiveData<>();

@Inject
FavouriteViewModel(ItemRepository itemRepository, ItemMotoRepository itemMotoRepository) {


    sendFavouritePostData = Transformations.switchMap(sendFavouriteDataPostObj, obj -> {

        if (obj == null) {
            return AbsentLiveData.create();
        }
        return itemRepository.uploadFavouritePostToServer(obj.itemId, obj.userId);
    });

    itemFavouriteData = Transformations.switchMap(itemFavouriteListObj, obj -> {
        if (obj == null) {
            return AbsentLiveData.create();
        }
        Utils.psLog("itemFavouriteData");
        return itemRepository.getFavouriteList(Config.API_KEY, obj.userId, obj.offset);
    });

 //Moto
    itemFavouriteDataMoto = 
 Transformations.switchMap(itemFavouriteListObjMoto, obj -> {
        if (obj == null) {
            return AbsentLiveData.create();
        }
        Utils.psLog("itemFavouriteData");
        return itemMotoRepository.getFavouriteList(Config.API_KEY,  
  obj.userId, obj.offset);
    });

    nextPageFavouriteLoadingData = Transformations.switchMap(nextPageLoadingFavouriteObj, obj -> {
        if (obj == null) {
            return AbsentLiveData.create();
        }
        Utils.psLog("nextPageFavouriteLoadingData");
        return itemRepository.getNextPageFavouriteProductList(obj.userId, obj.offset);
    });

}

///
public void setFavouritePostDataObj(String itemId, String userId) {

    if (!isLoading) {
        FavouriteViewModel.TmpDataHolder tmpDataHolder = new FavouriteViewModel.TmpDataHolder();
        tmpDataHolder.itemId = itemId;
        tmpDataHolder.userId = userId;

        sendFavouriteDataPostObj.setValue(tmpDataHolder);
        setLoadingState(true);
    }

}

//region Getter And Setter for item detail List
public void setItemFavouriteListObj(String loginUserId, String offset) {
    if (!isLoading) {
        FavouriteViewModel.TmpDataHolder tmpDataHolder = new FavouriteViewModel.TmpDataHolder();
        tmpDataHolder.userId = loginUserId;
        tmpDataHolder.offset = offset;
        itemFavouriteListObj.setValue(tmpDataHolder);

        // start loading
        setLoadingState(true);
    }
}

public LiveData<Resource<List<Item>>> getItemFavouriteData() {

    return itemFavouriteData;
}

public LiveData<Resource<List<ItemMoto>>> getItemFavouriteDataMoto() {

    return itemFavouriteDataMoto;
}

public LiveData<Resource<Boolean>> getFavouritePostData() {
    return sendFavouritePostData;
}

class TmpDataHolder {
    public String itemId = "";
    public String userId = "";
    public String cityId = "";
    public String offset = "";
}

}

当我检查最喜欢的产品时，我会看到 Item 的产品。我刷新，然后我看到 ItemMoto 的产品，但从来没有在一起。

Answer 1

为什么不使用“3rd LiveData”之类的简单方法作为组合值？

MutableLiveData<ItemMoto> mMotoLiveData = new MutableLiveData<>();
MutableLiveData<ItemFavorite> mFavoriteLiveData = new MutableLiveData<>();
MutableLiveData<Pair<ItemMoto,ItemFavorite>> mCombinedLiveData = new MutableLiveData<>();
ItemMoto mLastItemMoto = null;
ItemFavorite mLastItemFavorite = null;
...
mMotoLiveData.observe(..., onChanged(@Nullable ItemMoto moto){
  mLastItemMoto = moto;
  mCombinedLiveData.postValue(new Pair<>(mLastItemMoto, mLastItemFavorite));
});
mFavoriteLiveData.observe(..., onChanged(@Nullable ItemFavorite favorite){
  mLastItemFavorite = favorite;
  mCombinedLiveData.postValue(new Pair<>(mLastItemMoto, mLastItemFavorite));
});
mCombinedLiveData.observe(..., onChanged(@NonNull Pair<ItemMoto,ItemFavorite> pair){
  //pair.one and/or pair.second could be NULL, so pay attention!!
});

注意 LiveData "onChanged()" 事件不会被所有 "postValue()" 调用触发，因为如果它们过于频繁，只有少数（或最新的）会触发它们的 onChanged() 事件，所以如果您希望使用 LiveDatas 获得 ItemMoto+ItemFavorite 的 所有组合 对您没有用处。