How to make a PROPER main menu (or any other UI windows/modals) in Unity?

To address your problems:

• With default UnityEvent you cannot pass multiple parameters in the inspector. There are plugins that allow you to do that.
• You are probably referring to EventSystem. It has Standalone Input Module which which has settings for input. Event System has First Selected. It's an old input system module, you might want to write your own one to have full control over it. There probably exist ways to extend it, though.
• That is up to your created system or extended version of event system. I don't believe you can do this by default.

I would recommend to look into Divide-and-conquer. Try to split responsibilities as much as possible as long as it seems reasonable. If you are designing a system for reusable UI - you might want to split things like handling UI objects and how they react to changes of states in your game. Every game in some instances is different and in some it's similar. You can find things that are common for every/multiple games and create a system that uses those features only. And for specific features you would create game's unique behaviours.

Let's break down what each UI has in common or could have in most instances:

• Appear/Diappear.
• Animations [Explained in "Most instances"].
• Data to display. [This one is a bit tricky, there way too many ways to put data in required forms. But you can choose your own approach to displaying data which in your own opinions suits your needs the best, or is the most performant, whatever you choose].
• Fields to output data to. [Same story with data].
• Data communication. [How to send data to display/How to get required data to display].

(Most instances - because really what UI system needs to do is display information. I could just be Text, Open, Close. Not every UI needs animations, but well they feel nice :))

For this example, during game jam, I have created a project with reusable Views which is open to anyone on Github. In View I have tried to put everything that would usually be required for UI in Unity. Project has a little Main Menu demo. Like:

• Openning and closing.
• Playing animations.
• Generating animations for specific view.
• Makes use of Canvas Group which is commonly used in UI.
• Automatically creates/changes a few things on GameObject when script is applied.

You can modify it to your needs. But from my personal experience for every game that I have been using this system on I haven't removed any features, only added new ones or fixed some things. It does't really cover Data • because it is designed for every view to have it's own type of data communication route [to put it simply - just creation of new method to display data with strong type. Or any other way, if you develop one, but this one is the easiest].

Project is created in LWRP with Unity 2019.2.2f1 - Unity UI View System.

Small example with animations from the project

For other game specific things like:

• Inventory.
• Dialogues.
• User information.

Those things are specific which require its own approach to how UI will work for them. But they still have those common things, like Open/Close, etc. If you want those UI's to be reusable as well - then make the systems[Inv, Dialogue...] reusable as well. One should adapt to the other in some way. (If someone knows some magic how to avoid that - I will gladly listen to theirs solution)

P.S. I've put this in another answer because it describes a different approach to solving the problem. I still use MVC alike pattern for personal projects which is a more complicated system to be described or developed.

General

There are many solutions for this approach. Mainly it would be just learning how to make every system reusable and modular. There are already a lot of solutions/patterns on the internet like: Micro-Services, Module Pattern...

After a bit of time and experience you would understand how to make everything modular and come up with your own solutions that work for your needs the best.

Specific

I am going to describe the main system that I am using for UI.

I have a system that makes use of MVC pattern. I call it OMCVO - Object Model Controller View Output.

In Objects[aka Components] I have data objects [Models] that are sent to a Controller which decides to what view to assign the data. Some views that I have are Singletons, I usually don't use them through Controller.

[Some of the method naming isn't the actual names of methods in system, they are just to show an example].

Basically, I've split the Data, Output and Behaviour. The workflow is very simple:

• I create some DataType which I load with information and send this data to controller [Controller.Take(data)].
• Some View that accepts this DataType through Controller does calculations and displays them to Output.
• Some Data is linked to views for real-time changes.
• Views have standard API like Open, Close, Show, Hide, animations, relations to one another and other similar stuff.

Technical implementation is very tedious and complicated, so I won't go into it.

Part of old asnwer that some people might find useful

But for this purpose I would have different Views that are Models and Controllers of itself.

For example, Confirmation Window - it has a method public void ApplyConfirmation(UnityAction buttonConfirmationAction, string text); - and when you click on exit button wich has some method applied to if from MenuManager or whatever which has a call to Confirmation Window.

ConfirmationWindow.Instance.ApplyConfirmation(delegate { this.ExitGame(this.SaveDataBeforeExit) }, "Do you really want to exit?");

Now confirmation window pops up. ApplyConfirmation in my case has an "Yes" button which is asigned any method to be called when it's pressed and a text that changes the text on pop up window.

If you are sure that you are going to have only 1 UI element of that behavior and you can reuse it - you can go ahead and make it a Singleton to be able to call it easily from any class like UIElementClassName.Instance.PublicMethodToCall();

Hope you got the idea from it.

For a simpler and better described solution look at my other answer.

If you want a modular UI you can use and implement different fonts/texts, I'd write my own gui texture's fonts ect and have a OnGui holding all the options you want then simply either create functions regarding each option or have multiple scripts. Regarding 'noise' if it works it works, having a simple:

audioMenu = false; mainMenu = true; 

Can simply revert u back to the main screen.