Event Systems in Roblox Lua (Part 4 – Full Solution)

Click here for part 3 of this tutorial series, where we used a BindableEvent to take all the work out of creating our own event system. Ok, maybe not all the work, but still…The limitations are real with BindableEvents. Namely the arguments from firing them can’t include special tables, and that’s a problem when we’re working with our custom objects with metatables. Let’s fix it with a little bit of Lua magic.

First off, we need to save the original values when the event is fired. We don’t know what variables are sent to the fire  method, but we are able to save them with a table using the ...  operator. If you want to know the nitty-gritty, definitely read section 5.2 on variable arguments in Programming in Lua. Let’s add an argument table and an argument count fields in the event itself:

Note that we don’t have to add these here, by the way. Initializing them to nil does do anything except let us, the programmers, know that these are accessible fields that we’ll use in the future. It’s good practice, so you should get in the habit of showing these fields here.

Next, let’s save the arguments and argument count in these fields by revisiting our fire function:

Notice how we use ...  within a table to capture the values into argData . We can now use this table to inspect the possibly many arguments passed. Also, an important detail is the use of the select  function: if you give "#"  as the first argument, it returns the number of arguments passed after it. We just use   ...  again here to do that. Finally, we aren’t even sending anything to the BindableEvent anymore since we’re handling that ourselves. It’s best to remove it for efficiency’s sake.

Okay, now we have all the information saved! How do we use it? We have to get a bit creative with our connect function. Instead of connecting the function directly to the BindableEvent’s Event, we need to connect our own anonymous function. Inside, we’ll unpack  the arguments we saved while firing the event, and use that to call our function.

We use unpack  here in order to return each of the saved arguments from 1 to the number we saved. I’ll save the detailed explanation of unpack for section 5.1 in Programming in Lua. Just know that it’s like returning multiple values from a function, except for unpack  it’s the multiple values are all the indices of the table we give it.

And that should solve our problems with the BindableEvent! We get all the nice threading that Roblox provides internally while working around the table limitations by saving and unpacking them ourselves. Try out the final product with the test code from part 3 and you’ll see the improvement.

And there you have it – just a few ways you can create and use your own custom events within your Lua code on Roblox. You can download the final Event class code here. I had to whittle down my examples to just these 3, so if there’s interest I’ll post others. If you have questions, send me a direct message on Twitter or a message on Roblox. Thanks!

Event Systems in Roblox Lua (Part 3 – Better Solution)

Click here for part 2 of this tutorial series. In part 2, we created a simple solution to our problem of creating an event system. We saw there are some problems with this, namely the synchronous execution. One after another, our connected functions are called when the event is fired.

Often times, when you’re using a game engine it’s best to let the engine do the heavy lifting for you. Roblox provides the BindableEvent object to create events in (or out of) the game hierarchy, however it isn’t often that you’ll want to instantiate one every time you want to create a custom event. Let’s create an Event class which uses BindableEvent internally intstead. Let’s re-start with the boilerplate from part 2:

We’re no longer keeping track of the connections ourselves with a table. We’ll merely be passing the connect and fire behaviors onto the already existing functionality of the object. This is quite simple, actually:

The key benefit here: since we’re delegating the responsibility of calling the connected functions to the BindableEvent, it can handle the threading as well. Connected functions that yield will not block other connected functions, so this is a primary benefit from the first simple solution. Note the use of ...  to represent “any and all variables passed to this function”. We’ll talk more about it in part 4.

There is still a problem with this, though. BindableEvents were originally created for inter-script communication. When it comes to tables, there’s some “gotchas”. From the Roblox Wiki article on BindableEvent:

If a Table is passed as an argument to a BindableEvent it must be an array without missing entries or have string keys, not a mixture, or else the string keys will be lost.

This is a big drawback to using a BindableEvent. Your tables will not be the same table as the one you fired the event with. Here’s an example demonstrating this drawback:

You can see that we are firing the event with my_table , then in the connected function comparing that table to (supposedly) itself. This shouldn’t ever throw an error, but it always does due to the described limitations of the BindableEvent.

You’ll also lose metatables, too! So if you created an object using our object-oriented tricks from the previous tutorial, you won’t have an object anymore. Just an empty shell of a table, no purpose, no meaning in life… definitely something you want to avoid.

You can see that the key benefit of letting Roblox handle the threading with the cost of the BindableEvent’s limitations is definitely a trade-off. You can download the finished version of this Event class here. Is it possible to work around this limitation? Maybe. Click here to find out in part 4.

