管理和显示加载状态

Paging 库会跟踪分页数据的加载请求状态,并通过 LoadState 类将其公开。应用可以向 PagingDataAdapter 注册一个监听器,用于接收有关当前状态的信息并相应地更新界面。这些状态在该适配器中提供,因为它们与界面保持同步。这意味着,如果已对界面应用页面加载,您的监听器就会收到更新。

每个 LoadType 和数据源类型(PagingSourceRemoteMediator)都会获得一个单独的 LoadState 信号。监听器提供的 CombinedLoadStates 对象则会提供来自所有这些信号的加载状态的信息。您可以利用此详细信息向用户显示相应的加载指示器。

加载状态

Paging 库通过 LoadState 对象公开要在界面中使用的加载状态。LoadState 对象根据当前的加载状态采用以下三种形式之一:

可通过两种方法在界面中使用 LoadState:使用监听器,或使用特殊的列表适配器直接在 RecyclerView 列表中显示加载状态。

使用监听器获取加载状态

为了获取加载状态以用于界面中的一般用途,请使用 loadStateFlow 流或 PagingDataAdapter 提供的 addLoadStateListener() 方法。这些机制可用于获取 CombinedLoadStates 对象,该对象包含有关每个加载类型的 LoadState 行为的信息。

在以下示例中,PagingDataAdapter 根据刷新加载的当前状态显示不同的界面组件:

// Activities can use lifecycleScope directly, but Fragments should instead use
// viewLifecycleOwner.lifecycleScope.
lifecycleScope
.launch {
  pagingAdapter
.loadStateFlow.collectLatest { loadStates ->
    progressBar
.isVisible = loadStates.refresh is LoadState.Loading
    retry
.isVisible = loadState.refresh !is LoadState.Loading
    errorMsg
.isVisible = loadState.refresh is LoadState.Error
 
}
}
pagingAdapter.addLoadStateListener(loadStates -> {
  progressBar
.setVisibility(loadStates.refresh instanceof LoadState.Loading
   
? View.VISIBLE : View.GONE);
  retry
.setVisibility(loadStates.refresh instanceof LoadState.Loading
   
? View.GONE : View.VISIBLE);
  errorMsg
.setVisibility(loadStates.refresh instanceof LoadState.Error
   
? View.VISIBLE : View.GONE);
});
pagingAdapter.addLoadStateListener(loadStates -> {
  progressBar
.setVisibility(loadStates.refresh instanceof LoadState.Loading
   
? View.VISIBLE : View.GONE);
  retry
.setVisibility(loadStates.refresh instanceof LoadState.Loading
   
? View.GONE : View.VISIBLE);
  errorMsg
.setVisibility(loadStates.refresh instanceof LoadState.Error
   
? View.VISIBLE : View.GONE);
});

如需详细了解 CombinedLoadStates,请参阅获取更多加载状态信息

使用适配器显示加载状态

Paging 库提供了另一个名为 LoadStateAdapter 的列表适配器,用于直接在显示的分页数据列表中呈现加载状态。您可以通过此适配器获取列表的当前加载状态,将该状态传递给显示该信息的自定义 ViewHolder。

首先,创建一个 ViewHolder 类,用于保留对屏幕上的加载视图和错误视图的引用。创建一个接受 LoadState 作为参数的 bind() 函数。此函数应根据加载状态参数切换视图可见性:

class LoadStateViewHolder(
  parent
: ViewGroup,
  retry
: () -> Unit
) : RecyclerView.ViewHolder(
 
LayoutInflater.from(parent.context)
   
.inflate(R.layout.load_state_item, parent, false)
) {
 
private val binding = LoadStateItemBinding.bind(itemView)
 
private val progressBar: ProgressBar = binding.progressBar
 
private val errorMsg: TextView = binding.errorMsg
 
private val retry: Button = binding.retryButton
   
.also {
      it
.setOnClickListener { retry() }
   
}

 
fun bind(loadState: LoadState) {
   
if (loadState is LoadState.Error) {
      errorMsg
.text = loadState.error.localizedMessage
   
}

    progressBar
.isVisible = loadState is LoadState.Loading
    retry
.isVisible = loadState is LoadState.Error
    errorMsg
.isVisible = loadState is LoadState.Error
 
}
}
class LoadStateViewHolder extends RecyclerView.ViewHolder {
 
private ProgressBar mProgressBar;
 
private TextView mErrorMsg;
 
private Button mRetry;

