Unity 3d scripting tutorial

Introduction to scripting with Unity
Scripting is an essential part of Unity as it
defines the behaviour of your game. This
tutorial will introduce the fundamentals of
scripting using Javascript. No prior knowledge
of Javascript or Unity is required.

Time to complete: 2 hours.

Author: Graham McAllister

Contents 1. Aims of this tutorial
Scripting is how the user defines a game’s behaviour (or
1. Aims of this tutorial rules) in Unity. The recommended programming
language for Unity is Javascript, however C# or Boo
2. Prerequisites can also be used. This tutorial will cover the
3. Naming conventions fundamentals of scripting in Unity and also introduce key
elements of the Application Programming Interface (API).
4. Player Input You can think of the API as code that has already been
5. Connecting variables written for you which lets you concentrate on your game
design and also speeds up development time.
6. Accessing components
A good understanding of these basic principles is
7. Instantiate essential in order to harness the full power of Unity.
8. Debugging
9. Common Script Types
2. Prerequisites
This tutorial focuses on the scripting element of Unity, it
is assumed you are already familiar with Unity’s interface
(if not you should read the Unity GUI tutorial).
In order to make scripting easier to understand, it is
preferable to have a code editor that has syntax
highlighting support for Javascript. This means that
reserved words (syntax used by Javascript itself) are
coloured differently than user defined words. One such
editor is SubEthaEdit.

NB: any text that requires the user to take an action
begins with a ‘-’.

3. Naming Conventions
Before we begin, it is worth mentioning some conventions in Unity.
Variables - begin with a lowercase letter. Variables are used to store information about
any aspects of a game’s state.
Functions - begin with an uppercase letter. Functions are blocks of code which are
written once and can then be reused as often as needed.
Classes - begin with an uppercase letter. These can be thought of as collections of
functions.
Tip: When reading example code or the Unity API, pay close attention to the first letter of
words. This will help you better understand the relationship between objects.

4. Player Input
For our first program we’re going to allow the user
to move around in a simple game world.

Setting the scene
- Start Unity.
Firstly, let’s create a surface for the user to walk on.
The surface we’re going to use is a flattened cube
shape.
- Create a cube and scale its x,y,z dimensions to 5, 0.1, 5 respectively, it should now
resemble a large flat plane. Rename this object ‘Plane’ in the Hierarchy View.
- Create a 2nd cube and place it at the centre of this plane. If you can’t see the objects in
your Game View, alter the main camera so they’re visible. Rename the object to
Cube1.
- You should also create a point light and place it above the cubes so that they’re more
easily visible.
- Save the scene by selecting File->Save As and give the game a name.

Our first script
We’re now ready to start game programming. We’re going to allow the player to move
around the game world by controlling the position of the main camera. To do this we’re
going to write a script which will read input from the keyboard, then we attach (associate)
the script with the main camera (more on that in the next section).
- Begin by creating an empty script. Select Assets->Create->Javascript and rename this
script to Move1 in the Project Panel.
- Double-click on the Move1 script and it will open with the Update() function already
inserted (this is default behaviour), we’re going to insert our code inside this function.
Any code you insert inside the Update() function will be executed every frame.

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In order to move a game object in Unity we need to alter the position property of its
transform, the Translate function belonging to the transform will let us do this. The
Translate function takes 3 parameters, x, y and z movement. As we want to control the
main camera game object with the cursor keys, we simply attach code to determine if the
cursor keys are being pressed for the respective parameters:
function Update () {
transform.Translate(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
} 

The Input.GetAxis() function returns a value between -1 and 1, e.g. on the horizontal axis,
the left cursor key maps to -1, the right cursor key maps to 1.

Notice the 0 parameter for the y-axis as we’re not interested in moving the camera
upwards. The Horizontal and Vertical axis are pre-defined in the Input Settings, the names
and keys mapped to them can be easily changed in Edit->Project Settings->Input.
- Open the Move1 Javascript and type in the above code, pay close attention to capital
letters.

Attaching the script
Now that our first script is written, how do we tell Unity which game object should have this
behaviour? All we have to do is to attach the script to the game object which we want to
exhibit this behaviour.
- To do this, first click on the game object that you wish to have the behaviour as defined
in the script. In our case, this is the Main Camera, and you can select it from either the
Hierarchy View or the Scene View.
- Next select Components->Scripts->Move1 from the main menu. This attaches the
script to the camera, you should notice that the Move1 component now appears in the
Inspector View for the main camera.

Tip: You can also assign a script to an game object by dragging the script from the
Project View onto the object in the Scene View.

- Run the game (press the play icon at the lower left hand corner), you should be able to
move the main camera with the cursor keys or W,S,A,D.
You probably noticed that the camera moved a little too fast, let’s look at a better way to
control the camera speed.

