Use callbacks for bindings to enable async behavior

[nick-thompson]

Turns this:

m_webView->bind("example", [this](const choc::value::ValueView&) -> choc::value::Value {
    if (!isArrayWithNonZeroLength(args))
    {
        return {};
    }

    return doExample(args[0]);
});

into this

m_webView->bind("example", [this](const choc::value::ValueView& args, choc::ui::WebView::ReplyFn&& reply) {
    if (!isArrayWithNonZeroLength(args))
    {
        return reply({}, "Invalid arguments");
    }

    return reply(doExample(args[0]), {});
});

This leans into the already async behavior of choc's binding stuff, using a lambda to enable an async implementation that calls back when it has its value ready (i.e. after a bg thread has finished doing some work). This keeps the sync behavior already have, for cases where we need it, by just synchronously calling the replyFn when we have our value.

[awood314]

this is demystifying!

[awood314]

clarifying question: in this new world, all functions would be required to call reply (at their leisure), and not be required to return anything. is that right?

[nick-thompson]

Right!

[steve-mackinnon]

Non-blocking thought: I wonder if it's worth supporting both synchronous and async bind() calls. If this were to get upstreamed I think that would be necessary. Maybe have a separate bindWithAsyncReply() method.

That would make the implementation a little more complex, but you should be able to do something like:

// Pseudocode
using CallbackFn = std::function<choc::value::Value(const choc::value::ValueView& args)>;
using AsyncCallbackFn = std::function<void(const choc::value::ValueView& args, ReplyFn&& reply)>;
// ...
unordered_map<string, variant<CallbackFn, AsyncCallbackFn>> bindings;
// ...
invokeBinding() {
   if (holds_alternative<CallbackFn>(b)) {
       doSynchronousCallback();
    } else {
        doAsyncCallback();
    }
}

[steve-mackinnon]

So pumped about this! really solid solution 🔥