 
LoadStateViewHolder(
   
@NonNull ViewGroup parent,
   
@NonNull View.OnClickListener retryCallback) {
   
super(LayoutInflater.from(parent.getContext())
     
.inflate(R.layout.load_state_item, parent, false));

   
LoadStateItemBinding binding = LoadStateItemBinding.bind(itemView);
    mProgressBar
= binding.progressBar;
    mErrorMsg
= binding.errorMsg;
    mRetry
= binding.retryButton;
 
}

 
public void bind(LoadState loadState) {
   
if (loadState instanceof LoadState.Error) {
     
LoadState.Error loadStateError = (LoadState.Error) loadState;
      mErrorMsg
.setText(loadStateError.getError().getLocalizedMessage());
   
}
    mProgressBar
.setVisibility(loadState instanceof LoadState.Loading
     
? View.VISIBLE : View.GONE);
    mRetry
.setVisibility(loadState instanceof LoadState.Error
     
? View.VISIBLE : View.GONE);
    mErrorMsg
.setVisibility(loadState instanceof LoadState.Error
     
? View.VISIBLE : View.GONE);
 
}
}
class LoadStateViewHolder extends RecyclerView.ViewHolder {
 
private ProgressBar mProgressBar;
 
private TextView mErrorMsg;
 
private Button mRetry;

 
LoadStateViewHolder(
   
@NonNull ViewGroup parent,
   
@NonNull View.OnClickListener retryCallback) {
   
super(LayoutInflater.from(parent.getContext())
     
.inflate(R.layout.load_state_item, parent, false));

   
LoadStateItemBinding binding = LoadStateItemBinding.bind(itemView);
    mProgressBar
= binding.progressBar;
    mErrorMsg
= binding.errorMsg;
    mRetry
= binding.retryButton;
 
}

 
public void bind(LoadState loadState) {
   
if (loadState instanceof LoadState.Error) {
     
LoadState.Error loadStateError = (LoadState.Error) loadState;
      mErrorMsg
.setText(loadStateError.getError().getLocalizedMessage());
   
}
    mProgressBar
.setVisibility(loadState instanceof LoadState.Loading
     
? View.VISIBLE : View.GONE);
    mRetry
.setVisibility(loadState instanceof LoadState.Error
     
? View.VISIBLE : View.GONE);
    mErrorMsg
.setVisibility(loadState instanceof LoadState.Error
     
? View.VISIBLE : View.GONE);
 
}
}

首先,创建一个实现 LoadStateAdapter 的类,并定义 onCreateViewHolder()onBindViewHolder() 方法:这些方法会创建自定义 ViewHolder 的一个实例,并绑定关联的加载状态。

// Adapter that displays a loading spinner when
// state is LoadState.Loading, and an error message and retry
// button when state is LoadState.Error.
class ExampleLoadStateAdapter(
 
private val retry: () -> Unit
) : LoadStateAdapter<LoadStateViewHolder>() {

 
override fun onCreateViewHolder(
    parent
: ViewGroup,
    loadState
: LoadState
 
) = LoadStateViewHolder(parent, retry)

 
override fun onBindViewHolder(
    holder
: LoadStateViewHolder,
    loadState
: LoadState
 
) = holder.bind(loadState)
}
// Adapter that displays a loading spinner when
// state is LoadState.Loading, and an error message and retry
// button when state is LoadState.Error.
class ExampleLoadStateAdapter extends LoadStateAdapter<LoadStateViewHolder> {
 
private View.OnClickListener mRetryCallback;

 
ExampleLoadStateAdapter(View.OnClickListener retryCallback) {
    mRetryCallback
= retryCallback;
 
}

 
@NotNull
 
@Override
 
public LoadStateViewHolder onCreateViewHolder(@NotNull ViewGroup parent,
   
@NotNull LoadState loadState) {
   
return new LoadStateViewHolder(parent, mRetryCallback);
 
}

 
@Override
 
public void onBindViewHolder(@NotNull LoadStateViewHolder holder,
   
@NotNull LoadState loadState) {
    holder
.bind(loadState);
 
}
}
// Adapter that displays a loading spinner when
// state is LoadState.Loading, and an error message and retry
// button when state is LoadState.Error.
class ExampleLoadStateAdapter extends LoadStateAdapter<LoadStateViewHolder> {
 
private View.OnClickListener mRetryCallback;

 
ExampleLoadStateAdapter(View.OnClickListener retryCallback) {
    mRetryCallback
= retryCallback;
 
