Kotlin Jetpack Paging3 和Flow结合使用注意点

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在用DataBinding时要注意DataBinding访问的是静态方法

在kotlin中就要用companion object和@JvmStatic

class ImageViewBindingAdapter {//里面的BindingAdapter方法必须是静态方法，否则会编译会报错//DataBinding调用必须是静态方法companion object {@JvmStatic@BindingAdapter("image")fun setImage(imageView:ImageView, url: String){if (!TextUtils.isEmpty(url)){//加载网络图片}else{imageView.setBackgroundColor(Color.GRAY)}}}
}

paging3分页数据错乱的问题

在计算paging的prevKey和nextKey，也就是上一页，下一页的时候，需要考虑PagingConfig中的initialLoadSize参数

fun loadMovie(): Flow<PagingData<Movie>> {return Pager(config = PagingConfig(pageSize = 8,//第一次加载的数量，16也就是2页，默认是3*pageSizeinitialLoadSize = 16),pagingSourceFactory = {MoviePagingSource()}).flow
}class MoviePagingSource: PagingSource<Int, Movie>() {override fun getRefreshKey(state: PagingState<Int, Movie>): Int? {//第一次加载return 1}override suspend fun load(params: LoadParams<Int>): LoadResult<Int, Movie> {val currentPage = params.key ?: 1val pageSize = params.loadSizeval movies = RetrofitClient.createApi(MoviesApi::class.java).getMovies(currentPage, pageSize)var prevKey: Int? = nullvar nextKey: Int? = null//PagingConfig中的2个参数val realPageSize = 8val initialLoadSize = 16if (currentPage == 1){prevKey = nullnextKey = initialLoadSize / realPageSize + 1}else{prevKey = currentPage - 1nextKey = if (movies.hasMore) currentPage + 1 else null}//下面这样计算，在initialLoadSize不等于realPageSize的时候会有数据错乱//prevKey = if (currentPage == 1) null else currentPage - 1//nextKey = if (movies.hasMore) currentPage + 1 else nullreturn try {LoadResult.Page(data = movies.movieList,prevKey = prevKey,nextKey = nextKey)}catch (e: Exception){e.printStackTrace()return LoadResult.Error(e)}}}

给paging加上拉加载更多

recycleView.adapter = movieAdapter.withLoadStateFooter(MovieLoadMoreAdapter(this@MainActivity))

只需要加个适配器就可以

class MovieLoadMoreAdapter(private val context: Context): LoadStateAdapter<BindViewHolder>() {override fun onBindViewHolder(holder: BindViewHolder, loadState: LoadState) {}override fun onCreateViewHolder(parent: ViewGroup, loadState: LoadState): BindViewHolder {val binding = MovieLoadmoreBinding.inflate(LayoutInflater.from(context), null, false)return BindViewHolder(binding)}
}

paging加上下拉刷新

布局加上SwipeRefreshLayout

<androidx.swiperefreshlayout.widget.SwipeRefreshLayoutandroid\:id="@+id/swipeRefreshLayout"android\:layout\_width="match\_parent"android\:layout\_height="match\_parent"><androidx.recyclerview.widget.RecyclerViewandroid:id="@+id/recycleView"android:layout_width="match_parent"android:layout_height="match_parent"app:layoutManager="androidx.recyclerview.widget.LinearLayoutManager" /></androidx.swiperefreshlayout.widget.SwipeRefreshLayout>

加上刷新监听

swipeRefreshLayout.setOnRefreshListener {movieAdapter.refresh()
}

监听刷新结束

lifecycleScope.launchWhenCreated {movieAdapter.loadStateFlow.collectLatest { state ->mBinding.swipeRefreshLayout.isRefreshing = state.refresh is LoadState.Loading}
}

下拉刷新后，底部上拉加载更多的loadmore的动画不显示

PageConfig还有一个属性是prefetchDistance，预刷新的距离，距离最后一个item多远时加载数据，默认为pageSize
当prefetchDistance很小，并且initialLoadSize也很小时，就会出现上面的bug。比如initialLoadSize=8，prefetchDistance=1时
解决办法也比较简单，2个属性设置的大一点就行了

APP横竖屏切换之后paging加载的数据没有缓存起来

ViewModel缓存数据要在属性中
还有就是paging返回的是flow，需要用**cachedIn(viewModelScope)**来让paging的flow的生命周期和ViewModelScope的生命周期保持一致，也就是和activity保持一致

class MovieViewModel: ViewModel() {private val movies by lazy {Pager(config = PagingConfig(pageSize = 8,//第一次加载的数量，16也就是2页initialLoadSize = 16,//预刷新的距离，距离最后一个item多远时加载数据，默认为pageSizeprefetchDistance = 8),pagingSourceFactory = {MoviePagingSource()}).flow.cachedIn(viewModelScope)}fun loadMovie(): Flow<PagingData<Movie>> = movies}