Control the game’s UI with code

Intro

In this tutorial you will connect a character to a life bar and animate the health loss.

../../_images/lifebar_tutorial_final_result.gif

Here’s what you’ll create: the bar and the counter animate when the character takes a hit. They fade when it dies.

Here’s what you’ll create: the bar and the counter animate when the character takes a hit. They fade when it dies.

You will learn:

  • How to connect a character to a GUI with signals
  • How to control a GUI with GDscript
  • How to animate a life bar with the Tween node

Warning

This tutorial relies on a recent build of Godot 3’s master branch. You will find some differences with Godot 3 alpha 1, and some more changes might happen in Godot 3 beta. We’ll adapt the page after the beta release.

If you want to learn how to set up the interface instead, check out the step-by-step UI tutorials:

  • Create a main menu screen
  • Create a game user interface

When you code a game, you want to build the core gameplay first: the main mechanics, player input, win and loss conditions. The UI comes a bit later. You want to keep all the elements that make up your project separate if possible. Each character should be in its own scene, with its own scripts, and so should the UI elements. This prevents bugs, keeps your project manageable, and allows different team members to work on different parts of the game.

Once the core gameplay and the UI are ready, you’ll need to connect them somehow. In our example, we have the Enemy who attacks the Player at constant time intervals. We want the life bar to update when the Player takes damage.

To do this, we will use signals.

Note

Signals are Godot’s version of the Observer pattern. They allow us to send out some message. Other nodes can connect to the object that emits the signal and receive the information. It’s a powerful tool we use a lot for User Interface and achievement systems. You don’t want to use them everywhere though. Connecting two nodes adds some coupling between them. When there’s a lot of connections, they become hard to manage. For more information on check out the signals video tutorial on GDquest.

Download and explore the start project

Download the Godot project: ui_code_life_bar.zip. It contains all the assets and scripts you need to get started. Extract the .zip archive to get two folders:

  • tutorial/ contains the start and end godot games for this tutorial
  • exercise/ lets you practice what you’ll learn here by adding a new GUI element on your own. You’ll find more on that at the bottom of the page

Load the tutorial/start project in Godot. In the FileSystem dock double click on LevelMockup.tscn to open it. It’s an RPG game’s mockup where 2 characters face each other. The pink enemy attacks and damages the green square at regular time intervals, until its death. Feel free to try out the game: the basic combat mechanics already work. But as the character isn’t connected to the life bar the GUI doesn’t do anything.

Note

This is typical of how you’d code a game: you implement the core gameplay first, handle the player’s death, and only then you’ll add the interface. That’s because the UI listens to what’s happening in the game. So it can’t work if other systems aren’t in place yet. If you design the UI before you prototype and test the gameplay, chances are it won’t work well and you’ll have to re-create it from scratch.

The scene contains a background sprite, a GUI, and two characters.

../../_images/lifebar_tutorial_life_bar_step_tut_LevelMockup_scene_tree.png

The scene tree, with the GUI scene set to display its children

The GUI scene encapsulates all of the Game User Interface. It comes with a barebones script where we get the path to nodes that exist inside the scene:

onready var number_label = $Bars/LifeBar/Count/Background/Number
onready var bar = $Bars/LifeBar/TextureProgress
onready var tween = $Tween
  • number_label displays a life count as a number. It’s a Label node
  • bar is the life bar itself. It’s a TextureProgress node
  • tween is a component-style node that can animate and control any value or method from any other node

Note

The project uses a simple organisation that works for game jams and tiny games. At the root of the project, in the res:// folder, you will find the LevelMockup. That’s the main game scene and the one we will work with. All the components that make up the game are in the scenes/ folder. The assets/ folder contains the game sprites and the font for the HP counter. In the scripts/ folder you will find the enemy, the player, and the GUI controller scripts. Click the edit scene icon to the right of the node in the scene tree to open the scene in the editor. You’ll see the LifeBar and EnergyBar are sub-scenes themselves. You can also right click on the GUI node and activate the Editable Children checkbox at the bottom of the drop-down menu. .. image:: img/lifebar_tutorial_Player_with_editable_children_on.png

Set up the Lifebar with the Player’s max_health

We have to tell the GUI somehow what the player’s current health is, to update the lifebar’s texture, and to display the remaining health in the HP counter in the top left corner of the screen. To do this we send the player’s health to the GUI every time he takes damage. The GUI will then update the Lifebar and Number nodes with this value.

We could stop here to display the number, but we need to initialize the bar’s max_value for it to update in the right proportions. The first step is thus to tell the GUI what the green character’s max_health is.

