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Tools for creating list controllers. Supports infinite scroll, pagination, lazy loading, sorting, filtering, synchronization, huge lists, etc. Architecture and widgets agnostic.

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A Dart package for creating list controllers with asynchronous loading, pagination, filtering, data actualization, etc. The main objectives of the library: synchronization of processes of data loading and updating, minimizing the load on the data source. The library does not impose any restrictions on the data types used, methods of state management, other packages used, widgets.

Live demo

Demo


Features #

The package provides the following features for creating list controllers:

  • retrieving, sorting and filtering list data (see RecordsLoader, example)
  • actualization of list data without the need to reload it (see HotList, example)
  • infinite scrolling with support for any pagination strategy: keyset (see KeysetPagination, example), offset (see OffsetPagination, example), etc.
  • supporting multiple directions of loading list data, for example, to create lists with endless forward and backward scrolling (example)
  • interrupting data loading, for example, if filter criteria have been changed (see RecordsLoader, example)
  • reloading data, for example, in case of an error during the previous load (see RecordsLoader, example)
  • displaying intermediate results of loading list data. For example, adding line-by-line records to the list as they are ready (see RecordsLoader, example)
  • tracking changes of a list records to perform arbitrary actions (see RecordsTracker, example)

Advantages #

  • ensures balance between the ease of implementation and flexibility;
  • minimizes the load on a data source;
  • architecturally independent and compatible with any state management approach;
  • compatible with any widgets and can be used even without them;
  • works like a constructor — functionality can be added when needed;
  • does not impose any restrictions on the object for storing the state;
  • supports complex scenarios and allows for implementating additional functionality;
  • automatically synchronizes the processes of loading and updating the data with each other;
  • provides auxiliary tools that may come in handy when developing a list controller.

Usage #

  1. Define the class of a list controller;
  2. Add a ListCore mixin into it;
  3. Add other mixins with required functionality;
  4. Implement the simple class functions required for the mixins you use.

Generic types #

The package uses the following Generic type names:

Record — a list entry

Query — criteria of the records to be displayed (see RecordsLoader). For all optimization mechanisms to work, it is important that different Query objects with the same contents are equivalent. To do this, use the equatable package or override the == operator.

LoadResult — representation of the result of loaded data (see RecordsLoader).

Key — unique identifier of a record (usually int or String) (see HotList).

Event — actions performed on records in the list (see HotList).

DecisionRecord — simplified representation of Record on the basis of which decision on inclusion of Record in the list is made. Usually has the same type as Record or Event (see HotList).

Update — change in a Record (see RecordsTracker).

Offset — representation of record offset in case of an offset pagination (for example, int in case of page number) (see OffsetPagination).

Storing the state of the list #

The package does not impose restrictions on the object for storing the list state. You can use any object, from List<any> to objects of any complexity.

The library provides the class called ListState to store the state. You can use it if it meets your needs. The class is defined as follows.

@immutable
class ListState<Record, Query> {
  ListState({
    required this.query,
    this.records = const [],
    this.stage = const IdleListStage(),
    this.isInitialized = false,
  });
}

query — description of the criteria of the records included in the list;

records — the list of records;

stage — stage the list is at. It may have a value: ListStage.[idle/error/loading/complete]();

isInitialized - whether the list is initialized. Typically, this means that the first data retrieval was successful.

This class will also be used in some of the examples below.

Mixins #

ListCore #

ListCore<Record>

Provides basic functionality for the other parts of the list controller (mixins), as well as the ability to synchronize actions between them.

In the list of mixins of the controller, the ListCore mixin must go first.

Methods

waitAllLoadsToComplete() → Future<void>

Returns Future, which completes either immediately if there are no other loads, or upon completion of all loads.

resetController() → void

Interrupts all processes occurring in the list controller: loading, updating records and so forth.

