A comprehensive iOS package for writing mobile applications that interact with the Internet Computer Protocol (ICP), written in Swift. IcpKit aims at facilitating the interaction between iOS apps and the ICP blockchain.

For more information about ICP Development, we recommend starting from https://internetcomputer.org/docs/current/references/

Contributors

The main developer of this package is Konstantinos Gaitanis.

This Package has been built by Bity SA with the help of the DFinity Foundation Developer Grant Program.

License

MIT License is applicable for all Swift Code (see LICENSE).

The BLS12381 Rust Library is licensed by Levi Feldman (see LICENSE).

Installation

Swift Package Manager

Adding IcpKit as a dependency to your Xcode project is as easy as adding it to the dependencies value of your Package.swift.

...
dependencies: [
    .package(url: "https://github.com/kosta-bity/IcpKit.git", .upToNextMajor(from: "0.2.0"))
]
...

The IcpKit package defines two libraries Candid and IcpKit. Candid implements the Candid specification and IcpKit implements the communication with canisters. To use in your code

import Candid  // not needed when using the CodeGenerator
import IcpKit

What does it do?

IcpKit will take care of all the encoding, serialisation and cryptography required to communicate with ICP allowing developers to focus on the real functionality of their app and bootstrapping their development cycle.

Main Functionalities

• CodeGenerator Command Line Tool for parsing .did files and generating Swift code that can be directly added to your project.
• CandidEncoder and CandidDecoder for converting any Encodable/Decodable to a CandidValue
• Candid binary serialisation/deserialisation
• CBOR serialisation
• Cryptographic methods applicable to ICP such as signing and signature verification.
• Basic ICP Models for transactions, accounts, self-authenticating principals etc.
• Ledger and Archive Canister implementation as sample code.

IcpKit package is split in 3 products :

• Candid Library
• IcpKit Library
• CodeGenerator command line executable

Candid Library Overview

Candid is the language used to communicate with any ICP canister. It describes the data types used by the canister, and the methods one can call on the canister. Candid is essentially a textual and a binary representation of these types. The binary representation is what is actually being sent/received to/from the canister while the textual representation is used in the Candid Interface Definition files ( .did) to describe the interface of a canister.

If you use the CodeGenerator tool then you will never have to use Candid directly to interact with a canister.

The main classes of the Candid Library are the following :

Class	Description
CandidValue	Represents a CandidValue in Swift. Eg. .bool(true)
CandidType	Represent the type of a CandidValue. Eg. .option(.integer)
CandidSerialiser and CandidDeserialiser	Convert any CandidValue to/from its binary representation which can be directly sent/received to/from a canister. NOTE: Serialising recursive candid values is not supported.
CandidEncoder and CandidDecoder	Convert any Encodable/ Decodable Swift object to/from a CandidValue. See Swift/Candid Encoding Rules
CandidParser	Parses the contents of a .did file and returns a CandidInterfaceDefinition, a CandidType or a CandidValue depending on the type of file parsed (see Candid Interface Definition files). The CandidParser is mostly meant to be used with the provided Command Line Tool but it can be directly called from the your app for cases such as reading values from an online .did file, or similar.

IcpKit Library Overview

The IcpKit Library is built on top of Candid and is responsible for the actual communication with the canisters. The role of IcpKit is to abstract away the underlying data structures used when calling canister functions.

The main classes of the IcpKit Library are the following :

Class	Description
ICPRequestClient	Responsible for making the HTTP requests to the methods of canisters. It receives the arguments of the method as a CandidValue and will return the result of the canister as a CandidValue. Internally, it will serialise the candid arguments using the CandidSerialiser, sign the request when a ICPSigningPrincipal is provided and finally wrap everything in CBOR. When a response is received the opposite process is performed to return the CandidValue.
ICPFunction<T,R>	Wraps the CandidEncoder around the ICPRequestClient so that a canister method can be called using Swift objects instead of a CandidValue. Similarly the canister's response is decoded with CandidDecoder to return a Swift object.
LedgerCanister	Is provided as sample code and is the code generated using CodeGenerator from the Ledger.did file. It allows to query the ICP balance of any ICP account and to fetch the details of any ICP Block.
ICPSigningPrincipal	Protocol that must be implemented by your application in order to sign requests. The library provides the function to sign arbitrary data with a private key ICPCryptography.ellipticSign() but it is the responsibility of your app to handle the private keys. Implementing a concrete ICPSigningPrincipal requires you to get the private key required and then call ellipticSign and return the result. See How can I create a Principal

The ICPRequestClient class is the communication workhorse of the library allowing to make method calls to any canister.