}

 
@NotNull
 
@Override
 
public LoadStateViewHolder onCreateViewHolder(@NotNull ViewGroup parent,
   
@NotNull LoadState loadState) {
   
return new LoadStateViewHolder(parent, mRetryCallback);
 
}

 
@Override
 
public void onBindViewHolder(@NotNull LoadStateViewHolder holder,
   
@NotNull LoadState loadState) {
    holder
.bind(loadState);
 
}
}

如需在页眉和页脚中显示加载进度,请从 PagingDataAdapter 对象调用 withLoadStateHeaderAndFooter() 方法:

pagingAdapter
 
.withLoadStateHeaderAndFooter(
    header
= ExampleLoadStateAdapter(adapter::retry),
    footer
= ExampleLoadStateAdapter(adapter::retry)
 
)
pagingAdapter
 
.withLoadStateHeaderAndFooter(
   
new ExampleLoadStateAdapter(pagingAdapter::retry),
   
new ExampleLoadStateAdapter(pagingAdapter::retry));
pagingAdapter
 
.withLoadStateHeaderAndFooter(
   
new ExampleLoadStateAdapter(pagingAdapter::retry),
   
new ExampleLoadStateAdapter(pagingAdapter::retry));

如果您想让 RecyclerView 列表仅在页眉中或仅在页脚中显示加载状态,可改为调用 withLoadStateHeader()withLoadStateFooter()

获取更多加载状态信息

PagingDataAdapter 中的 CombinedLoadStates 对象会提供 PagingSource 实现的加载状态信息,如果存在 RemoteMediator 实现,也会提供其加载状态信息。

为方便起见,您可以使用 CombinedLoadStates 中的 refreshappendprepend 属性访问相应加载类型的 LoadState 对象。这些属性通常取决于 RemoteMediator 实现(如有)中的加载状态;如没有该实现,这些属性将包含 PagingSource 实现中的相应加载状态。如需详细了解底层逻辑,请参阅 CombinedLoadStates 的参考文档。

lifecycleScope.launch {
  pagingAdapter
.loadStateFlow.collectLatest { loadStates ->
   
// Observe refresh load state from RemoteMediator if present, or
   
// from PagingSource otherwise.
    refreshLoadState
: LoadState = loadStates.refresh
   
// Observe prepend load state from RemoteMediator if present, or
   
// from PagingSource otherwise.
    prependLoadState
: LoadState = loadStates.prepend
   
// Observe append load state from RemoteMediator if present, or
   
// from PagingSource otherwise.
    appendLoadState
: LoadState = loadStates.append
 
}
}
pagingAdapter.addLoadStateListener(loadStates -> {
 
// Observe refresh load state from RemoteMediator if present, or
 
// from PagingSource otherwise.
 
LoadState refreshLoadState = loadStates.refresh;
 
// Observe prepend load state from RemoteMediator if present, or
 
// from PagingSource otherwise.
 
LoadState prependLoadState = loadStates.prepend;
 
// Observe append load state from RemoteMediator if present, or
 
// from PagingSource otherwise.
 
LoadState appendLoadState = loadStates.append;
});
pagingAdapter.addLoadStateListener(loadStates -> {
 
// Observe refresh load state from RemoteMediator if present, or
 
// from PagingSource otherwise.
 
LoadState refreshLoadState = loadStates.refresh;
 
// Observe prepend load state from RemoteMediator if present, or
 
// from PagingSource otherwise.
 
LoadState prependLoadState = loadStates.prepend;
 
// Observe append load state from RemoteMediator if present, or
 
// from PagingSource otherwise.
 
LoadState appendLoadState = loadStates.append;
});

不过必须注意,只有 PagingSource 加载状态一定会与界面更新保持同步。由于 refreshappendprepend 属性可能会从 PagingSource 获取加载状态,也可能会从 RemoteMediator 获取加载状态,因此不能保证它们一定会与界面更新保持同步。这可能会导致以下界面问题:在任何新数据添加到界面中之前,加载似乎就已完成。

因此,便利访问函数适用于在页眉或页脚中显示加载状态,但在其他用例中,您可能需要从 PagingSourceRemoteMediator 中明确获取加载状态。为此,CombinedLoadStates 提供了 sourcemediator 属性。这两个属性会各公开一个 LoadStates 对象,分别包含 PagingSourceRemoteMediatorLoadState 对象:

lifecycleScope.launch {
  pagingAdapter
.loadStateFlow.collectLatest { loadStates ->
   
// Directly access the RemoteMediator refresh load state.
    mediatorRefreshLoadState
: LoadState? = loadStates.mediator.refresh
   
