Use asyncQueue instead of Timers in CachedValue. (dart-lang#7196)

app/lib/search/top_packages.dart

@@ -55,7 +55,7 @@ class TopPackages {
       );
     }
     _running = true;
-    await Future.wait(_values.map((v) => v.start()));
+    await Future.wait(_values.map((v) => v.update()));
   }
 
   @visibleForTesting

app/lib/service/announcement/backend.dart

@@ -36,7 +36,7 @@ class AnnouncementBackend {
 
   /// Update announcements regularly.
   Future<void> start() async {
-    await _announcementHtml.start();
+    await _announcementHtml.update();
   }
 
   /// Cancel updates and free resources.

app/lib/service/youtube/backend.dart

@@ -43,7 +43,7 @@ class YoutubeBackend {
 
   /// Starts the initial and schedules the periodic updates.
   Future<void> start() async {
-    await _packageOfWeekVideoList.start();
+    await _packageOfWeekVideoList.update();
   }
 
   /// Sets the list of PoW videos to a fixed value.

app/lib/shared/cached_value.dart

@@ -7,6 +7,7 @@ import 'dart:async';
 import 'package:clock/clock.dart';
 import 'package:logging/logging.dart';
 import 'package:meta/meta.dart';
+import 'package:pub_dev/service/async_queue/async_queue.dart';
 
 import 'monitoring.dart';
 
@@ -40,10 +41,10 @@ class CachedValue<T> {
   final Duration _interval;
   final Duration _timeout;
   DateTime _lastUpdated = clock.now();
-  Timer? _timer;
   T? _value;
   Completer? _ongoingCompleter;
   bool _closing = false;
+  bool _scheduled = false;
 
   CachedValue({
     required String name,
@@ -59,10 +60,30 @@ class CachedValue<T> {
 
   DateTime get lastUpdated => _lastUpdated;
   Duration get age => clock.now().difference(_lastUpdated);
-  bool get isAvailable => _value != null && age <= _maxAge;
+
+  bool get isAvailable {
+    _scheduleIfNeeded();
+    return _value != null && age <= _maxAge;
+  }
 
   /// The cached value, may be null.
-  T? get value => _value;
+  T? get value {
+    _scheduleIfNeeded();
+    return _value;
+  }
+
+  void _scheduleIfNeeded() {
+    if (!_scheduled && !_closing && (_value == null || age > _interval)) {
+      _scheduled = true;
+      asyncQueue.addAsyncFn(() async {
+        try {
+          await _update();
+        } finally {
+          _scheduled = false;
+        }
+      });
+    }
+  }
 
   @visibleForTesting
   void setValue(T v) {
@@ -71,7 +92,6 @@ class CachedValue<T> {
   }
 
   /// Updates the cached value.
-  @visibleForTesting
   Future<void> update() async {
     if (_closing) {
       throw StateError('Cache `$_name` is already closed.');
@@ -106,28 +126,8 @@ class CachedValue<T> {
     }
   }
 
-  /// Starts a periodic Timer to update the cached value.
-  ///
-  /// If this is the first call of [start], and initial value
-  /// was not set, this will also call [update].
-  Future<void> start() async {
-    if (_closing) {
-      throw StateError('Cache `$_name` is already closed.');
-    }
-    if (!isAvailable && _timer == null) {
-      await _update();
-      // ignore: invariant_booleans
-      if (_closing) return;
-    }
-    _timer ??= Timer.periodic(_interval, (timer) {
-      _update();
-    });
-  }
-
   Future<void> close() async {
     _closing = true;
-    _timer?.cancel();
-    _timer = null;
     if (_ongoingCompleter != null) {
       await _ongoingCompleter!.future;
     }

app/lib/shared/popularity_storage.dart

@@ -56,7 +56,7 @@ class PopularityStorage {
   int lookupAsScore(String package) => (lookup(package) * 100).round();
 
   Future<void> start() async {
-    await _popularity.start();
+    await _popularity.update();
   }
 
   Future<void> close() async {

app/test/shared/cached_value_test.dart

@@ -2,11 +2,14 @@
 // 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 'package:pub_dev/service/async_queue/async_queue.dart';
 import 'package:pub_dev/shared/cached_value.dart';
 import 'package:test/test.dart';
 
+import 'test_services.dart';
+
 void main() {
-  test('set value immediately available', () {
+  testWithProfile('set value immediately available', fn: () async {
     final cv = CachedValue<String>(
       name: 'test',
       interval: Duration(hours: 1),
@@ -19,7 +22,7 @@ void main() {
     expect(cv.value, 'x');
   });
 
-  test('failing update does not crash', () async {
+  testWithProfile('failing update does not crash', fn: () async {
     var count = 0;
     final cv = CachedValue<String>(
       name: 'test',
@@ -30,8 +33,10 @@ void main() {
         throw Exception();
       },
     );
-    await cv.start();
-    await Future.delayed(Duration(seconds: 2));
+    for (var i = 0; i < 15; i++) {
+      await cv.update();
+    }
+    await asyncQueue.ongoingProcessing;
     expect(cv.isAvailable, isFalse);
     expect(count, greaterThan(10));
     await cv.close();