The Ledger Canister is provided as a sample implementation which also allows for easy creation of ICP Wallet apps.

CodeGenerator Command Line Tool Overview

To use the CodeGenerator, you must first clone the project

git clone https://github.com/kosta-bity/IcpKit
cd IcpKit

and then you can either use Swift to compile and run or build the executable yourself.

Build the executable

From the IcpKit root folder :

./compile_code_generator.sh

You will find the executable inside the folder .build/release/CodeGenerator. Running the CodeGenerator without any arguments will display the help.

Use Swift to execute

Alternatively you can use swift to run it from the IcpKit root folder :

swift run CodeGenerator

Candid Interface Definition files (.did)

A canister is defined by all the types and methods used to interact with it. Most canisters publish their definition in the .did format defined here.

IcpKit can parse these files and generate Swift code that can be included in your iOS/Mac Project to interact with the canister. This effectively abstracts away all technical details regarding the communication.

The .did files can be one of two types :

• A Service Interface definition including type definitions and service methods. These are called interface .did files.
• A single candid value. These are called value .did files.

Development Process for integrating canisters in your app

Assuming that you have a .did service interface definition file, the steps to interact with a canister are the following :

• Generate Swift code for the .did file using the CodeGeneratorTool.
• Add the generated Swift files in your project.
• Modify the generated Swift files as necessary. The kind of modifications that are allowed are :
  • Renaming of types and methods.
    • Candid naming standards are not the same as Swift naming standards. You might want to respect naming conventions
    • Not all generated types have an assigned name. These are generated as UnnamedType<n> and can be renamed to a more readable name.
  • Adding/Modifying documentation.
• Write code that calls the methods of the canister.

Generating code for interface .did files

Here is a simple interface MyDid.did:

type MyVector = vec opt bool;
type FooResult = record { name: text; count: int8 };
service: { 
    foo: (input: MyVector; sorted: bool) -> (FooResult) query
};

Using the command

CodeGenerator -n SimpleExample MyDid.did

we get the SimpleExample.swift file generated with these contents:

import IcpKit

enum SimpleExample {
	typealias MyVector = [Bool?]
    
	struct FooResult: Codable {
		let name: String
		let count: Int8
	}  
  
	class Service: ICPService {
		func foo(input: MyVector, sorted: Bool) async throws -> FooResult {
			let caller = ICPQuery<CandidTuple2<MyVector, Bool>, FooResult>(canister, "foo")
			let response = try await caller.callMethod(.init(input, sorted), client, sender: sender)
			return response
		}
	}
}

We can then add this file to our project and use it anywhere in our code as follows:

let client = ICPRequestClient()
let myService = SimpleExample.Service("aaaaa-aa", client)
let myVector: SimpleExample.MyVector = [true, false, nil]
let record = try await myService.foo(input: myVector, sorted: false)
print(record.name)
print(record.count)

The code generated is completely type safe and avoids common mistakes such as wrong number types etc...

Generating Code for value .did files

An example of a value .did file is the following phone_book.did:

(
	vec {
	   record { name = "MyName"; phone = 1234 : nat; };
	   record { name = "AnotherName"; phone = 4321 : nat; };
	}
)

These types of files can be parsed using the following command :

./CodeGenerator value -n PhoneBook phone_book.did

This will generate PhoneBook.swift with the following contents :

enum PhoneBook {
   struct UnnamedType0: Codable {
      let name: String
      let phone: BigUInt
   }
   
   let cValue: [UnnamedType0] = [
   		.init(name: "MyName", phone: 1234),
      .init(name: "AnotherName", phone: 4321),
   ]
}

Parsing .did files during runtime

Your app might require to download a .did file, parse it and extract some values from it. This can be done using the CandidParser. For example, if we wanted to download and read the above phone_book.did

let phoneBookDidContents: String = try await downloadDid() // download phone_book.did file and read it 
let phoneBookCandidValue: CandidValue = try CandidParser().parseValue(phoneBookContents)