Event Systems in Roblox Lua (Part 2 – Simple Solution)

Click here for part 1 of this tutorial series. You should be familiar with the content discussed in my Object-Oriented Programming tutorial.

In part 1, I discussed the pattern of an event system and the need for it in Roblox code. We frequently find ourselves needing to define our own gameplay defined events. Let’s dive into a simple way we can implement this pattern. Let’s use the OOP tricks we know in Lua to create the shell of an Event class.

So, at surface level we need a way to store some number of functions. We should be able to add/remove those functions. Crazy idea: let’s just use a numerically-indexed table of functions. We’ll use table.insert  to connect a function, and to disconnect we’ll iterate over each index and remove the first we find that matches. When we fire the event, just iterate over the list of functions, calling them in order. Let’s code it up.

First, let’s add a connections  table to our event. This will be our table of functions.

Next, let’s write the fire function. Simple iteration using a numeric for-loop:

That oughta do it! Finally, let’s write the connect function.

Notice how I added a line that will throw an error if we sent no function to connect. Finally, we need to add a means to disconnect. Let’s return a table with a disconnect function as a key:

And there we have it! We’ll soon find out that there are some issues with this. For simple uses cases, this works fine. However, if any connected functions yield (yielding essentially means waiting, like announceGameOver does to display the message), the next connected function will not run until the yielding function completes. This is a problem in the part 1 example because we might want our map to clean up first, then the game to announce the final score. It depends on the order in which we connected our events.

This event system calls connected functions in the order they were connected. In other words, first-in-first-out (FIFO). We could reverse this order (last-in-first-out, LIFO) by iterating backwards in the fire function:

Alternatively, we could just insert new functions at the beginning of the list in connect by specifying index 1 as the insertion point in the table.insert  call:

This is actually less efficient as Lua must move all the existing functions in the table over by 1 to make room for the one we’re inserting. For the computer science nerds in the audience, this is an O(n) operation. Simply inserting to the end of the list would be O(1). It goes without saying that making both of these changes (reverse iteration and insertion at the front of the list) would cancel each other out and we’d be back to original behavior of FIFO.

If this solves your problem, then I have good news: you can stop here. As for the rest of us, we have more options to explore in part 3. You can download the finished version of this Event class here.

Click here to advance to part 3 of this tutorial series.

Event Systems in Roblox Lua (Part 1 – The Problem)

Hey everyone. Lately I’ve been considering the many ways to create event systems for Roblox Lua. You may already be familiar with native events, a.k.a. RBXScriptSignal, in Roblox. You connect your function to one of these, and when the engine detects the occurrence, it calls your function with the relevant arguments. BasePart.Touched is one of the most common examples.

But what if you’re making your own objects, like in my object-oriented programming tutorial? If your custom objects want to define events, you have a ton of different options for implementing this pattern. In this tutorial series, I will explore the different methods you can use to implement an event-listener pattern.

Before we look at the different ways we could create a system like this, let’s define exactly what we’re trying to code up. We want to be able to create an event and connect any number of functions to it. In addition, we want to send arguments to the connected functions when this event is fired. Finally, we need to be able to disconnect a connected function from this event. So, no matter what systems we end up making, we should be able to do something like this (for example, in a round-based game):

A couple things to note:

  • It ought not matter in which order our functions announceGameOver and cleanUpTheGame will run; in a perfect world we’d have them happen at the same time. (In reality, one will always happen before the other and it’s the order in which we connect these functions that may ultimately play a part in what order they are actually called.)
  • Not all connected functions will need to use all the arguments sent by the event, such as cleanUpTheGame  not needing the final score we sent.
  • Not all connected functions will need to stay connected – we might need to disconnect functions as a cleanup measure.
  • For brevity, I will be skipping over writing a wait  method to yield until an event is fired.

With those things in mind, let’s see the different ways we can make event systems in Roblox Lua!

Click here to proceed to part 2 of this tutorial series.

Object Oriented Programming in Lua (Part 4 – Clean-up)

This final part of this tutorial series will talk about some ways to clean up the example from the previous part and apply it to Roblox. First thing’s first: let’s move the entire Car class defintion into its own ModuleScript called “Car” inside ServerScriptService. Now we can combine the CarMetatable and CarMethods into a single table. Just have __index refer to the table itself. Let’s call the table Car.

