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// Copyright (c) 2015, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.
import 'dart:async';
/// A class for running an asynchronous function exactly once and caching its
/// result.
/// An `AsyncMemoizer` is used when some function may be run multiple times in
/// order to get its result, but it only actually needs to be run once for its
/// effect. To memoize the result of an async function, you can create a
/// memoizer outside the function (for example as an instance field if you want
/// to memoize the result of a method), and then wrap the function's body in a
/// call to [runOnce].
/// This is useful for methods like `close()` and getters that need to do
/// asynchronous work. For example:
/// ```dart
/// class SomeResource {
/// final _closeMemo = AsyncMemoizer();
/// Future close() => _closeMemo.runOnce(() {
/// // ...
/// });
/// }
/// ```
class AsyncMemoizer<T> {
/// The future containing the method's result.
/// This can be accessed at any time, and will fire once [runOnce] is called.
Future<T> get future => _completer.future;
final _completer = Completer<T>();
/// Whether [runOnce] has been called yet.
bool get hasRun => _completer.isCompleted;
/// Runs the function, [computation], if it hasn't been run before.
/// If [runOnce] has already been called, this returns the original result.
Future<T> runOnce(FutureOr<T> Function() computation) {
if (!hasRun) _completer.complete(Future.sync(computation));
return future;