Example

Live example, code

class BasicListController extends ValueNotifier<List<int>>
    with ListCore<int> {
  BasicListController() : super(List<int>.generate(100, (i) => i + 1));

  @override
  void dispose() {
    closeList();
    super.dispose();
  }
}

RecordsLoader #

RecordsLoader<Record, Query, LoadResult>

Implements the loading of data for the list from a specified source. Whether it is a local database or a remote server.

To load records, use the function loadRecords(Query query, {LoadingKey loadingKey = defaultLoadingKey}).

Many functions of this mixin have a loadingKey parameter. It is used to identify a particular data load and to synchronize the loads with each other. Its value depends on the mixins used and/or the implementation of the list controller. Loads with the same key preempt each other (can not pass at the same time), and those with different preempt don't. See Synchronization scenarios for details.

Implementation

The following functions need to be implemented in the list controller:

onRecordsLoadStart({required Query query, required String loadingKey}) → void

Called at the beginning of loading. Usually, this function should set a loading status in your list state object (for example, it might be executing ...isLoading = true).

performLoadQuery({required Query query, required String loadingKey}) → Future<LoadResult>

The function is responsible for retrieving data from a source. If an error occurs during the loading of data, the function must handle this error (e.g. set error status ...hasError = true in the list state object and throw HandledLoadingException, example). If it is necessary to interrupt loading for some reason, this can be done by throwing InterruptLoading.

putLoadResultToState({required Query query, required LoadResult loadResult, required String loadingKey}) → void

The function should add loaded records to the list state. You may also need to implement resetting the loading status in the list state object (like ...isLoading = false).

onRecordsLoadCancel({required Query query, required String loadingKey}) → void

Called in case of a loading cancellation. This situation can occur when one data load is interrupted by another or by closing the controller (see Synchronization scenarios). In this function, you may need to implement resetting of the loading status in the list state object (like ...isLoading = false). The function can also be used when data should be put into the list in a separate Isolate in order to close it (example).

Methods

isRecordsLoading(LoadingKey loadingKey, {Query? query}) → bool

Tells whether the loading of records is identified by the loadingKey and the query is currently being executed.

isAnyRecordsLoading() → bool

Tells whether any records are being loaded at the moment.

isLoadSucessful(LoadingKey loadingKey) → bool?

Indicates whether the loading of the records identified with the loadingKey key has succeeded. If there has been no loading with this key, the result is null.

repeatQuery(String loadingKey) → Future<void>

Initiates a repeat of a failed loading attempt identified by the loadingKey key.

repeatUnsuccessfulQueries() → Future<void>

Initiates a repeat of all failed loading attempts.

Example

Live example, code

class ListState {
  const ListState({this.records = const [], this.isLoading = false});

  final List<int> records;
  final bool isLoading;
}

class AsyncRecordsLoadingListController extends ValueNotifier<ListState> 
    with
        ListCore<int>,
        RecordsLoader<int, void, List<int>> {
  AsyncRecordsLoadingListController() : super(const ListState()) {
    loadRecords(null);
  }

  @override
  void dispose() {
    closeList();
    super.dispose();
  }

  // RecordsLoader section:

  @override
  void onRecordsLoadStart({required void query, required LoadingKey loadingKey}) => value = const ListState(isLoading: true);

  @override
  Future<List<int>> performLoadQuery({required void query, required LoadingKey loadingKey}) async {
    await Future.delayed(const Duration(milliseconds: 1500));
    return List<int>.generate(100, (i) => i + 1);
  }

  @override
  void putLoadResultToState({required void query, required List<int> loadResult, required LoadingKey loadingKey}) => value = ListState(records: loadResult);
}

KeysetPagination #

KeysetPagination<Record, Query, LoadResult>

Implements the loading of the next page of records based on previously loaded records. In the list of mixins, it should be placed after RecordsLoader.

To load the first page of records, use the loadRecords() function. The next pages of records are loaded by calling the loadNextPage() function. If the list is at a stage where loading the next page makes no sense, this loading will not be done. If at the time of calling the function loadNextPage() the list is still being actualized, then the loading of the next page will be continued after the actualizing. This is necessary so that the buildNextPageQuery() function can build a query to load the next page based on the actual list.

