An MCSession object enables and manages communication among all peers in a Multipeer Connectivity session.


class MCSession : NSObject


Initiating a Session

To set up a session:

  1. Use the init(displayName:) method of the MCPeerID to create a peer ID that represents the local peer, or retrieve a peer ID that you previously archived (to maintain a stable peer ID over time).

  2. Use the peer ID with the method init(peer:) to initialize the session object.

  3. Invite peers to join the session using an MCNearbyServiceBrowser object, an MCBrowserViewController object, or your own peer discovery code. (Sessions currently support up to 8 peers, including the local peer.)

  4. Set up an MCNearbyServiceAdvertiser object or MCAdvertiserAssistant object to allow other devices to ask your app to join a session that they create.

If you use one of the framework’s browser objects for peer discovery, when a peer accepts an invitation, the session calls its delegate object’s session(_:peer:didChange:) method with MCSessionState.connected as the new state, along with an object that tells you which peer became connected. See Creating a Session for related methods.

If instead you write your own peer discovery code, you are responsible for managing the connection manually. See the Managing Peers Manually section for more information.

Communicating with Peers

Once you have set up the session, your app can send data to other peers by calling one of the following methods, found in Sending Data and Resources:

  • send(_:toPeers:with:) sends an NSData object to the specified peers.

    On each recipient device, the delegate object’s session(_:didReceive:fromPeer:) method is called with the data object when the data has been fully received.

  • sendResource(at:withName:toPeer:withCompletionHandler:) sends the contents from an NSURL object to the specified peer. The URL can be either a local file URL or a web URL. The completionHandler block is called when the resource is fully received by the recipient peer or when an error occurs during transmission.

    This method returns an NSProgress object that you can use to cancel the transfer or check the current status of the transfer.

    On the recipient device, the session calls its delegate object’s session(_:didStartReceivingResourceWithName:fromPeer:with:) method when the device begins receiving the resource, and calls its session(_:didFinishReceivingResourceWithName:fromPeer:at:withError:) method when the resource has been fully received or when an error occurs.

  • startStream(withName:toPeer:) creates a connected byte stream (NSOutputStream) that you can use to send data to the specified peer.

    On the recipient device, the session calls its delegate object’s session(_:didReceive:withName:fromPeer:) method with an NSInputStream object that represents the other endpoint of communication.

    On both sides, your code must set the stream’s delegate, schedule the stream on a run loop, and open the stream. Your code must also implement stream delegate methods to manage sending and receiving stream data.

Managing Peers Manually

If instead of using the framework’s browser and advertiser objects to perform peer discovery, you decide to write your own peer discovery code (with NSNetService or the Bonjour C API, for example), you can manually connect nearby peers into a session. To do this:

  1. Establish a connection from your app to nearby peers, and exchange peer IDs with those peers.

    Each peer should serialize its own local MCPeerID object with NSKeyedArchiver, and the receiving peer should unserialize it with NSKeyedUnarchiver.

  2. Exchange connection data. After you have obtained the nearby peer’s ID object, call nearbyConnectionData(forPeer:withCompletionHandler:) to obtain a connection data object specific to that nearby peer.

    When the completion handler block is called, send the resulting connection data object to that peer.

  3. When your app receives connection data from another peer, it must call connectPeer(_:withNearbyConnectionData:) to add that peer to the session.

You can also cancel an outstanding connection attempt by calling cancelConnectPeer(_:). These methods are described in the Managing Peers Manually group.


To leave a session, your app must call disconnect(). For more details, see Leaving a Session.


Creating a Session

init(peer: MCPeerID)

Creates a Multipeer Connectivity session.

init(peer: MCPeerID, securityIdentity: [Any]?, encryptionPreference: MCEncryptionPreference)

Creates a Multipeer Connectivity session, providing security information.

var delegate: MCSessionDelegate?

The delegate object that handles session-related events.

var encryptionPreference: MCEncryptionPreference

A value indicating whether the connection prefers encrypted connections, unencrypted connections, or has no preference.

var myPeerID: MCPeerID

A local identifier that represents the device on which your app is currently running.

var securityIdentity: [Any]?

The security identity of the local peer.

Managing Peers Manually

func connectPeer(MCPeerID, withNearbyConnectionData: Data)

Call this method to connect a peer to the session when using your own service discovery code instead of an MCNearbyServiceBrowser or MCBrowserViewController object.

func cancelConnectPeer(MCPeerID)

Cancels an attempt to connect to a peer.

var connectedPeers: [MCPeerID]

An array of all peers that are currently connected to this session.

Leaving a Session

func disconnect()

Disconnects the local peer from the session.


enum MCSessionSendDataMode

Indicates whether delivery of data should be guaranteed.

enum MCSessionState

Indicates the current state of a given peer within a session.

enum MCEncryptionPreference

Indicates whether a session should use encryption when communicating with nearby peers.

enum MCError.Code

Error codes found in MCErrorDomain error domain NSError objects returned by methods in the Multipeer Connectivity framework.

Multipeer Connectivity Error Domain

The error domain for errors specific to Multipeer Connectivity.

Minimum and Maximum Supported Peers

Constants that define the minimum and maximum number of peers supported in a session.


Inherits From

Conforms To