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Can't read from a file using JSPromise #121

@revolter

Description

@revolter
Contributor

I know that this is more of a support issue than a bug report, but I really don't know what else to try, and didn't find another help channel.

Here is the entire script using Tokamak:

import JavaScriptKit
import TokamakDOM

struct TokamakApp: App {
    var body: some Scene {
        WindowGroup("Tokamak App") {
            ContentView()
        }
    }
}

struct ContentView: View {
    var body: some View {
        VStack {
            HTML("input", [
                "id": "file",
                "type": "file"
            ])
            Button("Convert") {
                let document = JSObject.global.document
                let input = document.getElementById("file")
                let file = input.files.item(0).object!

                let promise = file.text.function!.callAsFunction().object!

                JSPromise<JSValue, Error>(promise)!.then { value in
                    let console = JSObject.global.console.object!
                    let log = console.log.function!

                    log(value)
                }
            }
        }
    }
}

// @main attribute is not supported in SwiftPM apps.
// See https://bugs.swift.org/browse/SR-12683 for more details.
TokamakApp.main()

which is throwing this error:

Unhandled Promise Rejection: TypeError: Can only call Blob.text on instances of Blob

Running

document.getElementById("file").files.item(0).text().then(text => console.log(text))

works though.

Activity

j-f1

j-f1 commented on Mar 6, 2021

@j-f1
Member

The problem is here:

let promise = file.text.function!.callAsFunction().object!

The use of callAsFunction is unnecessary and causes the text method to not get the proper this value internally. Here’s the standard way to do this in JSKit:

let promise = file.text().object!
revolter

revolter commented on Mar 6, 2021

@revolter
ContributorAuthor

Indeed, though I actually had to change it to:

let promise = file.text!().object!

But now I'm getting:

[Error] Fatal error: The function was already released: file JavaScriptKit/JSFunction.swift, line 292
[Error] Unhandled Promise Rejection: RuntimeError: Unreachable code should not be executed (evaluating 'exports.swjs_call_host_function(host_func_id, argv, argc, callback_func_ref)')

revolter

revolter commented on Mar 7, 2021

@revolter
ContributorAuthor

I fixed it by changing it to:

let jsPromise = JSPromise<JSValue, Error>(promise)!

jsPromise.then { value in
    let console = JSObject.global.console.object!
    let log = console.log.function!

    log(value)

    // Without this, `jsPromise` gets released before this
    // closure gets called.
    print(jsPromise)
}

but I feel like it's not the best solution.

j-f1

j-f1 commented on Mar 7, 2021

@j-f1
Member

I think that’s the best we can do for now. There isn’t currently a cross-browser supported way for us to keep an object alive on the Swift side as long as its corresponding JS object remains alive. So you have to keep a strong reference to the JSObjectRef inside Swift if you want JS to be able to call into it. Once Safari gains support for FinalizationRegistry, it should be possible for us to automatically hold onto Swift objects until they’re no longer reachable from either Swift or JS.

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          Can't read from a file using JSPromise · Issue #121 · swiftwasm/JavaScriptKit