Tip

The bar, a TextureProgress, has a max_value of 100 by default. If you don’t need to display the character’s health with a number, you don’t need to change its max_value property. You send a percentage from the Player to the GUI instead: health / max_health * 100. .. image:: img/lifebar_tutorial_TextureProgress_default_max_value.png

Click the script icon to the right of the GUI in the Scene dock to open its script. In the _ready function, we’re going to store the Player‘s max_health in a new variable and use it to set the bar‘s max_value:

func _ready():
    var player_max_health = $"../Characters/Player".max_health
    bar.max_value = player_max_health

Let’s break it down. $"../Characters/Player" is a shorthand that goes one node up in the scene tree, and retrieves the Characters/Player node from there. It gives us access to the node. The second part of the statement, .max_health, accesses the max_health on the Player node.

The second line assigns this value to bar.max_value. You could combine the two lines into one, but we’ll need to use player_max_health again later in the tutorial.

Player.gd sets the health to max_health at the start of the game, so we could work with this. Why do we still use max_health? There are two reasons:

  1. We don’t have the guarantee that health will always equal max_health: a future version of the game may load a level where the player already lost some health.
  2. Because GUI is higher than Player in the scene tree, Godot will call its _ready function first. If we got the health value then it would still be at 0.

Note

When you open a scene in the game, Godot creates nodes one by one, following the order in your Scene dock, from top to bottom. GUI and Player are not part of the same node branch. To make sure they both exist when we access each other, we have to use the _ready function. Godot calls _ready right after it loaded all nodes, before the game starts. It’s the perfect function to set everything up and prepare the game session. Learn more about _ready: Scripting (continued)

Update health with a signal when the player takes a hit

Our GUI is ready to receive the health value updates from the Player. To achieve this we’re going to use signals.

Note

There are many useful built-in signals like enter_tree and exit_tree, that all nodes emit when they are respectively created and destroyed. You can also create your own using the signal keyword. On the Player node, you’ll find two signals we created for you: died and health_changed.

Why don’t we directly get the Player node in the _process function and look at the health value? Accessing nodes this way creates tight coupling between them. If you did it sparingly it may work. As your game grows bigger, you may have many more connections. If you get nodes from a bad it’s becomes very complex very soon. Not only that: you need to listen to the changes state constantly in the _process function. The check happens 60 times a second and you’ll likely break the game because of the order in which the code runs.

On a given frame you may look at another node’s property before it was updated: you get a value that from the last frame. This leads to obscure bugs that are hard to fix. On the other hand, a signal is emitted right after a change happened. It guarantees you’re getting a fresh piece of information. And you will update the state of your connected node right after the change happened.

Note

The Observer pattern, that signals derive from, still adds a bit of coupling between node branches. But it’s generally lighter and more secure than accessing nodes directly to communicate between two separate classes. It can be okay for a parent node to get values from its children. But you’ll want to favor signals if you’re working with two separate branches. Read Game Programming Patterns for more information on the Observer pattern. The full book is available online for free.

With this in mind let’s connect the GUI to the Player. Click on the Player node in the scene dock to select it. Head down to the Inspector and click on the Node tab. This is the place to connect nodes to listen the one you selected.

The first section lists custom signals defined in player.GD:

  • died is emitted when the character just died. We will use it in a moment to hide the UI.
  • health_changed is emitted when the character got hit.
../../_images/lifebar_tutorial_health_changed_signal.png

We’re connecting to the took_damage signal

Select health_changed and click on the Connect button in the bottom right corner to open the Connect Signal window. On the left side you can pick the node that will listen to this signal. Select the GUI node. The right side of the screen lets you pack optional values with the signal. We already took care of it in player.GD. In general I recommend not to add too many arguments using this window as they’re less convenient than doing it from the code.

../../_images/lifebar_tutorial_connect_signal_window_health_changed.png

The Connect Signal window with the GUI node selected

Tip

You can optionally connect nodes from the code. But doing it from the editor has two advantages:

1. Godot can write new callback functions for you in the connected script 1. An emitter icon appears next to the node that emits the signal in the Scene dock

At the bottom of the window you will find the path to the node you selected. We’re interested in the second row called “Method in Node”. This is the method on the GUI node that gets called when the signal is emitted. This method receives the values sent with the signal and lets you process them. If you look to the right, there is a “Make Function” radio button that is on by default. Click the connect button at the bottom of the window. Godot creates the method inside the GUI node. The script editor opens with the cursor inside a new _on_player_health_changed function.