Next, we can package the newCar constructor into this table. Remember that setmetatable(t, mt) returns the table  t whose metatable we are setting in the function call. The constructor can be simplified to just this:

Notice that we are creating the new object table inside the setmetatable  function, which will both set the metatable and then return it from the constructor. Finally, we fix the method definitions as well.

Since this is inside a ModuleScript, return the Car table on the final line.

Since we are creating objects, we can make many of them at once with all unique identitiesstates and methods. Here’s an expansion of the previous part’s examples. This should go in a script somewhere inside your game.

And that’s it! You should download the code for the Car class and example script. Play around with creating your own classes. It’s a lot of fun and you’ll find object oriented programming is a fantastic way to make your code more organized. If you have questions, send me a direct message on Twitter or a message on Roblox.

Object Oriented Programming in Lua (Part 3 – Examples)

In this section, we will apply the information from the last two sections into an actual example. Let’s implement a Car class in which we will make Car objects. Cars have a state (like their color or speed) as well as behavior like accelerating or honking the horn. We will create fields and methods for both of these.

We start by defining a car metatable and a table of car methods. We set the __index  metamethod so when a new car object is given this metatable, we can access our methods through it.

Next, we write a function to create a new car of a certain color and set the metatable properly. Additionally, we can initialize the speed field to zero. A function like this is called a constructor.

Excellent, now write some methods for the car, like acceleration and honking the horn. Store these in the CarMethods  table. Remember that we’re going to call these methods with colon syntax, so the first argument is going to be the car we’re working with. Let’s call this first parameter  self .

An important thing to mention here is that the following function definitions are all equivalent:

For the last one, we are defining the function with a colon! This tells Lua that we’re going to implicitly create a local variable called self  which refers to the first argument to the function. Conveniently, when we call the method with the colon syntax, self is automatically filled with the car in question.

And this is all we need to have something workable. Create a new car with the newCar  function and use some of its methods (don’t forget to use a colon instead of a period)!

In the next article, we’ll talk about ways to clean up this code more.

Click here for Part 4 of this tutorial series.

Object Oriented Programming in Lua (Part 2 – Metatables)

Part 2! This is all about metatables, which will be the main tool to implement OOP in Lua. Metatables, put simply, are Lua tables that describe the behavior of other Lua tables. You can use getmetatable(tab)  and setmetatable(tab, meta)  to manage a table’s metatable:

Now that you’ve set a table’s metatable, you can now set metamethods. These are special keys in the metatable that define specific behavior when set to a function, for example: __index , __add , __sub , __tostring , __eq and others. Notice how their names all being with two underscores (_). These metamethod functions are called when you do things with the table in question. It’s important to take time and read up on the behavior of each metamethod on your own, like how __add  is called when you add two tables with the same metatable.

We’ll be using __index to implement classes and objects. This metamethod is called when you try to index a key (doing  tab[key] or tab.key, remember these are equivalent) that hasn’t been set in a table yet:

In the above example, the goodbye print statement attempts to index world in my_tab . Since that key hasn’t been set in  my_tab  (it is nil), Lua calls our  __index  function with  my_tab  and  world as arguments. The function returns the string  cruel world , which is returned to the goodbye print statement. This code will output:

This example is pretty crucial to understand what’s going on behind the scenes and why. A key feature of __index is that you don’t have to set it to a function. You can also set it to another table instead! The behavior is now as follows: if you index something that isn’t in the table, it checks the table in __index instead (like a backup table).

Why is this important? For our objects, we will be creating a table of the functions (methods) that every object of one kind should have. We’ll set the metatable of the objects to equal this table of functions so that we don’t have to copy functions over to every instance of an object when it is created.

However, before we get to that, we have to understand colon syntax for method calling (remember that a method and a function are essentially the same, except that the word method is used in the context of OOP). In Roblox Lua, you are required to call methods like Destroy by using a colon. How is this different from using a period? When you use a colon in a method call, Lua sends the table itself (the left of the colon) as the first argument to the function. For example, if you had a table foo  with a function bar , then calling foo:bar()  is the same as foo.bar(foo) . More info about this behavior can be found in Chapter 16 of Programming in Lua. It’s syntax sugar.

When you use a colon in a method call, Lua sends the table itself (the left of the colon) as the first argument to the function.

Let’s put this all together: we can store our object’s methods (functions) inside a table. We set this table as the __index for the metatable of newly created objects. We call these methods using colon syntax so our functions can access the object we’re talking about. In the next article, we will take all these abstract topics and apply the concepts to concrete examples.

