perform the initial poll for devices quicker (#11356)

* perform the initial poll for devices quicker

* add a Poller class

* test the new Poller class

packages/flutter_tools/lib/src/base/utils.dart

@@ -10,6 +10,7 @@ import 'package:crypto/crypto.dart';
 import 'package:intl/intl.dart';
 import 'package:quiver/time.dart';
 
+import '../globals.dart';
 import 'context.dart';
 import 'file_system.dart';
 import 'platform.dart';
@@ -216,3 +217,42 @@ class Uuid {
 }
 
 Clock get clock => context.putIfAbsent(Clock, () => const Clock());
+
+typedef Future<Null> AsyncCallback();
+
+/// A [Timer] inspired class that:
+///   - has a different initial value for the first callback delay
+///   - waits for a callback to be complete before it starts the next timer
+class Poller {
+  Poller(this.callback, this.pollingInterval, { this.initialDelay: Duration.ZERO }) {
+    new Future<Null>.delayed(initialDelay, _handleCallback);
+  }
+
+  final AsyncCallback callback;
+  final Duration initialDelay;
+  final Duration pollingInterval;
+
+  bool _cancelled = false;
+  Timer _timer;
+
+  Future<Null> _handleCallback() async {
+    if (_cancelled)
+      return;
+
+    try {
+      await callback();
+    } catch (error) {
+      printTrace('Error from poller: $error');
+    }
+
+    if (!_cancelled)
+      _timer = new Timer(pollingInterval, _handleCallback);
+  }
+
+  /// Cancels the poller.
+  void cancel() {
+    _cancelled = true;
+    _timer?.cancel();
+    _timer = null;
+  }
+}

packages/flutter_tools/lib/src/device.dart

@@ -116,35 +116,28 @@ abstract class PollingDeviceDiscovery extends DeviceDiscovery {
 
   final String name;
   ItemListNotifier<Device> _items;
-  Timer _timer;
+  Poller _poller;
 
   Future<List<Device>> pollingGetDevices();
 
   void startPolling() {
-    if (_timer == null) {
+    if (_poller == null) {
       _items ??= new ItemListNotifier<Device>();
-      bool _fetchingDevices = false;
+
-      _timer = new Timer.periodic(_pollingInterval, (Timer timer) async {
+      _poller = new Poller(() async {
-        if (_fetchingDevices) {
-          printTrace('Skipping device poll: already in progress');
-          return;
-        }
-        _fetchingDevices = true;
         try {
           final List<Device> devices = await pollingGetDevices().timeout(_pollingTimeout);
           _items.updateWithNewList(devices);
         } on TimeoutException {
           printTrace('Device poll timed out.');
-        } finally {
-          _fetchingDevices = false;
         }
-      });
+      }, _pollingInterval);
     }
   }
 
   void stopPolling() {
-    _timer?.cancel();
+    _poller?.cancel();
-    _timer = null;
+    _poller = null;
   }
 
   @override

packages/flutter_tools/test/utils_test.dart

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:async';
+
 import 'package:flutter_tools/src/base/utils.dart';
 import 'package:flutter_tools/src/base/version.dart';
 import 'package:test/test.dart';
@@ -115,4 +117,52 @@ baz=qux
       expect(new Version.parse('Preview2.2'), isNull);
     });
   });
+
+  group('Poller', () {
+    const Duration kShortDelay = const Duration(milliseconds: 100);
+
+    Poller poller;
+
+    tearDown(() {
+      poller?.cancel();
+    });
+
+    test('fires at start', () async {
+      bool called = false;
+      poller = new Poller(() {
+        called = true;
+      }, const Duration(seconds: 1));
+      expect(called, false);
+      await new Future<Null>.delayed(kShortDelay);
+      expect(called, true);
+    });
+
+    test('runs periodically', () async {
+      // Ensure we get the first (no-delay) callback, and one of the periodic callbacks.
+      int callCount = 0;
+      poller = new Poller(() {
+        callCount++;
+      }, new Duration(milliseconds: kShortDelay.inMilliseconds ~/ 2));
+      expect(callCount, 0);
+      await new Future<Null>.delayed(kShortDelay);
+      expect(callCount, greaterThanOrEqualTo(2));
+    });
+
+    test('no quicker then the periodic delay', () async {
+      // Make sure that the poller polls at delay + the time it took to run the callback.
+      final Completer<Duration> completer = new Completer<Duration>();
+      DateTime firstTime;
+      poller = new Poller(() async {
+        if (firstTime == null)
+          firstTime = new DateTime.now();
+        else
+          completer.complete(new DateTime.now().difference(firstTime));
+
+        // introduce a delay
+        await new Future<Null>.delayed(kShortDelay);
+      }, kShortDelay);
+      final Duration duration = await completer.future;
+      expect(duration, greaterThanOrEqualTo(new Duration(milliseconds: kShortDelay.inMilliseconds * 2)));
+    });
+  });
 }