We can go one step further and decode the parsed CandidValue into a Swift struct :

struct PhoneBookEntry: Decodable {
  let name: String
	let phone: BigUInt
}
let phoneBook: [PhoneBookEntry] = try CandidDecoder().decode(phoneBookCandidValue)

Swift/Candid Encoding Rules

Candid	Swift	Notes
bool	Bool
text	String	utf8 encoding
int	BigInt
nat	BigUInt
int<n>	Int<n>	n = 8, 16, 32, 64, Int corresponds to int64
nat<n>	UInt<n>	n = 8, 16, 32, 64, UInt corresponds to nat64
blob	Data
opt <candid_type>	<swift_type>?	Because of limitations in the Swift compiler, Optional Structs with nil value are encoded as opt empty. `CandidEncoder().encode(MyStruct?.none) // .option(.empty) Type of nil Structs can not be determined)
vec <candid_type>	[<swift_type>]
record	struct <name>: Codable { ... }	Every record is encoded/decoded to a Swift Codable struct. Each record item corresponds to a struct member value with the same name. If the candid item is keyed only by number then the name is _<number>. struct MyStruct: Codable { let a: Bool; let b: String? } is encoded to record { a: bool; b: opt text; } and record { bool; text; } decodes to struct MyStruct2: Codable { let _0: Bool; let _1: String }
variant	enum <name>: Codable { ... }	Every variant is encoded/decoded to a Swift Codable enum. Each variant case corresponds to an enum case with the same name. Associated values are attached to each case using their names when available. variant { winter, summer } encodes to enum Season: Codable { case winter, summer } variant { status: int; error: bool; } encodes to enum Status { case status(Int); case error(Bool) }
function	CandidFunctionProtocol	Automatic encoding of Swift functions to a Candid Value is not supported because we can not deduce the function signature from the Swift Types without a value. This is due to Swift's Type system limitations. Decoding is allowed however.
service	CandidServiceProtocol	Encoding is not supported for the same reasons as for functions.
principal	CandidPrincipalProtocol
null	nil
empty	nil
reserved	nil

null,reserved and empty vs nil

When decoding null,reserved or empty from a CandidValue to Swift, they are always decoded as nil in Swift.

When encoding a nil from Swift to CandidValue, it is always encoded as opt <sub_type>.

CodingKeys

The Candid Binary format does not support textual representation of keys for records, variants, functions etc... Rather, a simple hashing method defined here as :

hash(id) = ( Sum_(i=0..k) utf8(id)[i] * 223^(k-i) ) mod 2^32 where k = |utf8(id)|-1

This means that when a CandidValue is deserialised from the binary format, we do not have access to the original key that generated the hash.

The CandidDecoder takes care of this automatically for all Swift structs by hashing the swift name and then comparing the hashes.

However, due to the way the Swift Decoding System works, we can not do the same for Swift enums. This means that for a Swift enum to be decoded from a Deserialised CandidValue (essentially every value received via the ICPRequestClient) we need to define the CodingKeys as follows :

• If we are decoding from a variant with named values :

  type NamedVariant = variant { one: int8; two: int16; three: record { a: int8; b: int16 } };
  

  We use CandidCodingKey to perform the hashing :

  enum NamedVariant {
    case one(Int8)
    case two(Int16)
    case three(a: Int8, b: Int16)
      
    enum CodingKeys: String, CandidCodingKey {
      case one, two, three
    }
    enum ThreeCodingKeys: String, CandidCodingKey {
      case a, b
    }
  }

• If we are deocding from a variant with unnamed values :

  type UnnamedVariant = variant { int8; int16; record { int8, int16} };
  

  We use Int, CodingKey to assing integer values to the keys :

  enum UnnamedVariant {
    case anyName(Int8)
    case really(Int16)
    case weCanChoose(here: Int8, too: Int16)
      
    enum CodingKeys: Int, CodingKey {
      case anyName, really, weCanChoose	// as long as we keep the order here
    }
    enum WeCanChooseCodingKeys: Int, CodingKey {
      case here, too										// and here
    }
  }

See the Swift Documentation regarding Encoding, Decoding and Coding Keys for a deeper understanding.

Advanced Topics

How can I manually write code to interact with a canister?

There are several ways to perform a request. Depending if it is a simple query or if the request actually changes the state of the blockchain.