Click here for Part 3 of this tutorial series.

Object Oriented Programming in Lua (Part 1 – Concepts)

Hi everyone! This multi-part tutorial is all about object-oriented programming (OOP for short) in Lua. I’ll be applying this information to Roblox’s flavor of Lua. For this tutorial, you should be fairly comfortable with tables (read this chapter from Programming in Lua if needed) as well as how ModuleScripts work. You’ll learn what metatables are and how they can be used to simulate OOP in Lua. Many of the concepts you’ll see here are applicable to other programming languages, namely Java or C#. If you’re going into computer science later in life, this is good to expose yourself to early.

Let’s first talk about what object-oriented programming means: programming involving the use of objects. Objects are distinctly identifiable entities. For example, a car, a button on-screen, a player in-game, or a tool/weapon. Objects have their own individual identities, states and behaviors:

  • The identity of an object just means that it is different from other objects of its kind.
  • The state of an object, such as the current speed of a car or player’s score, are represented by fields.
  • The behavior of an object is defined through methods which are actions the object can perform. A car can accelerate, a player can earn more points, and a button can be pressed.

This should sound familiar to Roblox scripters because every object with which you’ve worked follows this model! A Part has a BrickColor property (a field representing state); a ParticleEmitter can emit particles (a method representing behavior).

In this tutorial, we’re going to make our own blueprints for objects. These are called classes. These blueprints define all the fields and methods by which our objects’ state/behaviors will be identified.

The next article will talk about metamethods and some Lua syntax subtleties that will help us implement these behaviors.

Click here for Part 2 of this tutorial series.

Scripting Dice on ROBLOX

Hi everyone. This is going to be a tutorial on scripting dice using ROBLOX Studio. Let’s start by creating the dice model.

dice
The dice model we’re going to create.

Creating the Dice Model

If you’re already comfortable using ROBLOX Studio to build things, you can skip this part and grab the model off ROBLOX here.

  1. Create a 4×4 part.
  2. Color the dice white. Set the Top and Bottom surfaces to Smooth.
  3. Add the following decals: Top (3), Bottom (4), Front (1), Back (6), Left (2), Right (5). Credit to game-icons.net for the dice face images. Remember, opposite face values of a dice will add to 7.
  4. Add a new Script to the part, as well as a StringValue named Face and an IntValue named Roll. We’ll use these as the output for our script.

The dice’s hierarchy should look like this:

tree

Scripting the Dice

Now for the fun part. Let’s start by getting some variable references to the objects in the game hierarchy (visible in the Explorer window). Remember to open the Output window when scripting! Small note: Unless otherwise mentioned, the given code should be added from top to bottom in the script.

Next, let’s define a function called getHighestFace(part) which will determine the highest face of given part and return it.

What’s going on here: We’re going over each NormalId (Top, Bottom, Left, etc). For each of these, we get a Vector3 value from the NormalId using Vector3.FromNormalId so Top translates to (0, 1, 0), Bottom is (0, -1, 0), etc. Then we’re transforming that point, which is a point local to the given part’s CFrame, into world space using pointToWorldSpace. Finally, we’re getting the Y component (height) of the resulting world point.

The rest of the this function is simply finding the face that gives the highest world point. There’s the meat of the program – now let’s hook it up into something practical by writing an update function that uses the getHighestFace function.

This is simple enough: get the highest face of the dice, then stuff it into the Face StringValue. We’ll revisit this function later to determine what number is on what side of the dice. For now, we’ll use this line to connect the update function with the RunService‘s Stepped event, which fires about 30 times a second when the game is unpaused.

You could also add a while-true-do loop and update on your own frequency if you wanted. Test your script by pressing play, then rotating your dice. If the Face StringValue’s Value changes accurately based on the highest face, then you’ve done got the basis of your dice! If not, check the Output window for any red error messages or yellow warnings. One little typo can mess up the entire script!

If your dice is working properly, let’s go back and update the update function so that it determines which number is on what side of the dice. Add this to the top of your script:

This is a table we’ll use to look up values of the dice decals that are on whatever side of the part is highest. Update your update function to this:

This will stuff the current face’s value into the Roll IntValue. This way, another script in your game could read the current dice roll without having to do any math.

And that’s it! Go ahead and play around with your dice in Play mode and make sure each side corresponds to the correct value. If not, you might have to switch some values in the faceValues table. If none of your values are changing, check the Output for errors.

Here’s the completed dice model in case you got stuck or lost.

dice