LiveData 分页列表大小始终为 0

Question

我使用 android 分页库实现了分页。 ViewModel returns a LiveData<PagedList<MyObject>> 我观察到在我设置适配器的片段中一切正常，除了当我想检查列表的大小时总是 0。

viewModel.getSearchResults().observe(this, mList -> {
           adapter.submitList(mList);
           Log.e("Count", String.valueOf(mList.size()));
       });

其中 mList 是 PagedList<MyObject>

Answer 1

如果您查看 PagedList 文档中的 Loading Data，您会注意到以下内容：

If you use LivePagedListBuilder to get a LiveData<PagedList>, it will initialize PagedLists on a background thread for you.

此外，Mutability and Snapshots 声明如下：

A PagedList is mutable while loading, or ready to load from its DataSource. As loads succeed, a mutable PagedList will be updated via Runnables on the main thread. You can listen to these updates with a PagedList.Callback. (Note that PagedListAdapter will listen to these to signal RecyclerView about the updates/changes).

如果您想监听事件 onInserted、onChanged 或 onRemoved，您可以执行以下操作：

viewModel.observableData.observe(viewLifecycleOwner, Observer { pagedList ->
    adapter.submitList(pagedList)
    pagedList.addWeakCallback(null, object: PagedList.Callback() {
        override fun onChanged(position: Int, count: Int) {}
        override fun onInserted(position: Int, count: Int) {
            println("count: $count")
        }
        override fun onRemoved(position: Int, count: Int) {}
    })
})

Answer 2

如果你使用 Rxjava3，

RxJava3CallAdapterFactory.create() ，默认不同步。

您需要使用RxJava3CallAdapterFactory.createSynchronous()。