Implementation

The following functions need to be implemented in the list controller:

getListStage(String loadingKey) → ListStage

The function must convert the current state of the list into an object representing the current stage of the list (ListStage.idle(), ListStage.error(), ListStage.loading(), ListStage.complete()).

buildNextPageQuery(String loadingKey) → Query

Based on loadingKey, the function should build a Query object that will be used to load the next page of the list by the performLoadingQuery() function.

Example

Live example, code

typedef ExListState = ListState<int, int>;

class KeysetPaginationListController extends ValueNotifier<ExListState>
    with
        ListCore<int>,
        RecordsLoader<int, int, LoadResult>,
        KeysetPagination<int, int, LoadResult> {

  ...

  // KeysetPagination section:

  @override
  ListStage getListStage(LoadingKey loadingKey) => value.stage;

  @override
  int buildNextPageQuery(LoadingKey loadingKey) => value.records.last;
}

OffsetPagination #

OffsetPagination<Record, Query, LoadResult, Offset>

Implements the loading of list records based on offset Offset (for example, it can be int with page order number). In the mixins list, it should go after RecordsLoader.

Use the loadPage(Offset offset) function to load pages with records.

Implementation

The following functions need to be implemented in the list controller:

offsetToLoadingKey(Offset offset) → String

This function should generate a loading key based on the offset Offset. If multiple pages can be loaded simultaneously, each Offset must have a unique loading key. Otherwise, it can have any value (example).

buildOffsetQuery(Offset offset) → Query

Based on offset, the function should build a Query object that will be used to load the list page by the performLoadingQuery() function.

Example

Live example, code

class OffsetPaginationListController extends ValueNotifier<ExListState>
    with
        ListCore<int>,
        RecordsLoader<int, ListQuery, LoadResult>,
        OffsetPagination<int, ListQuery, LoadResult, int> {
  ...

  // OffsetPagination section

  @override
  String offsetToLoadingKey(int offset) => 'page$offset';

  @override
  ListQuery buildOffsetQuery(int offset) => value.query.copyWith(page: offset);
}

HotList #

HotList<Key, Event, DecisionRecord, Record>

Implements the updating (actualization) of individual list entries.

If some records of the list have been changed, then — instead of updating the entire list — it is more efficient to update only changed records in order to keep the list up to date. HotList simplifies this work.

The following is the process of updating the list:

changes1 —[Event]⟶ buffer2 —[List<Event>]⟶ expandHotListEvents3 —[RecordUpdates<DecisionRecord, Key>]⟶ processing4 —[HotListChanges<DecisionRecord, Key>]⟶ convertDecisionRecords5 —[HotListChanges<Record, Key>]⟶ updateHotList6

  1. To start HotList, you need to call the function initHotList(Stream<Event> changes), specifying the notification stream of changes in the data source.
  2. If data loading is in progress, the controller will wait for the loading to complete by buffering events.
  3. The expandHotListEvents function should convert the event list into a RecordUpdates object. This object contains a simplified representation of the records. The simplified representation should be sufficient to decide whether to include a record in the list.
  4. Based on RecordUpdates, the controller makes decisions about including records in the list.
  5. The simplified record representation DecisionRecord is converted into a Record object.
  6. Updating of the list.

If you are working with transactional databases, it is important that events are processed only after the transaction is closed. To reduce the load on the list controller and data source, it is recommended to receive these events (which occurred during the transaction) in batches. In this case, Event will be a list of events and the definition of the initialization function HotList will have approximately the following form initHotList(Stream<List<MyEvent>> changes).

If the list managed by the controller contains records that are updated automatically (such as the list of Cubits), the initHotList function should not receive events related to record updates, and the expandHotListEvents function should return a RecordUpdates object with an empty updatedRecords parameter.

