NUI callbacks

NUI can also send calls back to the game using so-called 'NUI callbacks'. These are currently only fully supported in Lua, other languages can be used but need a bit of a tricky workaround as these predate function references in codegen.

Generally, you'll use the RegisterNUICallback function in Lua, and the REGISTER_NUI_CALLBACK_TYPE native along with an event handler in other languages.

Both work very similarly, and we'll describe both below:

Registering a NUI callback in Lua

RegisterNUICallback('getItemInfo', function(data, cb)
    -- POST data gets parsed as JSON automatically
    local itemId = data.itemId

    if not itemCache[itemId] then
        cb({ error = 'No such item!' })
        return
    end

    -- and so does callback response data
    cb(itemCache[itemId])
end)

Registering a NUI callback in C#/JS

// JS
RegisterNuiCallbackType('getItemInfo') // register the type

// register a magic event name
on('__cfx_nui:getItemInfo', (data, cb) => {
    const itemId = data.itemId;

    if (!itemCache[itemId]) {
        cb({ error: 'No such item!' });
        return;
    }

    cb(itemCache[itemId]);
});

// C#
RegisterNuiCallbackType("getItemInfo"); // register the type

// register the event handler with manual marshaling
EventHandlers["__cfx_nui:getItemInfo"] += new Action<IDictionary<string, object>, CallbackDelegate>((data, cb) =>
{
    // get itemId from the object
    // alternately you could use `dynamic` and rely on the DLR
    if (data.TryGetValue("itemId", out var itemIdObj))
    {
        cb(new 
        {
            error = "Item ID not specified!"
        });

        return;
    }

    // cast away
    var itemId = (itemIdObj as string) ?? "";

    // same as above
    if (!ItemCache.TryGetValue(itemId, out Item item))
    {
        cb(new 
        {
            error = "No such item!"
        });

        return;
    }

    cb(item);
});

Invoking the NUI callback

// browser-side JS
fetch(`https://${GetParentResourceName()}/getItemInfo`, {
    method: 'POST',
    headers: {
        'Content-Type': 'application/json; charset=UTF-8',
    },
    body: JSON.stringify({
        itemId: 'my-item'
    })
}).then(resp => resp.json()).then(resp => console.log(resp));

To prevent requests from stalling, you have to return the callback at all times - even if containing just an empty object, or {"ok":true}, or similar.