Note

When you connect nodes from the editor, Godot generates a method name with the following pattern: _on_EmitterName_signal_name. If you wrote the method already, the “Make Function” option will keep it. You may replace the name with anything you’d like.

../../_images/lifebar_tutorial_godot_generates_signal_callback.png

Godot writes the callback method for you and takes you to it

Inside the parens after the function name, add a player_health argument. When the player emits the health_changed signal it will send its current health alongside it. Your code should look like:

func _on_Player_health_changed(player_health):
    pass
../../_images/lifebar_tutorial_player_gd_emits_health_changed_code.png

In Player.gd, when the Player emits the took_damage signal, it also sends its health value

Inside _on_Player_health_changed let’s call a second function called update_health and pass it the player_health variable.

Note

We could directly update the health value on LifeBar and Number. There are two reasons to use this method instead:

  1. The name makes it very clear for our future selves and teammates that when the player took damage, we update the health count on the GUI
  2. We will reuse this method a bit later

Create a new update_health method below _on_Player_health_changed. It takes a new_value as its only argument:

func update_health(new_value):

This method needs to:

  • set the Number node’s text to new_value converted to a string
  • set the TextureProgress‘s value to new_value
func update_health(new_value):
    number_label.text = str(new_value)
    bar.value = new_value

Tip

str is a built-in function that converts about any value to text. Number‘s text property requires a string so we can’t assign it to new_value directly

Also call update_health at the end of the _ready function to initialize the Number node’s text with the right value at the start of the game. Press F5 to test the game: the life bar update with every attack!

../../_images/lifebar_tutorial_LifeBar_health_update_no_anim.gif

Both the Number node and the TextureProgress update when the Player takes a hit

Animate the loss of life with the Tween node

Our interface is functional, but it could use some animation. That’s a good opportunity to introduce the Tween node, an essential tool to animate properties. Tween animates anything you’d like from a start to an end state over a certain duration. For example it can animate the health on the TextureProgress from its current level to the Player‘s new health when the character takes damage.

The GUI scene already contains a Tween child node stored in the tween variable. Let’s now use it. We have to make some changes to update_health.

We will use the Tween node’s interpolate_property method. It takes seven arguments:

  1. A reference to the node who owns the property to animate
  2. The property’s identifier as a string
  3. The starting value
  4. The end value
  5. The animation’s duration in seconds
  6. The type of the transition
  7. The easing to use in combination with the equation.

The last two arguments combined correspond to an easing equation. This controls how the value evolves from the start to the end point.

Click the script icon next to the GUI node to open it again. The Number node needs text to update itself, and the Bar needs a float or an integer. We can use interpolate_property to animate a number, but not to animate text directly. We’re going to use it to animate a new GUI variable named animated_health.

At the top of the script, define a new variable and name it animated_health. Navigate back to the update_health method and clear its content. Let’s animate the animated_health value. Call the Tween node’s interpolate_property method:

func update_health(new_value):
    tween.interpolate_property(self, "animated_health", animated_health, new_value, 0.6, Tween.TRANS_LINEAR, Tween.EASE_IN)

Let’s break down the call:

tween.interpolate_property(self, "animated_health", ...

We target animated_health on self, that is to say the GUI node. Tween‘s interpolate_property takes the property’s name as a string. That’s why we write it as "animated_health".

... _health", animated_health, new_value, 0.6 ...

The starting point is the current value the bar’s at. We still have to code this part, but it’s going to be animated_health. The end point of the animation is the Player‘s health after he health_changed: that’s new_value. And 0.6 is the animation’s duration in seconds.

...  0.6, tween.TRANS_LINEAR, Tween.EASE_IN)

The last two arguments are constants from the Tween class. TRANS_LINEAR means the animation should be linear. EASE_IN doesn’t do anything with a linear transition, but we must provide this last argument or we’ll get an error.

The animation will not play until we activated the Tween node with tween.start(). We only have to do this once if the node is not active. Add this code after the last line:

if not tween.is_active():
    tween.start()

Note

Although we could animate the health property on the Player, we really shouldn’t. Characters should lose life instantly when they get hit. It makes it a lot easier to manage their state, like to know when one died. You always want to store animations in a separate data container or node. The tween node is perfect for code-controlled animations. For hand-made animations, check out AnimationPlayer.

Assign the animated_health to the LifeBar

Now the animated_health variable animates but we don’t update the actual Bar and Number nodes anymore. Let’s fix this.