Implementation

The following functions need to be implemented in the list controller:

expandHotListEvents(List<Event> events) → FutureOr<RecordUpdates<DecisionRecord, Key>>

Converts an Event notification list into an RecordUpdates object containing the lists of created, updated and deleted entries. The entries of this object will be used to make decisions about including specific entries in the list. The Event list included in the function is formed from the events that occurred during the time the records were loaded from the data source (see Synchronization scenarios). If no data was loaded, there will be only one item in this list.

getDecisionRecordKey(DecisionRecord record) → Key

The function should return the unique identifier of the DecisionRecord record.

hasListRecord(Key key) → bool

The function should tell if the current list contains a record with the unique identifier Key.

recordFits(DecisionRecord record) → bool

The function should tell if the DecisionRecord is suitable for being added to the list.

convertDecisionRecords(Set<DecisionRecord> records) → Future<List<Record>>

The function should convert the list of DecisionRecord into a list of Record.

updateHotList(HotListChanges<Record, Key> changes)

Called when the list of records needs to be modified. Changes are described by the HotListChanges object that contains records to be added to the list and unique identifiers (keys) of records to be removed from the list.

Example

Since demonstrating how HotList works requires a data source with rather complex functionality, we use ExampleRecordRepository as a data source to simplify the example. Its code is available at the link.

Live example, code

typedef ExListState = ListState<ExampleRecord, ExampleRecordQuery>;

class ActualizingListController extends ValueNotifier<ExListState>
    with
        ListCore<ExampleRecord>,
        RecordsLoader<ExampleRecord, ExampleRecordQuery, List<ExampleRecord>>,
        HotList<int, ExampleRecordEvent, ExampleRecord, ExampleRecord> {
  ActualizingListController(this.repository) : super(ExListState(query: const ExampleRecordQuery(weightLte: 100))) {
    loadRecords(value.query);

    initHotList(repository.dbEvents);
  }

  ...

  // HotList section:

  @override
  Future<RecordUpdates<ExampleRecord, int>> expandHotListEvents(List<ExampleRecordEvent> events) async {
    final Iterable<ID> createdIds = events.whereType<ExampleRecordCreatedEvent>().map((event) => event.id);
    final Iterable<ID> updatedIds = events.whereType<ExampleRecordUpdatedEvent>().map((event) => event.id);
    final List<ExampleRecord> retrievedRecords = await repository.getByIds({...createdIds, ...updatedIds});

    return RecordUpdates(
      insertedRecords: retrievedRecords.where((r) => createdIds.contains(r.id)),
      updatedRecords: retrievedRecords.where((r) => updatedIds.contains(r.id)),
      deletedKeys: events.whereType<ExampleRecordDeletedEvent>().map((e) => e.id),
    );
  }

  @override
  ID getDecisionRecordKey(ExampleRecord record) => record.id;

  @override
  bool hasListRecord(ID key) => value.records.map((record) => record.id).contains(key);

  @override
  bool recordFits(ExampleRecord record) => value.query.fits(record);

  @override
  Iterable<ExampleRecord> convertDecisionRecords(Set<ExampleRecord> records) => records;

  @override
  void updateHotList(HotListChanges<ExampleRecord, int> changes) {
    final List<ExampleRecord> newRecordsList = value.records.where((r) => !changes.recordKeysToRemove.contains(r.id)).toList()
      ..insertAll(0, changes.recordsToInsert)
      ..sort(value.query.compareRecords);

    value = value.copyWith(records: newRecordsList);
  }
}

RecordsTracker #

RecordsTracker<Record, Update>

Tracks changes of individual records in the list. The records must be able to provide a Stream for tracking (as records, for example, Cubits from package Bloc can be used). When the list is changed, the new records must be registered for tracking by calling registerRecordsToTrack(List<Record> records).

Implementation

The following functions need to be implemented in the list controller:

buildTrackingStream(Record record) → Stream<Update>

The function should return a Stream of record updates.

onTrackEventOccur(List<Update> updatedRecords)

Called when there were changes in the records. updatedRecords contains a list of changes during RecordsTracker.trackingPeriod.

