Hello World for Lua¶

A tradition in the programming community is to introduce a new language by having you write a simple program that just spits out "Hello, world!"

We will walk you through this process with CVR's Lua Scripting and explain what each part means. Don't worry, while it looks long and boring, it's relatively painless. Experienced programmers can skim through this and not miss much, but new programmers may wish to read the whole thing in it's entirety to understand concepts and terminology.

The Component¶

First, we need to create the component that actually hosts and runs the script.

1. In Unity Editor, create a new cube in the scene: GameObject > 3D Object > Cube
2. Click on the cube to select it.
3. Create a new CVRSpawnable component in the cube:
   1. At the bottom of the Inspector tab, press Add Component.
   2. Search for cvrlua and click on CVR Lua Client Behaviour.

The Script¶

Now that we have somewhere to put the script, let's make the actual script.

1. Create a new folder in your project called Lua. This isn't required, but may help you organize things.
2. Create a new folder in Lua called Hello World. Again, not entirely necessary.
3. Right-click the folder and select Create > CVR Lua Script.
4. Name it helloworld.lua.
5. Open your project folder (the folder above Assets) with VSCode.
6. In your VSCode project window, locate and open helloworld.lua.

Programming¶

You should now have a file that looks like this:

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-- Start is called before the first frame update
function Start()

end

-- Update is called once per frame
function Update()

end

Here, we see two features of Lua: Functions, and comments.

What am I Looking At?¶

Functions are sections of code that can be referenced and executed by other code by name. Each function must start with the keyword function, one or more whitespace characters, then an identifier that must follow some naming rules (no whitespace, can't start with a number), and is followed by zero or more arguments inside of two parentheses. After these, the function can contain zero or more statements. Finally, the keyword end marks the end of the function.

Functions can be passed different kinds of arguments of many different types, like numbers, sequences of characters (called strings), tables (which can be in the form of lists or with more structure), or other types. After processing the arguments, each function can return a value. In the script above, you see two function definitions: Start(), and Update().

You'll also notice two dashes (--) followed by some remark. These are called comments, and are ignored by the scripting engine. The purpose of comments are simply to provide a way for programmers to leave notes in their code. They are not required in any way, and can be placed pretty much anywhere. The most typical locations for these are to be on their own lines, or at the end of a line.

CVR Lua Behaviours are something that start once your prop, avatar, or world object is instantiated by CVR, and persist until the object in question is destroyed. They fill the same niche as a Unity MonoBehaviour.

When the script starts, Start() is called by Unity. Therafter, as long as the script is marked as active, it will receive Update() calls from Unity. These calls are made multiple times per second.

Completing the Objective¶

Let's move right along.

We want to just output "Hello, world!" to console. So, let's do that.

1. In Start(), between the end of the parentheses, type in print followed by a space.
   • print() just prints to console.
2. Type in "Hello, world!"
   • This is a string containing the characters we wish to display.

And you're done!

Let's take a step back and look at what we've accomplished and try to make sense of it:

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-- Start is called before the first frame update
function Start()
   print "Hello, world!" 
end

-- Update is called once per frame
function Update()

end

First, something to keep in mind: Lua allows for simplified function calls. That means for simple situations, it's OK to write print("Hello, world!") as print "Hello, world!" Features like this are called syntax sugar and exist to make life a bit easier. Under the hood, each way of writing this function call do the same thing: They are functionally and syntactically identical.

What we added is an expression called a "function call". We called a built-in function called print(), which takes a single string as a parameter.

If print were written in Lua, it'd look something like this:

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function print(input_string)
    -- Some code here to send the input to logs and console.
end

In reality, it's part of the scripting engine and written (in our case) in C#. All it does in CVR is log the provided message to your game log, and add a message to your in-game console.

Each argument is positional, so an argument specified at position 1 in your function call will arrive at position 1 in the called function. In this case, our call of print("Hello, world") will set the input_string argument to "Hello, world!", and the function can use that argument however it likes.

Uploading¶

Now, go through the usual process of adding a CVRSpawnable to the prop and uploading it to the game.

When you spawn it in-game, it should print to your game logs.

Congratulations! You've made your first script.