Swift Server/Client Implementation of CoAP - RFC 7252
Objective-C Swift
Latest commit 71cab10 Feb 27, 2017 @stuffrabbit committed on GitHub Merge pull request #9 from oneam/return-address
Added return address to SCMessage received from SCClient



Updated for Swift 3

NEW: Download the Client-Implementation myCoAP for iOS/watchOS which builds upon this library: AppStore-Link

This project is an implementation of the "Constrained Application Protocol" (CoAP - RFC 7252) in Swift. It is intended for Clients and Servers. This implementation provides the standard CoAP features (including Caching) along with the extensions:

  • Observe
  • Block transfer (Block1 and Block2)

A short manual is provided below. Feedback is highly appreciated!

Want an Objective-C implementation? Checkout iCoAP.

Getting Started

The Files:

  • Copy all files included in the SwiftCoAP_Library folder to your Xcode project
  • Make sure to add GCDAsyncUdpSocket.h to your Objective-C Bridging-File, as this project uses the Objective-C-Library CocoaAsyncSocket for UDP communication

The Code

This section gives you an impression on how to use the provided data structures.


SCMessage represents a CoAP message in SwiftCoAP. You can initialize a message with help of the designated initializer as follows: SCMessage(). Alternatively, SCMessage provides a convenience initializer (convenience init(code: SCCodeValue, type: SCType, payload: NSData?)) that lets you create an instance the following way:

SCMessage(code: SCCodeValue(classValue: 0, detailValue: 01)!, type: .Confirmable, payload: "test".dataUsingEncoding(NSUTF8StringEncoding))
  • The CoAP type is represented as SCType of type enum (refer to source code)
  • The CoAP code is represented as a struct named SCCodeValue. The struct lets you apply the CoAP code syntax c.dd (e.g. SCCodeValue(classValue: 0, detailValue: 01) equals 0.01 (note that this is a failable initializer, which fails when invalid class values (greater 7) or detail values (greater 31) are passed as arguments)).
  • The CoAP options are represented as Dictionary. The option number represents the key (as Int) and the respective value pair represents an Array with NSData objects (in case that the same option is present multiple times). To add an option safely, it is recommended to use the provided addOption(option: Int, data: NSData) method.

  • Checkout the source code and its comments for more information


This class represents a CoAP client, which can be initialized with the given designated initializer: init(delegate: SCClientDelegate?).


You can modify the following properties of an SCClient object to alter its behavior:

  • sendToken: Bool (default true) If true, a randomized token with at least 4 bytes length is generated upon transmission
  • autoBlock1SZX: UInt? (default 2) If not nil, Block1 transfer will be used automatically when the payload size exceeds the value 2^(autoBlock1SZX +4). Valid Values: 0-6
  • httpProxyingData: (hostName: String, port: UInt16)? (default nil) If not nil, all message will be sent via http to the given proxy address
  • cachingActive: Bool (default false) If true, caching is activiated

Send a message by calling the method sendCoAPMessage(message: SCMessage, hostName: String, port: UInt16) and implement the provided SCClientDelegate protocol to receive callbacks. This should be it.

let m = SCMessage(code: SCCodeValue(classValue: 0, detailValue: 01), type: .Confirmable, payload: "test".dataUsingEncoding(NSUTF8StringEncoding))
m.addOption(SCOption.UriPath.rawValue, data: "test".dataUsingEncoding(NSUTF8StringEncoding)!)
let coapClient = SCClient(delegate: self)
coapClient.sendCoAPMessage(m, hostName: "coap.me", port: 5683)
Other Methods
  • cancelObserve() Cancels observe directly, sending the previous message with an Observe-Option Value of 1. Only effective, if the previous message initiated a registration as observer with the respective server.
  • closeTransmission() Closes the transmission. It is recommended to call this method anytime you do not expect to receive a response any longer.

The class SCClient gives you the opportunity to send a message as HTTP via a proxy. Just add the following line after initiating an SCClientobject:

coapClient.httpProxyingData = ("localhost", 5683)

The Options of the CoAP-Message are sent in the HTTP-Header. It is required that the Proxy returns the CoAP-Type in the Header of HTTP-Response as well. The respective Header-Field is COAP_TYPE. The Request-URI has the following Format: http://proxyHost:proxyPort/coapHost:coapPort An Example: Sending your message to the CoAP-Server coap.me with the Port 5683 via a HTTP-Proxy located at localhost:9292, lets the SwiftCoAP library compose the follwoing Request-URI: http://localhost:9292/coap.me:5683

Custom Transport Layer Functionality

SCClient encapsulates the CoAP transport layer functionality into a separate object which implements the SCCoAPTransportLayerProtocol protocol. SCClient uses the provided SCCoAPUDPTransportLayer class by default, which uses UDP. However, if you want to replace it with your own class just do the following steps:

  • Create a custom class and adopt the SCCoAPTransportLayerProtocol
  • Pass an object of your class to the init method of SCClient: init(delegate: SCClientDelegate?, transportLayerObject: SCCoAPTransportLayerProtocol)
  • SCClient will set itself as a delegate of your class and notify you through the methods of SCCoAPTransportLayerProtocol when e.g. a data needs to be sent.
  • Whenever you receive a response to data you have sent, call methods of the protocol SCCoAPTransportLayerDelegate on your property transportLayerDelegate which will hold a weak reference to the resepective object of type SCClient(reference is automatically set through SCClient).
  • Checkout the source code and the implementation of the class SCCoAPUDPTransportLayer, to see the functionality in action.

An (real) example where using a custom transport layer functionality would be helpful: You cannot use UDP, e.g. when you bring this library to WatchOS 2. As UDP communcation is not available on this OS you can use WatchConnectiviy as transport layer object for your SCClient and let the iPhone execute the UDP sendings.