Example

Live example, code

The following data structures are used in the example:

import 'package:bloc/bloc.dart';

class RecordData {
  RecordData({required this.color, required this.weight, required this.initPosition});
  final Color color;
  final int weight;
  final int initPosition;
}

class RecordCubit extends Cubit<RecordData> {
  RecordCubit(int initPosition)
      : super(RecordData(
          color: Colors.primaries[Random().nextInt(Colors.primaries.length)],
          weight: 1,
          initPosition: initPosition,
        ));

  void increase() => emit(RecordData(weight: state.weight + 1, color: state.color, initPosition: state.initPosition));
}

Controller example:

class RecordTrackingListController extends ValueNotifier<List<RecordCubit>>
    with
        ListCore<RecordCubit>,
        RecordsTracker<RecordCubit, RecordData> {
  RecordTrackingListController()
      : super(List<RecordCubit>.generate(10, (i) {
          return RecordCubit(i);
        }).toList()) {
    registerRecordsToTrack(value);
  }

  @override
  void dispose() {
    for (final record in value) {
      record.close();
    }

    closeList();
    super.dispose();
  }

  // RecordsTracker section:

  @override
  Stream<RecordData> buildTrackingStream(Cubit<RecordData> record) => record.stream;

  @override
  void onTrackEventOccur(List<RecordData> updatedRecords) {
    value = List.of(value)..sort((a, b) => b.state.weight.compareTo(a.state.weight));
  }
}

Synchronization scenarios #

The table below shows how different sequences of actions on the list will be handled.

The first column shows the actions are being performed. The header shows the requests to perform the new action. The key_ and query_ in the brackets are given to understand whether the new action will be performed with new or the same keys and queries as the action being performed. The intersection describes the way each action will be handled:

continue — the action will continue;

ignore — the action will not be executed;

cancel — the action will be canceled;

wait — before starting a new action, it will wait for the completion of the previous action.

↓ action being performed; triggered action → A. Load (key1, query1) B. Load (key1, query2) C. Load (key2, query3) D. Actualize records
1. Loading (key1, query1) 🟩 1 — continue; A — ignore 🟥 1 — cancel; B — continue 🟦 1 — continue; C — continue 🟪 1 — wait; D — continue
2. Loading (key1, query2) 🟥 2 — cancel; A — continue 🟩 2 — continue; B — ignore 🟦 2 — continue; C — continue 🟪 2 — wait; D — continue
3. Loading (key2, query3) 🟦 3 — continue; A — continue 🟦 3 — continue; B — continue 🟩 3 — continue; C — ignore 🟪 3 — wait; D — continue
4. Actualizing records 🟦 4 — continue; A — continue 🟦 4 — continue; B — continue 🟦 4 — continue; C — continue 🟪 4 — wait; D — continue

Bloc list controller #

The list_controller package supports the Bloc library. To simplify the creation of event handlers, you can use the event classes provided by the package if they are relevant to your tasks.

LoadRecordsEvent<Query>

Initiates the loading of records. Mirrors the RecordsLoader.loadRecords function that should be called in the handler.

void _onLoadRecords(LoadRecordsEvent<ExampleRecordQuery> event, Emitter<ListState> emit) => loadRecords(event.query, loadingKey: event.loadingKey);

RecordsLoadStartEvent<Query>

The event must be called from the overridden function RecordsLoader.onRecordsLoadStart. The handler must perform the same actions as those for this function.

@override
void onRecordsLoadStart({required ExampleRecordQuery query, required LoadingKey loadingKey}) =>
    add(RecordsLoadStartEvent(query: query, loadingKey: loadingKey));

void _onRecordsLoadStart(RecordsLoadStartEvent<ExampleRecordQuery> event, Emitter<ListState> emit) {
  ...

