Extensions to Apple's JavaScriptCore framework.
JavaScriptCore is a great framework for allowing extensibility in your apps, at least if your users are technically inclined, as users can extend your app’s functionality with Javascript. This works great, but there’s a problem: JavaScriptCore provides no implementation for many common Javascript functions including console.log and fetch.
This package provides implementations of both console.log, fetch, and much more to come in the future. It also provides a protocol that acts as a universal mechanism for adding functions and code to a JSContext called JSContextInstallable.
You can add the console logger functions to a JSContext via:
let context = JSContext()!
try context.install([.consoleLogging])This will install console.log, console.trace, console.debug, console.info, console.warn, and console.error to the JSContext. When calling those functions in Javascript, you’ll see detailed log messages in standard output.
Additionally, you can customize the logging destination via the JSConsoleLogging protocol. For instance, you may want to log messages to a swift-log logger.
import Logging
struct SwiftLogLogger: JSConsoleLogger {
let logger: Logger
func log(level: JSConsoleLoggerLevel?, message: String) {
self.logger.log(level: level?.swiftLogLevel ?? .info, "\(message)")
}
}
extension JSConsoleLoggerLevel {
fileprivate var swiftLogLevel: Logger.Level {
switch self {
case .debug: .debug
case .error: .error
case .info: .info
case .trace: .trace
case .warn: .warning
}
}
}Then, you can install SwiftLogLogger to your JSContext to redirect console.log calls to your logger.
let context = JSContext()!
try context.install([SwiftLogLogger(logger: logger)])You can add Javascript’s fetch function to a JSContext like so.
let context = JSContext()!
try context.install([.fetch])Since fetch depends on many Javascript classes, implementations of those classes will also be installed to the context alongside fetch. Those classes include AbortController, AbortSignal, FormData, Headers, DOMException, Request, Response, Blob, and File. At the time of writing this, ReadableStream is not supported.
You can also configure a URLSession instance to use as the underlying driver of the fetch implementation like so.
let context = JSContext()!
let configuration = URLSessionConfiguration.ephemeral
configuration.protocolClasses = [MyURLProtocol.self]
let session = URLSession(configuration: configuration)
try context.install([.fetch(session: session)])📱
fetch(session:)is only available on iOS 15+ because the fetch implementation uses data task specific delegates under the hood. On older versions, you can usefetch(sessionConfiguration:)wheresessionConfigurationis aURLSessionConfiguration.
The previous examples show how to easily add Javascript code to a JSContext, and this functionality is brought to you by the JSContextInstallable protocol. You can conform a type to the protocol to specify how specific Javascript code should be added to a context.
struct MyInstaller: JSContextInstallable {
func install(in context: JSContext) {
let myFunction: @convention(block) () -> Void = {
// ...
}
context.setObject(myFunction, forPath: "myFunction")
}
}
extension JSContextInstallable where Self == MyInstaller {
static var myFunction: Self { MyInstaller() }
}
let context = JSContext()!
try context.install([.consoleLogging, .fetch, .myFunction])You can also install Javascript files from a Bundle or from the file system using the following.
let context = JSContext()!
try context.install([
.bundled(path: "myBundledFile.js"), // Installs from main bundle.
.bundled(path: "anotherBundledFile.js", in: .module),
.file(at: URL.documentsDirectory.appending("myFile.js")),
.files(at: [
URL.documentsDirectory.appending("someFile.js"),
URL.documentsDirectory.appending("otherFile.js")
])
])