xmpp: Add support for setting reconnection policies

lib/xmpp/reconnect.dart

@@ -0,0 +1,87 @@
+import "dart:async";
+import "dart:math";
+
+import "package:logging/logging.dart";
+import "package:meta/meta.dart";
+
+abstract class ReconnectionPolicy {
+  /// Function provided by [XmppConnection] that allows the policy
+  /// to perform a reconnection.
+  void Function()? performReconnect;
+
+  /// Called by [XmppConnection] to register the policy.
+  void register(void Function() performReconnect) {
+    this.performReconnect = performReconnect;
+
+    reset();
+  }
+  
+  /// In case the policy depends on some internal state, this state must be reset
+  /// to an initial state when [reset] is called. In case timers run, they must be
+  /// terminated.
+  void reset();
+
+  /// Called by the [XmppConnection] when the reconnection failed.
+  void onFailure();
+
+  /// Caled by the [XmppConnection] when the reconnection was successful.
+  void onSuccess();
+}
+
+/// A simple reconnection strategy: Make the reconnection delays exponentially longer
+/// for every failed attempt.
+class ExponentialBackoffReconnectionPolicy extends ReconnectionPolicy {
+  int _counter;
+  Timer? _timer;
+  Logger _log;
+
+  ExponentialBackoffReconnectionPolicy()
+  : _counter = 0,
+    _log = Logger("ExponentialBackoffReconnectionPolicy");
+
+  /// Called when the backoff expired
+  void _onTimerElapsed() {
+    performReconnect!();
+  }
+  
+  @override
+  void reset() {
+    _log.finest("Resetting internal state");
+    _counter = 0;
+
+    if (_timer != null) {
+      _timer!.cancel();
+      _timer = null;
+    }
+  }
+
+  @override
+  void onFailure() {
+    _log.finest("Failure occured. Starting exponential backoff");
+    _counter++;
+
+    if (_timer != null) {
+      _timer!.cancel();
+    }
+
+    _timer = Timer(Duration(seconds: pow(2, _counter).toInt()), _onTimerElapsed);
+  }
+
+  @override
+  void onSuccess() {
+    reset();
+  }
+}
+
+/// A stub reconnection policy for tests
+@visibleForTesting
+class TestingReconnectionPolicy extends ReconnectionPolicy {
+  @override
+  void onSuccess() {}
+
+  @override
+  void onFailure() {}
+
+  @override
+  void reset() {}
+}