Delta time
As the previous code was inside the Update() function, the camera was moving at a
velocity measured in meters per frame. It is better however to ensure that your game
objects move at the more predictable rate of meters per second. To achieve this we
multiply the value returned from the Input.GetAxis() function by Time.deltaTime and also
by the velocity we want to move per second:

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var speed = 5.0;

  var x = Input.GetAxis("Horizontal") * Time.deltaTime * speed;
  var z = Input.GetAxis("Vertical") * Time.deltaTime * speed;
  transform.Translate(x, 0, z);
}

- Update the Move1 script with the above code.
Notice here that the variable speed is declared outside of the function Update(), this is
called an exposed variable, as this variable will appear in the Inspector View for whatever
game object the script is attached to (the variable gets exposed to the Unity GUI).
Exposing variables are useful when the value needs to be tweaked to get the desired
effect, this is much easier than changing code.

5. Connecting Variables
Connecting variables via the GUI is a very powerful
feature of Unity. It allows variables which would
normally be assigned in code to be done via drag and
drop in the Unity GUI. This allows for quick and easy
prototyping of ideas. As connecting variables is done
via the Unity GUI, we know we always need to expose
a variable in our script code so that we can assign the
parameter in the Inspector View.
We’ll demonstrate the connecting variables concept
by creating a spotlight which will follow the player
(Main Camera) around as they move.
- Add a spotlight to the Scene View. Move it if necessary so it’s close to the other game
objects.
- Create a new Javascript and rename it to Follow.
Let’s think what we want to do. We want our new spotlight to look at wherever the main
camera is. As it happens, there’s a built in function in Unity to do this, transform.LookAt().
If you were beginning to think ‘how do I do this?’ and were already imagining a lot of code,
then it’s worth remembering to always check the Unity API for a function that already
exists. We could also make a good guess at looking in the ‘transform’ section of the API
as we’re interested in altering the position or rotation of a game object.
Now we come to the connecting variables section; what do we use as a parameter for
LookAt()? Well we could hardcode a game object, however we know we want to assign
the variable via the GUI, so we’ll just use an exposed variable (of type Transform). Our
Follow.js script should look like this:
var target : Transform;

transform.LookAt(target);
}

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- Attach the script to the spotlight and notice when the component gets added, the
“target” variable is exposed.
- With the spotlight still selected, drag the Main Camera from the Hierarchy View onto the
“target” variable in the Inspector View. This assigns the target variable, i.e. the
spotlight will now follow the Main Camera. If we wanted the spotlight to follow a
different game object we could just drag in a different object (as long as it was of type
Transform of course).
- Play the game. If you watch the Scene View you should see the spotlight following the
Main Camera around. You may want to change the position of the spotlight to improve
the effect.

6. Accessing Components
As a game object can have multiple scripts (or other components) attached, it is often
necessary to access other component’s functions or variables. Unity allows this via the
GetComponent() function.
We’re now going to add another script to our spotlight which will make it look at Cube1
whenever the jump button (spacebar by default) is pressed.
Let’s think about this ﬁrst, what do we want to do:
1. Detect when the jump button has been pressed.
2. When jump has been pressed make the spotlight look at Cube1. How do we do this?
Well, the Follow script contains a variable “target” whose value determines which game
object the spotlight should look at. We need to set a new value for this parameter. We
could hardcode the value for the cube (see the section ‘Doing it with code’ later), however
we know that exposing the variable and assigning this via the GUI is a better way of doing
this.
- Create a new Javascript and name it Switch. Add the following code to Switch.js:
var switchToTarget : Transform;
  if (Input.GetButtonDown("Jump"))
    GetComponent(Follow).target = switchToTarget;
}

Notice in particular how Follow is the parameter to GetComponent(), this returns a
reference to the Follow script which we can then use to access its “target” variable.
- Add the Switch script to the spotlight and assign Cube1 to the switchToTarget
parameter in the Inspector View.
- Run the game. Move around and verify that the spotlight follows you as usual, then hit
the spacebar and the spotlight should focus on the Cube1.

Doing it with code
Earlier in the tutorial we mentioned that it would be possible to assign the variables via
code (as opposed to the Unity GUI), let’s take a look at how you would do that.

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Remember this is only for comparison, assigning
variables via the GUI is the recommended approach.
The problem we were interested in earlier was how
do we tell the spotlight to look at Cube1 when the
jump button was pressed. Our solution was to
expose a variable in the Switch script which we could
then assign by dropping Cube1 onto it from the Unity
GUI. There are two main ways to do this in code:
1. Use the name of the game object.
2. Use the tag of the game object.

1. Game object name
A game object’s name can be seen in the Hierarchy View. To use this name with code we
use it as a parameter in the GameObject.Find() function. So if we want the jump button to
switch the spotlight from Main Camera to Cube1, the code is as follows:
  {
    var newTarget = GameObject.Find("Cube").transform;
    GetComponent(Follow).target = newTarget;
}
}

Notice how no variable is exposed as we name it directly in code. Check the API for more
options using Find().