Define the method you wish to call :

let method = ICPMethod(
  canister: ICPSystemCanisters.ledger,
  methodName: "account_balance",
  arg: .record([
    "account": .blob(account.accountId)
  ])
)

Make the a simple query request:

let response = try await client.query(method, effectiveCanister: ICPSystemCanisters.ledger)

equivalently we can also do

let response = try await client.query(.uncertified, method, effectiveCanister: ICPSystemCanisters.ledger)

Make a call request and then poll for the response

let requestId = try await client.call(method, effectiveCanister: ICPSystemCanisters.ledger)
let response = try await client.pollRequestStatus(requestId: requestId, effectiveCanister: ICPSystemCanisters.ledger)

equivalently we can also do

let response = try await client.callAndPoll(requestId: requestId, effectiveCanister: ICPSystemCanisters.ledger)

or even

let response = try await client.query(.certified, method, effectiveCanister: ICPSystemCanisters.ledger)

All these have the exact same result.

Hot to use ICPFunction

The same can be achieved with the following code, only this time by using the ICPFunction. This allows us to directly feed an Encodable to the function and receive back a Decodable. ICPFunction will make sure the input is correctly encoded from the input Swift type to a CandidValue and the output is decoded from the CandidValue to the output Swift type.

struct Result: Decodable {
	let name: String
	let count: Int?
}
// equivalent Candid definition:
// function (nat) -> ( record { name: text; count: opt int64; } )
let function = ICPFunction<BigUInt, Result>(canister, "foo")
let result = try await function.callMethod(12345)

How can I create a Principal?

Starting from a seed

We recommend using the HdWalletKit.Swift from HorizontalSystems in order to derive the public/private Key Pair from the seed. The ICP derivation path is m/44'/223'/0'/0/0

Starting from a public/private Key Pair

The public key must be in uncompressed form.

1. Create a ICPPrincipal instance using ICPCryptography.selfAuthenticatingPrincipal(uncompressedPublicKey:).
2. If you need to sign requests (eg. to send transactions) you also need to create a ICPSigningPrincipal.
3. The main account of this principal can be created using ICPAccount.mainAccount(of:).

Implementing a simple ICPSigningPrincipal

Here is a minimal ICPSigningPrincipalimplementation. This is given as an example only. Do not use this code in production as keeping private keys in memory is generally not a good practice.

The sign method is purposely async to allow for any type of delayed fetching of the private keys (eg. remote access or access to iOS Secure Enclave).

class SimpleSigningPrincipal: ICPSigningPrincipal {
  private let privateKey: Data
  let rawPublicKey: Data
  let principal: ICPPrincipal
  
  init(publicKey: Data, privateKey: Data) throws {
    self.privateKey = privateKey
    self.rawPublicKey = publicKey
    self.principal = try ICPCryptography.selfAuthenticatingPrincipal(uncompressedPublicKey: publicKey)    
  }
  
  func sign(_ message: Data, domain: ICPDomainSeparator) async throws -> Data {
    return try ICPCryptography.ellipticSign(message, domain: domain, with: privateKey)
  }
  
  static func fromMnemonic(_ words: [String]) throws -> SimplePrincipal {
    let seed = HdWalletKit.Mnemonic.seed(mnemonic: words)!
    let xPrivKey = HDExtendedKeyVersion.xprv.rawValue
    let privateKey = try HDPrivateKey(seed: seed, xPrivKey: xPrivKey)
      .derived(at: 44, hardened: true)
      .derived(at: 223, hardened: true)
      .derived(at: 0, hardened: true)
      .derived(at: 0, hardened: false)
      .derived(at: 0, hardened: false)
    let publicKey = privateKey.publicKey(compressed: false)
    return try SimplePrincipal(privateKey: privateKey.raw, uncompressedPublicKey: publicKey.raw)
  }
}

Known Limitations

• Serialisation of recursive candid values leads to infinite loop.
• Encoding of CandidFunctionProtocol is not supported as there is no easy way to infer the CandidTypes used in the arguments and results of the function. This means that the automatic code generation will fail when calling a canister method that expects another function as input.
• Encoding of CandidServiceProtocol is not supported because functions can not be encoded.
• Encoding of optional Structs can not infer the members of the struct. These are encoded as opt empty which should be accepted by all canisters according to Candid specifications.