Lesson 8
Lambda Functions, UI
Download the week 8 project
We have already created and used several functions in our scripts. A function is a block of code that can be called from somewhere else by name, such as hit(), and optionally take arguments for the function to use, such as add_points(1). So far, we have only defined named functions, but there's another type of function you can define inline (i.e., where you call it) called a lambda function.
A lambda function is just like a normal named function in every way, except that it doesn't have a name. Instead, you define the function where you would otherwise call a function by name. Consider the example below, where we connect the screen_exited signal of a VisibleOnScreenNotifier's to a function that emits our own finished signal. We cannot connect screen_exited directly to the finished signal because finished requires an argument which we need to explicitly pass.
Lambda Functions
User Interface (UI)
We will never call _on_screen_exited directly by name; it will only ever by called by the VisibleOnScreenNotifier2D's signal. We can achieve the same result using a lambda function instead:
The lambda function is defined within the call to connect() and looks exactly like any other function definition, except that it does not have a name. Now, when the screen_exited signal is emitted, it will call this lambda function, which emits the finished signal.
Lambda functions are incredibly useful when you need a function that will only be used in one place and doesn't need to be called by name.
Homework
Create the UI
Create the default UI Theme.
In the FileSystem tab, right-click and Create New/Resource.
Search for the resource type Theme and click Create.
Save your new resource as ui_theme.tres.
Double-click ui_theme.tres in the FileSystem tab to edit it.
In the Inspector, select the Default Font (one is provided in the game folder, or you can provide your own).
In the Inspector, set the Default Font Size to 20.
Open the Project Settings. On the GUI/Theme tab, set your ui_theme.tres as the Custom theme for your project.
Save & Restart the editor for the settings to take effect.
Update Globals script.
Add new variables playerScore and playerDistance to Globals.gd
Update Player script.
Get rid of local score variable.
Update add_points() function to update the Globals.playerScore instead.
Add a new const variable PixelsPerMeter, give it a value such as 50.
In the _physics_process() function, update the Globals.playerDistance variable to track how far the player has run.
Create a new scene with a Control root, name it UI.
Add a Label child node, name it Score.
Position the Score label somewhere along the top of the screen.
Set the initial text to "Score: 0"
Add a Label child node, name it Distance.
Position the Distance label somewhere along the top of the screen.
Set the initial text to "Distance: 0m"
Add a script to the UI root node.
In the _process() function, update the text of the Score and Distance labels.
(OPTIONAL) Add UI for Player Health
Create a new scene with a Control root named health_ui.
Add an HBoxContainer child node.
Add 3 TextureRect children nodes named Heart1, Heart2, and Heart3.
Set the Texture for each TextureRect to your default empty heart image.
Set the Expand Mode of each TextureRect to Ignore Size.
Set the Stretch Mode of each TextureRect to Keep Aspect.
Add a script to the health_ui root node.
Add export vars for the full hear and empty heart textures.
Add export var for an Array of TextureRects for the heart UI elements.
In the Inspector, drag all three of your Heart TextureRects into the array.
Add a local variable for maxHealth.
In the _ready() function, set maxHealth = hearts.size().
Add a set_health(health) function.
Add a while loop, iterating through each heart. Increment an index within the loop. If the current index is less than the health value passed in, assign the full heart texture to the TextureRect, otherwise assign the empty heart texture.
Update Globals script.
Add a playerHealth variable.
Update Player script.
Update the hit() function to reduce the Globals.playerHealth by 1, and call die() when it reaches 0.
Update the main UI.
Add the hearts_ui scene as a child to the main UI scene, position it where you like.
In the main UI script's _process() function, call set_health(Globals.playerHealth).