2. Game object tag
A game object’s tag is a string which can be used to identify a component. To see the
built-in tags click on the Tag button in the Inspector View, notice you can also create your
own. The function for ﬁnding a component with a speciﬁc tag is GameObject.FindWithTag
() and takes a string as a parameter. Our complete code to do this is:
  {
    var newTarget = GameObject.FindWithTag("Cube").transform;
    GetComponent(Follow).target = newTarget;
}
}

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7. Instantiate
It is often desirable to create objects during run-time (as the game is being played). To do
this, we use the Instantiate function.
Let’s show how this works by instantiating (creating) a new game object every time the
user presses the fire button (either the left mouse button or left ctrl on the keyboard by
default).
So what do we want to do? We want the user to move around as usual, and when they hit
the fire button, instantiate a new object. A few things to think about:
1. Which object do we instantiate?
2. Where do we instantiate it?
Regarding which object to instantiate, the best way of solving this is to expose a variable.
This means we can state which object to instantiate by using drag and drop to assign a
game object to this variable.
As for where to instantiate it, for now we’ll just create the new game object wherever the
user (Main Camera) is currently located whenever the fire button is pressed.
The Instantiate function takes three parameters; (1) the object we want to create, (2) the
3D position of the object and (3) the rotation of the object.
The complete code to do this is as follows (Create.js):
var newObject : Transform;

  if (Input.GetButtonDown("Fire1")) {
  Instantiate(newObject, transform.position, transform.rotation);
  }
}

Don’t forget that transform.position and transform.rotation are the position and rotation of
the transform that the script is attached to, in our case this will be the Main Camera.
However, when an object is instantiated, it is usual for that object to be a prefab. We’ll
now turn the Cube1 game object into a prefab.
- Firstly, let’s create an empty prefab. Select Assets->Create->Prefab. Rename this
prefab to Cube.
- Drag the Cube1 game object from the Hierarchy View onto the Cube prefab in the
Project view. Notice the prefab icon changes.
Now we can create our Javascript code.
- Create a new Javascript and name it Create. Insert the above code.
- Attach this script to the Main Camera and assign the Cube prefab to the newObject
variable of Main Camera.
- Play the game and move around as usual. Each time the fire button is clicked (LMB or
left ctrl) and you should notice a new cube appearing.

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8. Debugging
Debugging is the skill of finding and fixing human errors in your code (ok let’s call them
mistakes!). Unity provides help via the Debug class, we’ll now look at the Debug.Log()
function.

Log
The Log() function allows the user to send a message to the Unity Console. Reasons for
doing this might include:
1. To prove that a certain part of the code is being reached during run-time.
2. To report the status of a variable.
We’ll now use the Log() function to send a message to the Unity Console when the user
clicks the fire button.
- Open the Create script and add the following line after the ‘Instantiate’ code inside the
‘if’ block:
  Debug.Log("Cube created");

- Run the game and click the fire button, you should see a line appear at the bottom of
the Unity GUI saying “Cube created”, you can click on this to examine the Unity
Console.

Watch
Another useful feature for debugging is exposing a private variable. This makes the
variable visible in the Inspector View when the Debug mode is selected, but it cannot be
edited.
To demonstrate this, we’ll expose a private variable to count the number of cubes that we
instantiate.
- Open the Create script again and add two lines:
(1) Add a private variable called cubeCount
(2) Increment this variable whenever a cube is instantiated.
The complete code is a follows (Create.js):
var newObject : Transform;
private var cubeCount = 0;


  if (Input.GetButtonDown("Fire1")) {
  Instantiate(newObject, transform.position, transform.rotation);
  Debug.Log("Cube created");
  cubeCount++;
  }

}

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- Run the game and click the ﬁre button to create some cubes. Notice in the Inspector
View how the cubeCount variable is incremented whenever a new cube is instantiated.
Notice also how the number appears greyed out, this denotes that it’s a read-only
variable (cannot be edited).

9. Common script types
Whenever a new Javascript is created, by default it contains an Update() function. This
section will discuss other common options available, simply replace the name of the
Update() function with one from the list below.

FixedUpdate()
Code placed inside this function is executed at regular intervals (a ﬁxed framerate). It is
common to use this function type when applying forces to a Rigidbody.
// Apply a upwards force to the rigid body every frame

function FixedUpdate () {
rigidbody.AddForce (Vector3.up);
}

Awake()
Code inside here is called when the script is initialized.

Start()
This is called before any Update() function, but after Awake(). The difference between the
Start () and Awake() functions is that the Start() function is only called if the script is
enabled (if its checkbox is enabled in the Inspector View).

OnCollisionEnter()
Code inside here is executed when the game object the script belongs to collides with
another game object.

OnMouseDown()
Code inside here is executed when the mouse hovers over a game object which contains
a GUIElement or a Collider.
// Loads the level named "SomeLevel" as a response
// to the user clicking on the object

function OnMouseDown () {
Application.LoadLevel ("SomeLevel");
}

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OnMouseOver()
Code inside here is executed when the mouse hovers over a game object which contains
a GUIElement or a Collider.
// Fades the red component of the material to zero
// while the mouse is over the mesh

function OnMouseOver () {
renderer.material.color.r -= 0.1 * Time.deltaTime;
}

Check the Unity API for more information on all of these functions.

Summary
This tutorial has introduced the essential scripting concepts in Unity. You
should now read other Unity tutorials or try experimenting yourself!

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Unity 3d scripting tutorial

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