Paging 3 简单使用

需求： *常规列表，有刷新，加载（异常，底部）

依赖：

	//Paging
    def paging_version = "3.0.1"
    implementation "androidx.paging:paging-runtime:$paging_version"
    implementation "androidx.paging:paging-guava:$paging_version"
    testImplementation "androidx.paging:paging-common:$paging_version"

布局

展示代码

		recyclerView.setLayoutManager(new LinearLayoutManager(requireContext()));
        _adapter = new InnerAdapter();
        //刷新UI
        _adapter.addLoadStateListener(new SmartLoadStateListener(_adapter, _binding.refresh));
        //加载更多UI
        ConcatAdapter adapter = _adapter.withLoadStateFooter(new LoadMoreStateAdapter(getLayoutInflater(), _adapter));
        _binding.recyclerView.setAdapter(adapter);

        _viewModel.getOrders().observe(getViewLifecycleOwner(), result -> {
            _adapter.submitData(getLifecycle(), result);
        });

SmartLoadStateListener：响应刷新UI

@Override
    public Unit invoke(CombinedLoadStates combinedLoadStates) {
        LoadState refresh = combinedLoadStates.getRefresh();
        if (refresh instanceof LoadState.Loading) {
            _layout.autoRefreshAnimationOnly();
        } else {
            _layout.finishRefresh();
        }
        return null;
    }

LoadMoreStateAdapter：让没有更多的状态可以展示

@Override
    public boolean displayLoadStateAsItem(@NonNull LoadState loadState) {
        return super.displayLoadStateAsItem(loadState)
                || (loadState instanceof LoadState.NotLoading && loadState.getEndOfPaginationReached());
    }

ViewModel：提供数据给View

public LiveData> getOrders() {
        if (null == data) {
            Pager pager = this.orderRepository.getExceptionOrders(30);
            CoroutineScope viewModelScope = ViewModelKt.getViewModelScope(this);
            LiveData> source = PagingLiveData.cachedIn(PagingLiveData.getLiveData(pager), viewModelScope);
            data = Transformations.map(source, pagingData -> PagingDataTransforms.map(pagingData, AppAgent.getInstance().getWorkExecutor(), ExceptionOrderModel::new));
        }
        return data;
    }

PagingSource

 @Nullable
    @Override
    public Integer getRefreshKey(@NonNull PagingState pagingState) {
        return null;
    }

    @NonNull
    @Override
    public ListenableFuture> loadFuture(@NonNull LoadParams loadParams) {
        int page = loadParams.getKey();
        int size = loadParams.getLoadSize();
        ListenableFuture>> task = Futures.submit(() -> dataSource.loadExceptionOrders(page, size), executor);
        return Futures.transform(task, input -> toLoadResult(input, page, size), executor);
    }
    
    private LoadResult toLoadResult(Result> result, int page, int size) {
        if (result instanceof Result.Success) {
            Result.Success> success = (Result.Success>) result;
            if (success.getData().size() == size) {
                return new LoadResult.Page<>(
                        success.getData()
                        , null
                        , page + 1
                );
            } else {
                return new LoadResult.Page<>(
                        success.getData()
                        , null
                        , null
                );
            }
        } else {
            Result.Fail> fail = (Result.Fail>) result;
            return new LoadResult.Error<>(new LoadErrorException(fail.getMsg()));
        }
    }

需要注意的一点，_adapter.withLoadStateFooter() 会返回一个Adapter，将这个Adapter绑定给recyclerView，才能展示Footer 或者 Headter