Introduction

Echo is a Swift package for consuming RESTful and REST-like web services. It provides a lightweight, API-centric wrapper around the more general URLSesssion class provided by the Foundation framework. The project's name comes from the nautical E or Echo flag:

For example, the following code uses Echo's WebServiceProxy class to access a simple web service that returns the first n values of the Fibonacci sequence:

let webServiceProxy = WebServiceProxy(session: URLSession.shared, baseURL: baseURL)

do {
    // GET test/fibonacci?count=8
    let response: [Int] = try await webServiceProxy.invoke(.get, path: "test/fibonacci", arguments: [
        "count": 8
    ])
    
    print(response) // [0, 1, 1, 2, 3, 5, 8, 13]
} catch {
    print(error.localizedDescription)
}

iOS 18 or later is required.

WebServiceProxy

The WebServiceProxy class is used to issue API requests to the server. This class provides a single initializer that accepts the following arguments:

• session - a URLSession instance
• baseURL - the base URL of the service

Service operations are initiated via one of the following methods:

public func invoke(_ method: Method, path: String, 
    arguments: [String: Sendable] = [:],
    content: Data? = nil, 
    contentType: String? = nil) async throws { ... }

public func invoke<B: Encodable>(_ method: Method, path: String, 
    arguments: [String: Sendable] = [:],
    body: B) async throws { ... }

public func invoke<T: Decodable & SendableMetatype>(_ method: Method, path: String,
    arguments: [String: Sendable] = [:],
    content: Data? = nil, 
    contentType: String? = nil) async throws -> T { ... }

public func invoke<B: Encodable, T: Decodable & SendableMetatype>(_ method: Method, path: String,
    arguments: [String: Sendable] = [:],
    body: B) async throws -> T { ... }

public func invoke<T>(_ method: Method, path: String,
    arguments: [String: Sendable] = [:],
    content: Data? = nil, 
    contentType: String? = nil,
    responseHandler: @escaping ResponseHandler<T>) async throws -> T { ... }

All variants accept the following arguments:

• method - the HTTP method to execute
• path - the path to the requested resource, relative to the base URL
• arguments - a dictionary containing the query arguments as name/value pairs

The first two versions execute a service method that does not return a value. The following two versions deserialize a service response of type T using JSONDecoder. The final version accepts a ResponseHandler callback to facilitate decoding of custom response content:

public typealias ResponseHandler<T> = @Sendable (_ content: Data, _ contentType: String?) throws -> T

Three of the methods accept the following arguments for specifying custom request body content:

• content - an optional Data instance representing the body of the request
• contentType - an optional string value containing the MIME type of the content

The other two methods accept a body argument of type B that is serialized using JSONEncoder. JSON data is encoded and decoded using a date strategy of millisecondsSince1970.

Query Arguments

Any sendable value may be used as a query argument and will generally be encoded using its string representation. However, Date instances are first converted to a 64-bit integer value representing epoch time. Additionally, array instances represent multi-value parameters and behave similarly to <select multiple> tags in HTML forms.

The undefined property of the WebServiceProxy class can be used to represent unspecified or unknown argument values.

Return Values

A value representing the server response is returned upon successful completion of an operation. If an operation does not complete successfully, a WebServiceError will be thrown. The error's statusCode property can be used to determine the nature of the error. If the type of the error response is "text/plain", the response content will be provided in the error's localized description:

if let webServiceError = error as? WebServiceError {
    print(webServiceError.statusCode)
}

print(error.localizedDescription)