JavaScriptCore call function with callback Swift

Question

Im trying to call a Javascript Function with an callback from Swift and i dont get it working.

What i have is:

Javascript:

        global.calculateWithCb = function(temp,cb){
            cb(5 * temp)
        }

        global.calculate = function(temp) {
            return 5 * temp
        }

Swift:

        let context = JSContext()
        let scriptURL =        NSURL(fileURLWithPath: String("XXX/bundle.js"))
        let script : String = try String(contentsOfURL:scriptURL, encoding: NSUTF8StringEncoding)

        var jsSource: String! = script
        jsSource = "var window = this; \(jsSource)"
        context.evaluateScript(jsSource)

        let calculate = context.objectForKeyedSubscript("calculate")
        let output = calculate.callWithArguments([40]).toNumber()

So the calculate function is working like expected but i really don't know how to pass a block as a callback argument. Im searching a lot in the web but i don't find any hint maybe somebody can point me in the right direction?

May be duplicate [http://stackoverflow.com/questions/24595692/swift-blocks-not-working](http://stackoverflow.com/questions/24595692/swift-blocks-not-working) — Qichang He, Apr 01 '16 at 04:04
Did you ever figure this out? I'm trying to do this right now. — datWooWoo, May 24 '16 at 21:53
I have it completely working with Objective C, would this help you? — Sprotte, May 25 '16 at 08:14

score 3 · Answer 1 · answered May 04 '19 at 10:42

3

You need to manually init a JSValue with a @convention(block) swift block before calling as argument.

let testBlock : @convention(block) (JSValue?) -> Void = { calledBackValue in
    print("calledBackValue:", calledBackValue)
}
let callback = JSValue.init(object: testBlock, in: context)
// ...
jsFunctionToCallWithCallback.call(withArguments: [42, callback])

var theJSFunction = function(val, callback) {
    // Do smt...
    callback(val);
}

answered May 04 '19 at 10:42

barissenkal

31
3

I tried implementing this solution, but I can't get it to work. Is the context that you pass to the callback just the generic JSCore context previously set up? – Julia Dec 18 '22 at 12:45
@Julia Yes it's the same JSContext as theJSFunction is running in. I remember calling a 3 arg swift block with 2 args on js side being a problem, might want to make sure number of args is same on both sides. Also in my old code I end up using `(JSValue) -> Void` instead of `(JSValue?) -> Void` for some reason – barissenkal Dec 25 '22 at 21:42

JavaScriptCore call function with callback Swift

1 Answers1