So far, the update_health method looks like this:

func update_health(new_value):
    Tween.interpolate_property(self, "animated_health", animated_health, new_value, 0.6, Tween.TRANS_LINEAR, Tween.EASE_IN)
    if not Tween.is_active():
        Tween.start()

In this specific case, because number_label takes text, we need to use the _process method to animate it. Let’s now update the Number and TextureProgress nodes like before, inside of _process:

func _process(delta):
    number_label.text = str(animated_health)
    bar.value = animated_health

Note

number_label and bar are variables that store references to the Number and TextureProgress nodes.

Play the game to see the bar animate smoothly. But the text displays decimal number and looks like a mess. And considering the style of the game, it’d be nice for the life bar to animate in a choppier fashion.

../../_images/lifebar_tutorial_number_animation_messed_up.gif

The animation is smooth but the number is broken

We can fix both problems by rounding out animated_health. Use a local variable named round_value to store the rounded animated_health. Then assign it to number_label.text and bar.value:

func _process(delta):
    var round_value = round(animated_health)
    number_label.text = str(round_value)
    bar.value = round_value

Try the game again to see a nice blocky animation.

../../_images/lifebar_tutorial_number_animation_working.gif

By rounding out animated_health we hit two birds with one stone

Every time the player takes a hit, the `GUI` calls `_on_Player_health_changed`, which in turn calls `update_health`. This updates the animation and the `number_label` and `bar` follow in `_process`.
The animated life bar that shows the health going down gradually is just a trick. It makes the GUI feel alive. If the `Player` takes 3 damage, it happens in an instant.

Fade the bar when the Player dies

When the green character dies, it plays a death animation and fades out. At this point, we shouldn’t show the interface anymore. Let’s fade the bar as well when the character died. We will reuse the same Tween node as it manages multiple animations in parallel for us.

First, the GUI needs to connect to the Player‘s died signal to know when it just died. Press :kbd:F1 to jump back to the 2D Workspace. Select the Player node in the Scene dock and click on the Node tab next to the Inspector.

Find the died signal, select it, and click the Connect button.

../../_images/lifebar_tutorial_player_died_signal_enemy_connected.png

The signal should already have the Enemy connected to it

In the Connecting Signal window, connect to the GUI node again. The Path to Node should be ../../GUI and the Method in Node should show _on_Player_died. Leave the Make Function option on and click Connect at the bottom of the window. This will take you to the GUI.gd file in the Script Workspace.

../../_images/lifebar_tutorial_player_died_connecting_signal_window.png

You should get these values in the Connecting Signal window

Note

You should see a pattern by now: every time the GUI needs a new piece of information, we emit a new signal. Use them wisely: the more connections you add, the harder they are to track.

To animate a fade on a UI element, we have to use its modulate property. modulate is a Color that multiplies the colors of our textures.

Note

modulate comes from the CanvasItem class, All 2D and UI nodes inherit from it. It lets you toggle the visibility of the node, assign a shader to it, and modify it using a color with modulate.

modulate takes a Color value with 4 channels: red, green, blue and alpha. If we darken any of the first three channels it darkens the interface. If we lower the alpha channel our interface fades out.

We’re going to tween between two color values: from a white with an alpha of 1, that is to say at full opacity, to a pure white with an alpha value of 0, completely transparent. Let’s add two variables at the top of the _on_Player_died method and name them start_color and end_color. Use the Color() constructor to build two Color values.

func _on_Player_died():
    var start_color = Color(1.0, 1.0, 1.0, 1.0)
    var end_color = Color(1.0, 1.0, 1.0, 0.0)

Color(1.0, 1.0, 1.0) corresponds to white. The fourth argument, respectively 1.0 and 0.0 in start_color and end_color, is the alpha channel.

We then have to call the interpolate_property method of the Tween node again:

Tween.interpolate_property(self, "modulate", start_color, end_color, 1.0, Tween.TRANS_LINEAR, Tween.EASE_IN)

This time we change the modulate property and have it animate from start_color to the end_color. The duration is of one second, with a linear transition. Here again, because the transition is linear, the easing does not matter. Here’s the complete _on_Player_died method:

func _on_Player_died():
    var start_color = Color(1.0, 1.0, 1.0, 1.0)
    var end_color = Color(1.0, 1.0, 1.0, 0.0)
    tween.interpolate_property(self, "modulate", start_color, end_color, 1.0, Tween.TRANS_LINEAR, Tween.EASE_IN)

And that is it. You may now play the game to see the final result!

../../_images/lifebar_tutorial_final_result.gif

The final result. Congratulations for getting there!

Note

Using the exact same techniques, you can change the color of the bar when the Player gets poisoned, turn the bar red when its health drops low, shake the UI when he takes a critical hit... the principle is the same: emit a signal to forward the information from the Player to the GUI and let the GUI process it.