add MailClient.disconnect() to close connections

Close #71

lib/mail/mail_client.dart

@@ -55,10 +55,22 @@ class MailClient {
   //Future<MailResponse<List<MimeMessage>>> poll(Mailbox mailbox) {}
 
   /// Connects and authenticates with the specified incoming mail server.
+  /// Also compare [disconnect()].
   Future<MailResponse> connect() {
     return _incomingMailClient.connect();
   }
 
+  /// Disconnects from the mail service.
+  /// Also compare [connect()].
+  Future disconnect() async {
+    if (_incomingMailClient != null) {
+      await _incomingMailClient.disconnect();
+    }
+    if (_outgoingMailClient != null) {
+      await _outgoingMailClient.disconnect();
+    }
+  }
+
   // Future<MailResponse> tryAuthenticate(
   //     ServerConfig serverConfig, MailAuthentication authentication) {
   //   return authentication.authenticate(this, serverConfig);
@@ -202,6 +214,8 @@ abstract class _IncomingMailClient {
 
   Future<MailResponse> connect();
 
+  Future disconnect();
+
   Future<MailResponse<List<Mailbox>>> listMailboxes();
 
   Future<MailResponse<Mailbox>> selectMailbox(Mailbox mailbox,
@@ -378,6 +392,14 @@ class _IncomingImapClient extends _IncomingMailClient {
     return response;
   }
 
+  @override
+  Future disconnect() {
+    if (_imapClient != null) {
+      return _imapClient.closeConnection();
+    }
+    return Future.value();
+  }
+
   @override
   Future<MailResponse<List<Mailbox>>> listMailboxes() async {
     var mailboxResponse = await _imapClient.listMailboxes(recursive: true);
@@ -574,6 +596,14 @@ class _IncomingPopClient extends _IncomingMailClient {
     return authResponse;
   }
 
+  @override
+  Future disconnect() {
+    if (_popClient != null) {
+      return _popClient.closeConnection();
+    }
+    return Future.value();
+  }
+
   @override
   Future<MailResponse<List<Mailbox>>> listMailboxes() {
     _config.pathSeparator = '/';
@@ -683,6 +713,8 @@ class _IncomingPopClient extends _IncomingMailClient {
 
 abstract class _OutgoingMailClient {
   Future<MailResponse> sendMessage(MimeMessage message);
+
+  Future disconnect();
 }
 
 class _OutgoingSmtpClient extends _OutgoingMailClient {
@@ -734,4 +766,12 @@ class _OutgoingSmtpClient extends _OutgoingMailClient {
     }
     return MailResponseHelper.success(sendResponse.code);
   }
+
+  @override
+  Future disconnect() {
+    if (_smtpClient != null) {
+      return _smtpClient.closeConnection();
+    }
+    return Future.value();
+  }
 }

lib/pop/pop_client.dart

@@ -239,7 +239,7 @@ class PopClient {
     }
   }
 
-  Future<dynamic> close() {
+  Future<dynamic> closeConnection() {
     _isSocketClosingExpected = true;
     return _socket?.close();
   }

lib/smtp/smtp_client.dart

@@ -190,7 +190,8 @@ class SmtpClient {
     }
   }
 
-  Future<dynamic> close() {
+  /// Closes the connection to the remote SMTP server.
+  Future<dynamic> closeConnection() {
     _isSocketClosingExpected = true;
     return _socket?.close();
   }

lib/src/pop/commands/pop_quit_command.dart

@@ -8,7 +8,7 @@ class PopQuitCommand extends PopCommand<String> {
 
   @override
   String nextCommand(PopResponse response) {
-    _client.close();
+    _client.closeConnection();
     return null;
   }
 }

lib/src/smtp/commands/smtp_quit_command.dart

@@ -8,7 +8,7 @@ class SmtpQuitCommand extends SmtpCommand {
 
   @override
   String nextCommand(SmtpResponse response) {
-    _client.close();
+    _client.closeConnection();
     return null;
   }
 }