How to call an async javascript function from a javascript SDK in Swift with JavaScriptCore

Question

I am trying to execute an async function from a javascript SDK in Swift using JSCore. I have been able to accomplish evaluating and viewing the contents of the script but have been unsuccessful in actually calling the async function. Here is my code to verify I have even read the script. Any thoughts on how to accomplish this?

lazy var context: JSContext? = {
    let context = JSContext()

    guard let
        commonJSPath = Bundle.main.path(forResource: "test", ofType: "js") else {
            print("Unable to read resource files.")
            return nil
    }

    do {
        let common = try String(contentsOfFile: commonJSPath, encoding: String.Encoding.utf8)
        _ = context?.evaluateScript(common)
    } catch (let error) {
        print("Error while processing script file: \(error)")
    }

    return context
}()

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.
    setupJS()

}

func setupJS(){
    guard let context = context else {
        print("JSContext not found.")
        return
    }

    let dictionary = context.objectForKeyedSubscript("MainObject").toDictionary()
    print(dictonary)
    //let script = "const {variableName} = FooJS;";
    //let result = context.evaluateScript(script)

}

I'd like to do something like async function doSomething(){ return 'hello world!'; } console.log(await doSomething());

How do you know it hasn't been successful? Is the problem that you expect the Swift runtime to wait for some synchronous "finish" or something? — Pointy
– Pointy, Commented Oct 30, 2017 at 21:17
i have 2 problems, one subscripting to the actual function to call, and then awaiting the result in Swift — iPhonig
– iPhonig, Commented Oct 30, 2017 at 21:30

Predrag Samardzic · Accepted Answer · 2017-10-30 22:41:42Z

4

To call the javascript function from swift:

context.objectForKeyedSubscript("functionName_in_Javascript_file").call(withArguments: [])

In order to get control back to swift, you could define a block to be called from javascript like this:

//someString could be a result you need from javascript.

let swiftHandler: @convention(block) (String) -> Void = {(someString) in
        //do something
    }

//make block available to javasript like this:

let swiftBlock = unsafeBitCast(swiftHandler, to: AnyObject.self)

context.setObject(swiftBlock, forKeyedSubscript: "swiftCallback" as (NSCopying & NSObjectProtocol)!)

//and lastly in your javascript file, when you obtain the result you need, just call swift function using name you gave it in previous step:

swiftCallback(result);

edited Oct 30, 2017 at 22:41

answered Oct 30, 2017 at 22:06

Predrag Samardzic

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3 Comments

Byurhan Beyzat Over a year ago

Does there no way to await JS functions without callbacks?

Julia Over a year ago

can someone give a full example for this solution? I can't seem to be able to adapt it to my scenario. Thanks

Julia Over a year ago

when I try to use these code snippets, I get the error about the context.setObject() line: "Value of optional type 'JSContext?' must be unwrapped to refer to member 'setObject' of wrapped base type 'JSContext'"

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