Question

我正在尝试使用Mediator Live数据来合并两个Livedata。用ViewModel和Fragment编写的方法如下所示。我收到“类错误”强制转换异常错误（详细信息在下面列出）。您能否建议出问题了。可能我认为这可能是MediatorLive从两个实时数据中获取值的方式，但我无法弄清楚，因此无法向编码人员提出要求。。请提前告知并感谢您的帮助。

// LiveData -1

public LiveData<GrpMsgTimestamp> getlastRec(Long lastbookID, String groupID)
    {
    return bookRepository.getlatest(lastbookID,groupID);
    }

// LiveData -2

    public LiveData<Integer> getNoOfUnreadBook(Long lastbookID, String groupID)//Final to be kept
    {
    return bookRepository.getNoOfUnreadBook(lasbookID,groupID);
    }

//调解方法

    MediatorLiveData liveDataMerger = new MediatorLiveData<GrpIDandLatestMsg>();

    public MediatorLiveData getLiveDataMerger(Long lastmsgID,String groupID) {
    liveDataMerger.addSource(getlastRec(lastbookID,groupID),value->liveDataMerger.setValue(getlastRec(lastbookID,groupID)));
        liveDataMerger.addSource(getNoOfUnreadBook(lastgroupID,groupID), value -> liveDataMerger.setValue(getNoOfUnreadBook(lastbookID,groupID)));

    return liveDataMerger;
    }

//片段中的观察者

mViewModel.getLiveDataMerger(chatList.getLastbookID(),chatList.getGroupID()).observe(getActivity(), new Observer<GrpIDandLatestBook>() {
                    @Override
                    public void onChanged(@Nullable final GrpIDandLatestBook newName) {
                        // Update the UI, in this case, a TextView.
                        if(newName != null) {
                            chatList.setLast_msg(newName.getBook());
                            chatList.setLasttimestamp(String.valueOf(newName.getTimestamp()));
                            chatList.setNoofunreadBook(String.valueOf(newName.getMsgcount()));

                        }
                    }

错误：-

java.lang.ClassCastException: android.arch.lifecycle.ComputableLiveData$1 cannot be cast to         com.support.android.designlibdemo.database.inernal.ModelClass.GrpIDandLatestMsg
    at com.support.android.designlibdemo.View.UI.FragmentChatList$1$1.onChanged(FragmentChatList.java:61)
    at android.arch.lifecycle.LiveData.considerNotify(LiveData.java:109)
    at android.arch.lifecycle.LiveData.dispatchingValue(LiveData.java:126)
    at android.arch.lifecycle.LiveData.setValue(LiveData.java:282)
    at android.arch.lifecycle.MutableLiveData.setValue(MutableLiveData.java:33)
    at com.support.android.designlibdemo.ViewModel.ChatListViewModel.lambda$getLiveDataMerger$1$ChatListViewModel(ChatListViewModel.java:65)

Thanks

Answer 1

这是我创建的用于融合2个不同祖先列表的类。

class MelterFavorites extends MediatorLiveData<List<FavoriteEntity>> {
    // ...
    private List<FavoriteEntity>   tmpFavoriteCities        = null;
    private List<FavoriteEntity>   tmpFavoriteSellPoints    = null;

    private final Observer<List<FavoriteCity>> cityListObserver = new Observer<List<FavoriteCity>>() {
        @Override
        public void onChanged(final List<FavoriteCity> favoriteCityList) {
            // ...
            tmpFavoriteCities.clear();

            // ...
            if (favoriteCityList != null) {
                tmpFavoriteCities.addAll(favoriteCityList);
            }

            // melt results
            final List<FavoriteEntity> meltedFavorites = meltFavorites(tmpFavoriteCities, tmpFavoriteSellPoints);

            // ...
            setValue(meltedFavorites);
        }
    };

    private final Observer<List<FavoriteSellPoint>> sellPointListObserver = new Observer<List<FavoriteSellPoint>>() {
        @Override
        public void onChanged(final List<FavoriteSellPoint> favoriteSellPointList) {
            // ...
            tmpFavoriteSellPoints.clear();

            // ...
            if (favoriteSellPointList != null) {
                tmpFavoriteSellPoints.addAll(favoriteSellPointList);
            }

            // melt results
            final List<FavoriteEntity> meltedFavorites = meltFavorites(tmpFavoriteCities, tmpFavoriteSellPoints);

            // ...
            setValue(meltedFavorites);
        }
    };

    private List<FavoriteEntity> meltFavorites(final List<FavoriteEntity> favoriteCities, final List<FavoriteEntity> favoriteSellPoints) {
        final TreeSet<FavoriteEntity> allFavorites = new TreeSet<>();

        allFavorites.addAll(favoriteCities);
        allFavorites.addAll(favoriteSellPoints);

        return new ArrayList<>(allFavorites);
    }

    public MelterFavorites(final LiveData<List<FavoriteCity>> favoriteCities, LiveData<List<FavoriteSellPoint>> favoriteSellPoints) {
        // build internal temporary list
        tmpFavoriteCities       = new ArrayList<FavoriteEntity>();
        tmpFavoriteSellPoints   = new ArrayList<FavoriteEntity>();

        addSource(favoriteCities,       cityListObserver);
        addSource(favoriteSellPoints,   sellPointListObserver);
    }
}

实现Mediator LiveData时发生ClassCastException错误

1 个答案: