Add a reason argument to the disconnect handler

docs/getting_started.rst

@@ -267,8 +267,8 @@ following example shows how to register handlers for them::
         emit('my response', {'data': 'Connected'})
 
     @socketio.on('disconnect')
-    def test_disconnect():
-        print('Client disconnected')
+    def test_disconnect(reason):
+        print('Client disconnected, reason:', reason)
 
 The ``auth`` argument in the connection handler is optional. The client can
 use it to pass authentication data such as tokens in dictionary format. If the
@@ -288,6 +288,10 @@ the exception are returned to the client in the error packet. Examples::
         if not self.authenticate(request.args):
             raise ConnectionRefusedError('unauthorized!')
 
+The disconnection event handler receives a ``reason`` argument that indicates
+the cause of the disconnection. The :attr:`flask_socketio.SocketIO.reason`
+member includes constants for all the possible reasons.
+
 Note that connection and disconnection events are sent individually on each
 namespace used.
 
@@ -305,7 +309,7 @@ create class-based namespaces::
         def on_connect(self):
             pass
 
-        def on_disconnect(self):
+        def on_disconnect(self, reason):
             pass
 
         def on_my_event(self, data):

example/app.py

@@ -118,8 +118,8 @@ def connect():
 
 
 @socketio.on('disconnect')
-def test_disconnect():
-    print('Client disconnected', request.sid)
+def test_disconnect(reason):
+    print('Client disconnected', request.sid, reason)
 
 
 if __name__ == '__main__':

src/flask_socketio/__init__.py

@@ -165,6 +165,7 @@ class SocketIO:
                             fatal errors are logged even when
                             ``engineio_logger`` is ``False``.
     """
+    reason = socketio.Server.reason
 
     def __init__(self, app=None, **kwargs):
         self.server = None

src/flask_socketio/namespace.py

@@ -22,8 +22,16 @@ def trigger_event(self, event, *args):
             # there is no handler for this event, so we ignore it
             return
         handler = getattr(self, handler_name)
-        return self.socketio._handle_event(handler, event, self.namespace,
-                                           *args)
+        try:
+            return self.socketio._handle_event(handler, event, self.namespace,
+                                               *args)
+        except TypeError:
+            if event == 'disconnect':
+                # legacy disconnect events do not have the reason argument
+                return self.socketio._handle_event(
+                    handler, event, self.namespace, *args[:-1])
+            else:
+                raise
 
     def emit(self, event, data=None, room=None, include_self=True,
              namespace=None, callback=None):

test_socketio.py

@@ -26,7 +26,7 @@ def on_connect(auth):
 
 
 @socketio.on('disconnect')
-def on_disconnect():
+def on_disconnect(reason):
     global disconnected
     disconnected = '/'