Function Templates with callback functions in Go

[rogchap]

Basic FunctionTemplate support, which allows global functions that callback to Go functions:

iso, _ := v8go.NewIsolate()
global, _ := v8go.NewObjectTemplate(iso)
printfn, _ := v8go.NewFunctionTemplate(iso, func(info *v8go.FunctionCallbackInfo) *v8go.Value {
	fmt.Printf("%+v\n", info.Args())
	return nil
})
global.Set("print", printfn, v8go.ReadOnly)
ctx, _ := v8go.NewContext(iso, global)
ctx.RunScript("print('foo', 'bar', 0, 1)", "")
// Output:
// [foo bar 0 1]

Due to the complexities of Go -> C -> Go there are two registries:

1. Callback registry attached to the Isolate
2. A global registry for the Context

To make sure that created Contexts can be GC'd we have a register/deregister surrounding script execution; other wise a map[int]*Context would never GC

[tmc]

There's a lot going on here and I can't say I grok every line but I'm going to approve as the tests look good and I don't see glaring errors!

[choonkeat]

how does vm.TerminateExecution interact with callback of v8go.NewFunctionTemplate?

would it be possible to include context.Context as first argument of callback?

[rogchap]

Once the VM is terminated, you will no longer be able to fire the callback. These callbacks are connected to the isolate, so once the Isolate is GC'd so will be the callback functions (unless attached to some other struct etc)

You can get the current context via info.Context()

[rogchap]

context.Context will get confusing with v8go.Context (as I just did)
The V8 API does not have such a concept as the Go Context; if we introduce the Go Context, this would have to exist throughout the API, which would likely be a breaking change.

[rogchap]

I had a look at your code (https://github.com/choonkeat/try-v8go/blob/main/main.go)
I see what you are trying to do... you want to be able to check if the Go Context is Done within the callback?
Maybe we could attach a context in a way that is non-breaking ie. something like info.GoContext() ??

[rogchap]

@choonkeat Would you be able to submit this Issue as a feature request? It's maybe something we could look to supporting.

[choonkeat]

context.Context will get confusing with v8go.Context (as I just did)

ya :-P

Would you be able to submit this Issue as a feature request?

it was more of a comment, since idiomatic Go callback function usually comes with context.Context. But if it doesn't fit v8, I don't mind passing my own context.Context via closure

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i'm still trying to wrap my head around how things "should" work. e.g. implementing fetch i'd need to return a Promise i think and thus will be tracking #61 closely

[rogchap]

Yes, to implement true fetch you need to return the Promise; A poor man's version is here: https://github.com/rogchap/v8go/blob/master/function_template_test.go#L52

[rogchap]

FYI. this is better supported now that we have #76