Paging 程式庫會追蹤分頁資料的載入要求狀態,並透過 LoadState
類別公開資料。應用程式可透過 PagingDataAdapter
註冊事件監聽器,以便接收目前狀態的相關資訊,並據此更新使用者介面。這些狀態是由轉接程式提供,因為其與使用者介面同步。這表示事件監聽器在使用者介面套用載入網頁時會收到更新。
針對每個 LoadType
和資料來源類型 (PagingSource
或 RemoteMediator
) 會提供獨立的 LoadState
信號。此事件監聽器提供的 CombinedLoadStates
物件會提供所有信號的載入狀態資訊。您可以使用此詳細資訊,向使用者顯示適用的載入指標。
載入狀態
Paging 程式庫會透過 LoadState
物件公開使用者介面中的載入狀態。根據目前的載入狀態,LoadState
物件採用下列其中一種格式:
- 如果沒有運作中的載入作業且沒有錯誤,則
LoadState
是LoadState.NotLoading
物件。這個子類別也包含endOfPaginationReached
屬性,表示是否已觸及分頁終點。 - 如果有執行中的載入作業,則
LoadState
是LoadState.Loading
物件。 - 如果發生錯誤,則
LoadState
是LoadState.Error
物件。
在您的使用者介面中有兩種使用 LoadState
的方法:使用事件監聽器,或使用特殊的清單轉接程式,直接在 RecyclerView
清單顯示載入狀態。
透過事件監聽器存取載入狀態
如要在使用者介面中查看一般用途的載入狀態,請使用 loadStateFlow
串流或由 PagingDataAdapter
提供的 addLoadStateListener()
方法。這些機制提供 CombinedLoadStates
物件的存取權,其中包含每種載入類型的 LoadState
行為相關資訊。
在下列範例中,PagingDataAdapter
會根據重新整理載入的目前狀態顯示不同的使用者介面元件:
// 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);
});
如要進一步瞭解 CombinedLoadStates
,請參閱「存取其他載入狀態資訊」。
使用轉接程式顯示載入狀態
Paging 程式庫提供另一個名為 LoadStateAdapter
的清單轉接程式,可讓您在目前顯示的分頁資料清單中直接顯示載入狀態。此轉接程式可存取清單的目前載入狀態,方便您傳遞給顯示資訊的自訂檢視畫面持有人。
首先,建立檢視畫面持有人類別,以持續參考螢幕上的載入和錯誤檢視畫面。建立可接受 LoadState
做為參數的 bind()
函式。此函式應根據載入狀態參數來切換檢視畫面的瀏覽權限:
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);
}
}
接下來,請建立導入 LoadStateAdapter
的類別,並定義 onCreateViewHolder()
和 onBindViewHolder()
方法。這些方法會建立自訂檢視畫面持有人的執行個體,並繫結相關聯的載入狀態。
// 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);
}
}
以標頭或頁尾顯示載入狀態
如要在標頭和頁尾顯示載入進度,請透過 PagingDataAdapter
物件呼叫 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 RecyclerView
清單僅在標頭或僅在頁尾會顯示載入狀態,可呼叫 withLoadStateHeader()
或 withLoadStateFooter()
。
存取其他載入狀態資訊
PagingDataAdapter
的 CombinedLoadStates
物件會提供 PagingSource
導入的載入狀態資訊,以及 RemoteMediator
導入的載入狀態資訊 (如果有的話)。
為了方便,您可以使用 CombinedLoadStates
的 refresh
、append
和 prepend
屬性,即可存取適當載入類型的 LoadState
物件。一般來說,這些屬性會延遲 RemoteMediator
導入的載入狀態 (如果有的話);除此之外,其中包含 PagingSource
導入的適當載入狀態。如要進一步瞭解基礎邏輯,請參閱 CombinedLoadStates
的參考說明文件。
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;
});
但請注意,只有 PagingSource
載入狀態可以保證與使用者介面更新保持同步。由於 refresh
、append
以及 prepend
屬性可能會從 PagingSource
或 RemoteMediator
取得載入狀態,故無法保證會與使用者介面更新保持同步。因此,在使用者介面新增任何新資料之前,可能會導致載入顯示完成的使用者介面問題。
基於這個原因,便利存取子針對在標頭或頁尾中顯示載入狀態運作良好,但針對其他用途,您可能需要透過 PagingSource
或 RemoteMediator
特別存取載入狀態。CombinedLoadStates
會提供適用於此目的的 source
和 mediator
屬性。這些屬性會各自公開 LoadStates
物件,其中分別包含 PagingSource
或 RemoteMediator
的 LoadState
物件:
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 的串連運算子
CombinedLoadStates
物件提供載入狀態中所有變更的存取權,因此請務必根據特定事件篩選載入狀態串流。如此可確保您在適當時機更新使用者介面,以避免斷斷續續與不要必要的使用者介面更新。
舉例來說,假設您想顯示空白的檢視畫面,但只有在初始資料載入完成後才能顯示。此用途會要求您確認資料重新整理載入作業已開始,然後等待 NotLoading
狀態來確認重新整理作業已完成。您必須篩除所有您不需要的信號:
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);
}
});
此範例會等到更新重新整理載入狀態,但僅在狀態為 NotLoading
時才會觸發。這能確保在所有使用者介面更新發生前,遠端重新整理作業已全部完成。
Stream API 可讓您執行此類型作業。您的應用程式可以指定所需的載入事件,並在符合適當條件時處理新資料。
目前沒有任何建議。
建議登入 Google 帳戶。