RecordsLoadCancelEvent<Query>

The event must be called from the overridden function RecordsLoader.onRecordsLoadCancel. The handler must perform the same actions as those for this function.

@override
void onRecordsLoadCancel({required ExampleRecordQuery query, required LoadingKey loadingKey}) =>
    add(RecordsLoadCancelEvent(query: query, loadingKey: loadingKey));

void _onRecordsLoadCancel(RecordsLoadCancelEvent<ExampleRecordQuery> event, Emitter<ListState> emit) {
  ...

LoadingErrorEvent<Query>

The event can be used when an error occurs in the overridden RecordsLoader.performLoadQuery function. It should be used to set an error in the list state. Normally, a HandledLoadingException should be thrown after this event is called.

@override
Future<List<ExampleRecord>> performLoadQuery({required ExampleRecordQuery query, required String loadingKey}) async {
  try {
    return await loadMyRecords(...);
  } on SomeException {
    add(LoadingErrorEvent<ExampleRecordQuery>(query: query, loadingKey: loadingKey));
    throw HandledLoadingException();
  }
}

PutLoadResultToStateEvent<Query, LoadResult>

The event must be called from the overridden function RecordsLoader.putLoadResultToState. The handler must perform the same actions as those for this function.

@override
void putLoadResultToState({required ExampleRecordQuery query, required List<ExampleRecord> loadResult, required LoadingKey loadingKey}) =>
    add(PutLoadResultToStateEvent(query: query, loadResult: loadResult, loadingKey: loadingKey));

void _onPutLoadResultToState(PutLoadResultToStateEvent<ExampleRecordQuery, List<ExampleRecord>> event, Emitter<ListState> emit) {
  ...

RepeatQueryEvent

Triggers the reloading of records in case the previous attempt failed. Mirrors the RecordsLoader.repeatQuery function that should be called in the handler.

void _onRepeatQuery(RepeatQueryEvent event, Emitter<ListState> emit) => repeatQuery(event.loadingKey);

LoadNextPageEvent

Triggers the loading of the next page of records. Usually used together with the KeysetPagination or OffsetPagination mixins and used to call the KeysetPagination.loadNextPage or OffsetPagination.loadPage functions respectively.

void _onLoadNextPage(LoadNextPageEvent event, Emitter<ListState> emit) => loadNextPage();

LoadNextPageDirectedEvent

The purpose is the same as the LoadNextPageEvent event, except that the constructor contains the loadingKey parameter. It is used for handling lists that have two or more directions of records to load.

UpdateHotListEvent<Record, Key>

The event must be called from an overridden function HotList.updateHotList. The handler must perform the same actions as for this function.

@override
void updateHotList(HotListChanges<ExampleRecord, PK> changes) => add(UpdateHotListEvent(changes: changes));

void _onUpdateHotList(UpdateHotListEvent<ExampleRecord, PK> event, Emitter<ListState> emit) {
  ...

Additional information #

Pagination with Flutter's ListView #

There are different ways of initiating the loading of the next data page. The simplest is when a certain item from the end of the list is displayed on the screen:

@override
Widget build(BuildContext context) {
  return ListView.builder(
    itemCount: listState.records.length,
    itemBuilder: (context, index) {
      // The number of the record from the end of the list, the display of which initiates the next page load:
      const recordsLoadThreshold = 1;

      if (index >= listState.records.length - recordsLoadThreshold) {
        WidgetsBinding.instance.addPostFrameCallback((_) {
          loadNextPage(); // From KeysetPagination mixin
        });
      }

      return ListTile(title: Text(listState.records[index]));
    },
  );
}

Project status #

This package is currently under development and the API can change in the future.

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Publisher

verified publisherastonio.com

Tools for creating list controllers. Supports infinite scroll, pagination, lazy loading, sorting, filtering, synchronization, huge lists, etc. Architecture and widgets agnostic.

Repository (GitHub)
View/report issues

Documentation

API reference

License

MIT (license)

Dependencies

async, collection, meta, rxdart

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