Livedata并未始终如一地更新视图

Question

recycleView在初始加载时没有从网络上更新结果。

recycleView：

    override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)
        mRecyclerAdapter = MovieListAdapter(context)
        rvMovieList.apply {
            // Dedicated layouts for Screen Orientation
            if (resources.configuration.orientation == Configuration.ORIENTATION_PORTRAIT) {
                layoutManager = LinearLayoutManager(context)
            } else {
                layoutManager = GridLayoutManager(context, 2)
            }
            adapter = mRecyclerAdapter
        }
    }

并使用ViewModel的Livedata收听网络结果。

LiveData侦听摘要下面的片段：

    override fun onResume() {
        super.onResume()
        // Listen to data change
        viewModel.getMovies().observe(this, mMovieListObserver)
    }
    private val mMovieListObserver: Observer<PagedList<MovieItem>> = Observer { movieItems ->
        Log.d(TAG, "MovieItems: ${movieItems.size}")
        showEmptyList(movieItems?.size == 0)
        mRecyclerAdapter.submitList(movieItems)
    }
    private fun showEmptyList(isEmpty: Boolean) {
        tvEmptyListView.visibility = if (isEmpty) View.VISIBLE else View.GONE
        rvMovieList.visibility = if (isEmpty) View.GONE else View.VISIBLE
    }
    override fun onPause() {
        viewModel.getMovies().removeObserver(mMovieListObserver)
        super.onPause()
    }

具有讽刺意味的是，结果填充了后续载荷的回收景观。我觉得Livedata无法正常工作。引入空浏览时的期望是根据网络的结果显示/隐藏recycleView/emptyview。

ViewModel粘贴下面：

class MovieListViewModel : ViewModel() {
    private val PAGE_SIZE = 10
     internal var movies: LiveData<PagedList<MovieItem>>
    init {
        val dataSourceFactory = MovieDataSourceFactory()
        val pagedListConfig = PagedList.Config.Builder()
                .setInitialLoadSizeHint(PAGE_SIZE)
                .setPageSize(PAGE_SIZE)
                .setEnablePlaceholders(true)
                .build()
        movies = LivePagedListBuilder(dataSourceFactory, pagedListConfig)
                // .setBoundaryCallback() TODO
                .build()
    }
    fun getMovies(): LiveData<PagedList<MovieItem>> {
        return movies
    }
}

感谢您的时间，感谢解决方案或最佳实践的任何意见。谢谢。

回购：https：//gitlab.com/faisalm/moviedirect

////----

更新了datasourceFactory和dataSource。

class MovieDataSourceFactory : DataSource.Factory<Int, MovieItem>() {
    private val mutableLiveData = MutableLiveData<MovieDataSource>()
    override fun create(): DataSource<Int, MovieItem> {
        val dataSource = MovieDataSource()
        mutableLiveData.postValue(dataSource)
        return dataSource
    }
}

---

class MovieDataSource internal constructor() : PageKeyedDataSource<Int, MovieItem>() {
    private val movieDbService: MovieDbService = RetrofitFactory.create()
    override fun loadInitial(params: LoadInitialParams<Int>, callback: LoadInitialCallback<Int, MovieItem>) {
        val moviesListCall = movieDbService.fetchLatestMoviesPaged(Constants.API_KEY, 1)
        moviesListCall.enqueue(object : Callback<MoviesList> {
            override fun onResponse(call: Call<MoviesList>, response: Response<MoviesList>) {
                if (response.isSuccessful) {
                    val moviesLists = response.body()?.results
                    callback.onResult(moviesLists!!, 1, 2)
                }
            }
            override fun onFailure(call: Call<MoviesList>, t: Throwable) {}
        })
    }
    override fun loadBefore(params: LoadParams<Int>, callback: LoadCallback<Int, MovieItem>) {}
    override fun loadAfter(params: LoadParams<Int>, callback: LoadCallback<Int, MovieItem>) {
        val moviesListCall = movieDbService.fetchLatestMoviesPaged(Constants.API_KEY, params.key)
        moviesListCall.enqueue(object : Callback<MoviesList> {
            override fun onResponse(call: Call<MoviesList>, response: Response<MoviesList>) {
                if (response.isSuccessful) {
                    val moviesLists = response.body()?.results
                    callback.onResult(moviesLists!!, params.key + 1)
                }
            }
            override fun onFailure(call: Call<MoviesList>, t: Throwable) {}
        })
    }
}

Answer 1

我认为问题是您添加和删除Livedata的观察者的方式。

而不是在onResume中添加并在onPause中删除，而只需在片段中的onActivityCreated中观察它。Livedata的观察方法吸收了生命周期的人(这是您在片段中使用this通过的内容(，并且它将确保在该生命周期中正确观察它。

。

所以删除这些行：

viewModel.getMovies().removeObserver(mMovieListObserver) viewModel.getMovies().addObserver(this, mMovieListObserver)

并添加此：

override fun onActivityCreated(savedInstanceState: Bundle?) {
    super.onActivityCreated(savedInstanceState)
    viewModel.getMovies().observe(this, Observer { movieItems ->
        Log.d(TAG, "MovieItems: ${movieItems.size}")
        showEmptyList(movieItems?.loadedCount == 0)
        mRecyclerAdapter.submitList(movieItems)
    })
}