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README.md

Multipeer Connectivity for Pythonista

This is a Pythonista wrapper around iOS Multipeer Connectivity.

Multipeer connectivity allows you to find and exchange information between 2-8 iOS and Mac devices in the same network neighborhood (same wifi or bluetooth), without going through some server.

Sample use cases include games, chats, file exchange (like AirDrop) and so on.

Here's a minimal usage example, a line-based chat. You need to be running the same code on all devices participating in the chat.

import multipeer

my_name = input('Name: ')

mc = multipeer.MultipeerConnectivity(display_name=my_name, service_type='chat')

try:
  while True:
    chat_message = input('Message: ')
    mc.send(chat_message)
finally:
  mc.end_all()

This example is functional, even though the prompts and incoming messages tend to get mixed up. You can try it out by running the multipeer.py file. There is also a cleaner Pythonista UI version of the chat in multipeer_chat.

Here are the things to note when starting to use this library:

Peer-to-peer, not client-server

This wrapper around the MC framework makes no assumptions regarding the relationships between peers. If you need client-server roles, you can build them on top.

Expected usage

  1. Create a subclass of MultipeerCommunications to handle messages from the framework (see separate topic, below).
  2. Instantiate the subclass with your service type, peer display name and optional initial context data (see the class description).
  3. Wait for peers to connect (see the peer_added and get_peers methods).
  4. Optionally, have each participating peer stop accepting further peers, e.g. for the duration of a game (see the stop_looking_for_peers method).
  5. Send and receive messages (see a separate topic, below).
  6. Potentially react to additions and removal of peers.
  7. Optionally, start accepting peers again, e.g. after a previous game ends (see the start_looking_for_peers method).
  8. Before your app exits, call the end_all method to make sure there are no lingering connections.

What's in a message?

Messages passed between peers are UTF-8 encoded text. This wrapper JSON-serializes the message you give to the send method (probably a str or a dict), then encodes it in bytes. Receiving peers reconstitute the message and pass it to the receive callback.

Streaming

There are methods to use streaming instead of simple messages. Streamed data is received in 1024 byte chunks. There is a constructor option initialize_streams that can be used to set up a stream with each connected peer; otherwise, the streams are initialized when needed.

Performance

Pythonista forum user mithrendal ran some ping tests with very small data payload and 1000 repeats. Observed average times for a two-way messages were:

  • 11.85 ms - send method with reliable=False
  • 11.94 ms - send method with reliable=True (the default)
  • 6.19 ms - stream method

Tentative conclusions from these results:

  • Connections are likely to be good enough that reliable messaging is not a performance concern.
  • Streaming may be significantly better if communications delay is an issue.

What is a peer ID?

Peer IDs passed around by the wrapper have a display_name member that gives you the display name of that peer. There is no guarantee that these names are unique between peers.

The IDs act also as identifier objects for specific peers, and can be used to send messages to individual peers.

You cannot create peer IDs for remote peers manually.

Why do I need to subclass?

This wrapper chooses to handle callbacks via subclassing rather than requiring a separate delegate class. Subclass should define the following methods; see the API for the method signatures:

  • peer_added
  • peer_removed
  • receive
  • stream_receive

The versions of these methods in the MultipeerConnectivity class just print out the information received.

Note that if these method update the UI, you should decorate them with objc_util.on_main_thread.

Additional details

  • This implementation uses automatic invite of all peers (until you call stop_looking_for_peers). Future version may include a callback for making decisions on which peers to accept.
  • Related to the previous point, including discovery info while browsing for peers is not currently supported.
  • Also, there is no way to explicitly kick a specific peer out of a session. This seems to be a limitation of the Apple framework.
  • Following defaults are used and are not currently configurable without resorting to ObjC:
    • Secure - Encryption is required on all connections.
    • Not secure - A specific security identity cannot be set.

API

Class: MultipeerConnectivity

Multipeer communications. Subclass this class to define how you want to react to added or removed peers, and to process incoming messages from peers.

Constructor:

mc = MultipeerConnectivity(display_name='Peer', service_type='dev-srv')

Arguments:

  • display_name - This peer's display name (e.g. a player name). Must not be None or an empty string, and must be at most 63 bytes long (UTF-8 encoded).
  • service_type - String that must match with that of the peers in order for a connection to be established. Must be 1-15 characters in length and contain only a-z, 0-9, or '-'.
  • initial_data - Any JSON-serializable data that can be requested by peers with a call to get_initial_data().
  • initialize_streams - If True, a stream is set up to any peer that connects.

Created object will immediately start advertising and browsing for peers.

Methods

peer_added(self, peer_id)

Override handling of new peers in a subclass.

peer_removed(self, peer_id)

Override handling of lost peers in a subclass.

get_peers(self)

Get a list of peers currently connected.

get_initial_data(self, peer_id)

Returns initial context data provided by the peer, or None.

start_looking_for_peers(self)

Start conmecting to available peers.

stop_looking_for_peers(self)

Stop advertising for new connections, e.g. when you have all the players and start a game, and do not want new players joining in the middle.

send(self, message, to_peer=None, reliable=True)

Send a message to some or all peers.

  • message - to be sent to the peer(s). Must be JSON-serializable.
  • to_peer - receiver peer IDs. Can be a single peer ID, a list of peer IDs, or left out (None) for sending to all connected peers.
  • reliable - indicates whether delivery of data should be guaranteed (enqueueing and retransmitting data as needed, and ensuring in-order delivery). Default is True, but can be set to False for performance reasons.

stream(self, byte_data, to_peer=None)

Stream message string to some or all peers. Stream per receiver will be set up on first call. See constructor parameters for the option to have streams per peer initialized on connection.

  • byte_data - data to be sent to the peer(s). If you are sending a string, call its encode() method and pass the result to this method.
  • to_peer - receiver peer IDs. Can be a single peer ID, a list of peer IDs, or left out (None) for sending to all connected peers.

receive(self, message, from_peer)

Override in a subclass to handle incoming messages.

stream_receive(self, byte_data, from_peer)

Override in a subclass to handle incoming streamed data. byte_data is a bytearray; call its decode() method if you expect a string.

disconnect(self)

End your games or similar sessions by calling this method.

end_all(self)

Disconnects from the multipeer session and removes internal references. Further communications will require instantiating a new MultipeerCommunications (sub)class.

Functions

get_self(manager_object)

Expects a 'manager object', i.e. one of session, advertiser or browser, and uses the contained peer ID to locate the right Python manager object.

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