Attention: Here be dragons

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Listening to player input

Building upon the previous lesson, Creating your first script, let's look at another important feature of any game: giving control to the player. To add this, we need to modify our sprite_2d.gd code.

You have two main tools to process the player's input in Redot:

1. The built-in input callbacks, mainly _unhandled_input(). Like _process(), it's a built-in virtual function that Redot calls every time the player presses a key. It's the tool you want to use to react to events that don't happen every frame, like pressing Space to jump. To learn more about input callbacks, see Using InputEvent.

2. The Input singleton. A singleton is a globally accessible object. Redot provides access to several in scripts. It's the right tool to check for input every frame.

We're going to use the Input singleton here as we need to know if the player wants to turn or move every frame.

For turning, we should use a new variable: direction. In our _process() function, replace the rotation += angular_speed * delta line with the code below.

GDScriptC#

var direction = 0
if Input.is_action_pressed("ui_left"):
    direction = -1
if Input.is_action_pressed("ui_right"):
    direction = 1

rotation += angular_speed * direction * delta

Our direction local variable is a multiplier representing the direction in which the player wants to turn. A value of 0 means the player isn't pressing the left or the right arrow key. A value of 1 means the player wants to turn right, and -1 means they want to turn left.

To produce these values, we introduce conditions and the use of Input. A condition starts with the if keyword in GDScript and ends with a colon. The condition is the expression between the keyword and the end of the line.

To check if a key was pressed this frame, we call Input.is_action_pressed(). The method takes a text string representing an input action and returns true if the action is pressed, false otherwise.

The two actions we use above, "ui_left" and "ui_right", are predefined in every Redot project. They respectively trigger when the player presses the left and right arrows on the keyboard or left and right on a gamepad's D-pad.

Note

You can see and edit input actions in your project by going to Project -> Project Settings and clicking on the Input Map tab.

Finally, we use the direction as a multiplier when we update the node's rotation: rotation += angular_speed * direction * delta.

Comment out the lines var velocity = Vector2.UP.rotated(rotation) * speed and position += velocity * delta like this:

GDScriptC#

#var velocity = Vector2.UP.rotated(rotation) * speed

#position += velocity * delta

This will ignore the code that moved the icon's position in a circle without user input from the previous exercise.

If you run the scene with this code, the icon should rotate when you press Left and Right.

Moving when pressing "up"

To only move when pressing a key, we need to modify the code that calculates the velocity. Uncomment the code and replace the line starting with var velocity with the code below.

GDScriptC#

var velocity = Vector2.ZERO
if Input.is_action_pressed("ui_up"):
    velocity = Vector2.UP.rotated(rotation) * speed

We initialize the velocity with a value of Vector2.ZERO, another constant of the built-in Vector type representing a 2D vector of length 0.

If the player presses the "ui_up" action, we then update the velocity's value, causing the sprite to move forward.

Complete script

Here is the complete sprite_2d.gd file for reference.

GDScriptC#

extends Sprite2D

var speed = 400
var angular_speed = PI


func _process(delta):
    var direction = 0
    if Input.is_action_pressed("ui_left"):
        direction = -1
    if Input.is_action_pressed("ui_right"):
        direction = 1

    rotation += angular_speed * direction * delta

    var velocity = Vector2.ZERO
    if Input.is_action_pressed("ui_up"):
        velocity = Vector2.UP.rotated(rotation) * speed

    position += velocity * delta

If you run the scene, you should now be able to rotate with the left and right arrow keys and move forward by pressing Up.

Summary

In summary, every script in Redot represents a class and extends one of the engine's built-in classes. The node types your classes inherit from give you access to properties, such as rotation and position in our sprite's case. You also inherit many functions, which we didn't get to use in this example.

In GDScript, the variables you put at the top of the file are your class's properties, also called member variables. Besides variables, you can define functions, which, for the most part, will be your classes' methods.

Redot provides several virtual functions you can define to connect your class with the engine. These include _process(), to apply changes to the node every frame, and _unhandled_input(), to receive input events like key and button presses from the users. There are quite a few more.

The Input singleton allows you to react to the players' input anywhere in your code. In particular, you'll get to use it in the _process() loop.

In the next lesson, Using signals, we'll build upon the relationship between scripts and nodes by having our nodes trigger code in scripts.