86 lines
2.4 KiB
Dart
86 lines
2.4 KiB
Dart
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import 'dart:async';
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import 'dart:collection';
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/// A utility for batching multiple requests together, to improve application performance.
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///
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/// Enqueues batches of requests until the next tick, when they are processed in bulk.
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///
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/// Port of Facebook's `DataLoader`:
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/// https://github.com/graphql/dataloader
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class DataLoader<Id, Data> {
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/// Invoked to fetch a batch of keys simultaneously.
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final FutureOr<Iterable<Data>> Function(Iterable<Id>) loadMany;
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/// Whether to use a memoization cache to store the results of past lookups.
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final bool cache;
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var _cache = <Id, Data>{};
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var _queue = Queue<_QueueItem<Id, Data>>();
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bool _started = false;
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DataLoader(this.loadMany, {this.cache = true});
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Future<void> _onTick() async {
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if (_queue.isNotEmpty) {
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var current = _queue.toList();
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_queue.clear();
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var data = await loadMany(current.map((i) => i.id));
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for (int i = 0; i < current.length; i++) {
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var item = current[i];
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var value = data.elementAt(i);
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if (cache) _cache[item.id] = value;
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item.completer.complete(value);
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}
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}
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_started = false;
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// if (!_closed) scheduleMicrotask(_onTick);
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}
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/// Clears the value at [key], if it exists.
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void clear(Id key) => _cache.remove(key);
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/// Clears the entire cache.
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void clearAll() => _cache.clear();
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/// Primes the cache with the provided key and value. If the key already exists, no change is made.
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///
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/// To forcefully prime the cache, clear the key first with
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/// `loader..clear(key)..prime(key, value)`.
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void prime(Id key, Data value) => _cache.putIfAbsent(key, () => value);
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/// Closes this [DataLoader], cancelling all pending requests.
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void close() {
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while (_queue.isNotEmpty) {
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_queue.removeFirst().completer.completeError(
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StateError('The DataLoader was closed before the item was loaded.'));
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}
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_queue.clear();
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}
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/// Returns a [Future] that completes when the next batch of requests completes.
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Future<Data> load(Id id) {
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if (cache && _cache.containsKey(id)) {
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return Future<Data>.value(_cache[id]);
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} else {
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var item = _QueueItem<Id, Data>(id);
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_queue.add(item);
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if (!_started) {
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_started = true;
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scheduleMicrotask(_onTick);
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}
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return item.completer.future;
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}
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}
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}
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class _QueueItem<Id, Data> {
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final Id id;
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final Completer<Data> completer = Completer();
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_QueueItem(this.id);
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}
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