// Directly access the RemoteMediator append load state.
    mediatorAppendLoadState
: LoadState? = loadStates.mediator.append
   
// Directly access the RemoteMediator prepend load state.
    mediatorPrependLoadState
: LoadState? = loadStates.mediator.prepend
   
// Directly access the PagingSource refresh load state.
    sourceRefreshLoadState
: LoadState = loadStates.source.refresh
   
// Directly access the PagingSource append load state.
    sourceAppendLoadState
: LoadState = loadStates.source.append
   
// Directly access the PagingSource prepend load state.
    sourcePrependLoadState
: LoadState = loadStates.source.prepend
 
}
}
pagingAdapter.addLoadStateListener(loadStates -> {
 
// Directly access the RemoteMediator refresh load state.
 
LoadState mediatorRefreshLoadState = loadStates.mediator.refresh;
 
// Directly access the RemoteMediator append load state.
 
LoadState mediatorAppendLoadState = loadStates.mediator.append;
 
// Directly access the RemoteMediator prepend load state.
 
LoadState mediatorPrependLoadState = loadStates.mediator.prepend;
 
// Directly access the PagingSource refresh load state.
 
LoadState sourceRefreshLoadState = loadStates.source.refresh;
 
// Directly access the PagingSource append load state.
 
LoadState sourceAppendLoadState = loadStates.source.append;
 
// Directly access the PagingSource prepend load state.
 
LoadState sourcePrependLoadState = loadStates.source.prepend;
});
pagingAdapter.addLoadStateListener(loadStates -> {
 
// Directly access the RemoteMediator refresh load state.
 
LoadState mediatorRefreshLoadState = loadStates.mediator.refresh;
 
// Directly access the RemoteMediator append load state.
 
LoadState mediatorAppendLoadState = loadStates.mediator.append;
 
// Directly access the RemoteMediator prepend load state.
 
LoadState mediatorPrependLoadState = loadStates.mediator.prepend;
 
// Directly access the PagingSource refresh load state.
 
LoadState sourceRefreshLoadState = loadStates.source.refresh;
 
// Directly access the PagingSource append load state.
 
LoadState sourceAppendLoadState = loadStates.source.append;
 
// Directly access the PagingSource prepend load state.
 
LoadState sourcePrependLoadState = loadStates.source.prepend;
});

LoadState 中的链操作符

由于 CombinedLoadStates 对象可以获取加载状态的所有更改,因此必须根据特定事件过滤加载状态流。这样可以确保在适当的时间更新界面,以避免卡顿和不必要的界面更新。

例如,假设您想显示一个空视图,但仅在初始数据加载完成后显示。此用例需要验证数据刷新加载是否已开始,然后等待 NotLoading 状态以确认刷新已完成。您必须过滤掉除所需信号外的所有信号:

lifecycleScope.launchWhenCreated {
  adapter
.loadStateFlow
   
// Only emit when REFRESH LoadState for RemoteMediator changes.
   
.distinctUntilChangedBy { it.refresh }
   
// Only react to cases where REFRESH completes, such as NotLoading.
   
.filter { it.refresh is LoadState.NotLoading }
   
// Scroll to top is synchronous with UI updates, even if remote load was
   
// triggered.
   
.collect { binding.list.scrollToPosition(0) }
}
PublishSubject<CombinedLoadStates> subject = PublishSubject.create();
Disposable disposable =
  subject
.distinctUntilChanged(CombinedLoadStates::getRefresh)
 
.filter(
    combinedLoadStates
-> combinedLoadStates.getRefresh() instanceof LoadState.NotLoading)
 
.subscribe(combinedLoadStates -> binding.list.scrollToPosition(0));

pagingAdapter
.addLoadStateListener(loadStates -> {
  subject
.onNext(loadStates);
});
LiveData<CombinedLoadStates> liveData = new MutableLiveData<>();
LiveData<LoadState> refreshLiveData =
 
Transformations.map(liveData, CombinedLoadStates::getRefresh);
LiveData<LoadState> distinctLiveData =
 
Transformations.distinctUntilChanged(refreshLiveData);

distinctLiveData
.observeForever(loadState -> {
 
if (loadState instanceof LoadState.NotLoading) {
    binding
.list.scrollToPosition(0);
 
}
});

此示例将一直等到刷新加载状态更新,但仅在状态为 NotLoading 时触发。这样可以确保在发生任何界面更新之前,远程刷新已完全完成。

流 API 使此类操作成为可能。应用可以指定需要的加载事件,并在满足相应条件时处理新数据。

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