TriggerEventAsyncInternal<T>

ClientScript.cs

using CitizenFX.Core;
using System;
using System.Threading.Tasks;

using static CitizenFX.Core.Native.API;

public abstract class ClientScript : BaseScript
{
    private async Task<T> TriggerEventAsyncInternal<T>(string eventName, int timeout, bool server, params object[] args)
    {
        var responseEventName = $"{eventName}:response";
        Action<T> eventHandler = null;
        bool hasResult = false;
        T result = default;
        
        eventHandler = new Action<T>((T tResult) =>
        {
            EventHandlers[responseEventName] -= eventHandler;
            result = tResult;
            hasResult = true;
        });
        EventHandlers[responseEventName] += eventHandler;
      
        if (server)
            TriggerServerEvent(eventName, args);
        else
            TriggerEvent(eventName, args);

        var t = DateTime.Now;
        while (!hasResult && (DateTime.Now - t).TotalMilliseconds < timeout)
            await Delay(0);

        return result;
    }

    protected async Task<T> TriggerEventAsync<T>(string eventName, params object[] args)
        => await TriggerEventAsyncInternal<T>(eventName, 250, false, args);

    protected async Task<T> TriggerServerEventAsync<T>(string eventName, params object[] args)
        => await TriggerEventAsyncInternal<T>(eventName, 2500, true, args);
}

ServerScript.cs

using CitizenFX.Core;
using System;
using System.Threading.Tasks;

using static CitizenFX.Core.Native.API;

public abstract class ServerScript : BaseScript
{
    private async Task<T> TriggerEventAsyncInternal<T>(string eventName, int timeout, bool server, Player target, params object[] args)
    {
        var responseEventName = $"{eventName}:response";
        Action<T> eventHandler = null;
        bool hasResult = false;
        T result = default;

        eventHandler = new Action<T>((T tResult) =>
        {
            EventHandlers[responseEventName] -= eventHandler;
            result = tResult;
            hasResult = true;
        });
        EventHandlers[responseEventName] += eventHandler;

        if (server)
            TriggerEvent(eventName, args);
        else
        {
            if (target != null)
                target.TriggerEvent(eventName, args);
            else
                TriggerClientEvent(eventName, args);
        }

        var t = DateTime.Now;
        while (!hasResult && (DateTime.Now - t).TotalMilliseconds < timeout)
            await Delay(1);

        return result;
    }

    protected async Task<T> TriggerEventAsync<T>(string eventName, params object[] args)
        => await TriggerEventAsyncInternal<T>(eventName, 250, true, null, args);

    protected async Task<T> TriggerClientEventAsync<T>(string eventName, params object[] args)
        => await TriggerEventAsyncInternal<T>(eventName, 2500, false, null, args);

    protected async Task<T> TriggerClientEventAsync<T>(Player target, string eventName, params object[] args)
        => await TriggerEventAsyncInternal<T>(eventName, 2500, false, target, args);
}