This class represents a CoAP server, which can be initialized with the standard designated initializer init(). The given convenience initializer init?(port: UInt16) initializes a server instance and automatically starts listening on the given port. This initialization can fail if a UDP-socket error occurs.


You can modify the following properties of an SCServer object to alter its behavior:

  • autoBlock2SZX: UInt? (default nil) If not nil, Block2 transfer will be used automatically in responses when the payload size exceeds the value 2^(autoBlock1SZX +4). Valid Values: 0-6
  • resources: [SCResourceModel] Array of SCResourceModel objects which represent a resource of the server (see below)
  • autoWellKnownCore: Bool (default true) If set to true, the server will automatically provide responses for the resource well-known/core with its current resources.

  • start(port: UInt16 = 5683) -> Bool Starts the server manually on the given port

  • close() Closes Udp socket listening
  • reset() Resets the context of the server (including added resources, cached message contexts, registered observers for resources and data uploads for Block1)
  • didCompleteAsynchronousRequestForOriginalMessage(message: SCMessage, resource: SCResourceModel, values:(statusCode: SCCodeValue, payloadData: NSData?, contentFormat: SCContentFormat!, locationUri: String!)) Call this method when your resource is ready to process a separate response. The concerned resource must return true for the method willHandleDataAsynchronouslyForGet(...). It is necessary to pass the original message and the resource (both received in willHandleDataAsynchronouslyForGet) so that the server is able to retrieve the current context. Additionay, you have to pass the typical values tuple which form the response (as described in SCResourceModel)
  • updateRegisteredObserversForResource(resource: SCResourceModel) Call this method when the given resource has updated its data representation in order to notify all registered observers (and has observable set to true).

Resource representation SCResourceModel

SwiftCoAP provides the base class SCResourceModel to represent a CoAP resource in the server implementation. To create your own resources with custom behavior, you just have to subclass SCResourceModel. You must use the designated initializer init(name: String, allowedRoutes: UInt) which requires you to set the name and the routes (GET, POST, PUT, DELETE) which you want to support (see explanation below).

Resource properties

SCResourceModel has the following properties which can be modified/set on initialization:

  • name: String The name of the resource
  • allowedRoutes: UInt Bitmask of allowed routes (see SCAllowedRoutes enum) (you can pass for example SCAllowedRoute.Get.rawValue | SCAllowedRoute.Post.rawValue to support GET and POST)
  • maxAgeValue: UInt! (default nil) If not nil, every response will contain the provided MaxAge value
  • etag: NSData! (default nil and read-only) If not nil, every response will contain the provided eTag. The etag is generated automatically whenever you update the value dataRepresentation of the resource (is represented as hashvalue of your data representation).
  • dataRepresentation: NSData! The current data representation of the resource. Needs to stay up to date
  • observable: Bool (default false) If true, a response will contain the Observe option, and endpoints will be able to register as observers in SCServer. Call updateRegisteredObserversForResource(self), anytime the value of your dataRepresentation changes.
Resource methods

The following methods are used for data reception of your allowed routes. SCServer will call the appropriate message upon the reception of a reqeuest. Override the respective methods, which match your allowedRoutes.

SCServer passes a queryDictionary containing the URI query content (e.g ["user_id": "23"]) and all options contained in the respective request. The POST and PUT methods provide the message's payload as well. Please, refer to the example resources in the SwiftCoAPServerExample project for implementation examples.

  • willHandleDataAsynchronouslyForRoute(route: SCAllowedRoute, queryDictionary: [String : String], options: [Int : [NSData]], originalMessage: SCMessage) -> Bool This method lets you decide whether the current request shall be processed asynchronously, i.e. if true will be returned, an empty ACK will be sent, and you can provide the actual content in a separate response by calling the servers didCompleteAsynchronousRequestForOriginalMessage(...). Note: dataForGet(...), dataForPost(...), etc. will not be called additionally if you return true.

The following methods require data for the given routes GET, POST, PUT, DELETE and must be overriden if needed. If you return nil, the server will respond with a Method not allowed error code (Make sure that you have set the allowed routes in the allowedRoutes bitmask property). You have to return a tuple with a statuscode, optional payload, optional content format for your provided payload and (in case of POST and PUT) an optional locationURI.

  • dataForGet(#queryDictionary: [String : String], options: [Int : [NSData]]) -> (statusCode: SCCodeValue, payloadData: NSData?, contentFormat: SCContentFormat!)?
  • dataForPost(#queryDictionary: [String : String], options: [Int : [NSData]], requestData: NSData?) -> (statusCode: SCCodeValue, payloadData: NSData?, contentFormat: SCContentFormat!, locationUri: String!)?
  • dataForPut(#queryDictionary: [String : String], options: [Int : [NSData]], requestData: NSData?) -> (statusCode: SCCodeValue, payloadData: NSData?, contentFormat: SCContentFormat!, locationUri: String!)?
  • dataForDelete(#queryDictionary: [String : String], options: [Int : [NSData]]) -> (statusCode: SCCodeValue, payloadData: NSData?, contentFormat: SCContentFormat!)?
Server with Resources Example
let server = SCServer(port: 5683)

let resource = TestResourceModel(name: "test", allowedRoutes: SCAllowedRoute.Get.rawValue | SCAllowedRoute.Post.rawValue | SCAllowedRoute.Put.rawValue | SCAllowedRoute.Delete.rawValue, 
text: "This is a very long description text, I hope that all of you will like it")

server?.delegate = self

Don't hesitate to contact me if something is unclear!


Make sure to take a look at the examples, which show the library in action. Let me know if you have questions, or other issues.

Used Libraries:

This version uses the public domain licensed CocoaAsyncSocket library for UDP-socket networking. Click here for more information.