Background loading

When switching the main scene of your game (for example going to a new level), you might want to show a loading screen with some indication that progress is being made. The main load method (ResourceLoader::load or just load from gdscript) blocks your thread while the resource is being loaded, so It’s not good. This document discusses the ResourceInteractiveLoader class for smoother load screens.

ResourceInteractiveLoader

The ResourceInteractiveLoader class allows you to load a resource in stages. Every time the method poll is called, a new stage is loaded, and control is returned to the caller. Each stage is generally a sub-resource that is loaded by the main resource. For example, if you’re loading a scene that loads 10 images, each image will be one stage.

Usage

Usage is generally as follows

Obtaining a ResourceInteractiveLoader

Ref<ResourceInteractiveLoader> ResourceLoader::load_interactive(String p_path);

This method will give you a ResourceInteractiveLoader that you will use to manage the load operation.

Polling

Error ResourceInteractiveLoader::poll();

Use this method to advance the progress of the load. Each call to poll will load the next stage of your resource. Keep in mind that each stage is one entire “atomic” resource, such as an image, or a mesh, so it will take several frames to load.

Returns OK on no errors, ERR_FILE_EOF when loading is finished. Any other return value means there was an error and loading has stopped.

Load progress (optional)

To query the progress of the load, use the following methods:

int ResourceInteractiveLoader::get_stage_count() const;
int ResourceInteractiveLoader::get_stage() const;

get_stage_count returns the total number of stages to load. get_stage returns the current stage being loaded.

Forcing completion (optional)

Error ResourceInteractiveLoader::wait();

Use this method if you need to load the entire resource in the current frame, without any more steps.

Obtaining the resource

Ref<Resource> ResourceInteractiveLoader::get_resource();

If everything goes well, use this method to retrieve your loaded resource.

Example

This example demostrates how to load a new scene. Consider it in the context of the Singletons (AutoLoad) example.

First we setup some variables and initialize the current_scene with the main scene of the game:

var loader
var wait_frames
var time_max = 100 # msec
var current_scene

func _ready():
    var root = get_tree().get_root()
    current_scene = root.get_child(root.get_child_count() -1)

The function goto_scene is called from the game when the scene needs to be switched. It requests an interactive loader, and calls set_progress(true) to start polling the loader in the _progress callback. It also starts a “loading” animation, which can show a progress bar or loading screen, etc.

func goto_scene(path): # game requests to switch to this scene
    loader = ResourceLoader.load_interactive(path)
    if loader == null: # check for errors
        show_error()
        return
    set_process(true)

    current_scene.queue_free() # get rid of the old scene

    # start your "loading..." animation
    get_node("animation").play("loading")

    wait_frames = 1

_process is where the loader is polled. poll is called, and then we deal with the return value from that call. OK means keep polling, ERR_FILE_EOF means load is done, anything else means there was an error. Also note we skip one frame (via wait_frames, set on the goto_scene function) to allow the loading screen to show up.

Note how use use OS.get_ticks_msec to control how long we block the thread. Some stages might load really fast, which means we might be able to cram more than one call to poll in one frame, some might take way more than your value for time_max, so keep in mind we won’t have precise control over the timings.

func _process(time):
    if loader == null:
        # no need to process anymore
        set_process(false)
        return

    if wait_frames > 0: # wait for frames to let the "loading" animation to show up
        wait_frames -= 1
        return

    var t = OS.get_ticks_msec()
    while OS.get_ticks_msec() < t + time_max: # use "time_max" to control how much time we block this thread

        # poll your loader
        var err = loader.poll()

        if err == ERR_FILE_EOF: # load finished
            var resource = loader.get_resource()
            loader = null
            set_new_scene(resource)
            break
        elif err == OK:
            update_progress()
        else: # error during loading
            show_error()
            loader = null
            break

Some extra helper functions. update_progress updates a progress bar, or can also update a paused animation (the animation represents the entire load process from beginning to end). set_new_scene puts the newly loaded scene on the tree. Because it’s a scene being loaded, instance() needs to be called on the resource obtained from the loader.

func update_progress():
    var progress = float(loader.get_stage()) / loader.get_stage_count()
    # update your progress bar?
    get_node("progress").set_progress(progress)

    # or update a progress animation?
    var len = get_node("animation").get_current_animation_length()

    # call this on a paused animation. use "true" as the second parameter to force the animation to update
    get_node("animation").seek(progress * len, true)

func set_new_scene(scene_resource):
    current_scene = scene_resource.instance()
    get_node("/root").add_child(current_scene)

Using multiple threads

ResourceInteractiveLoader can be used from multiple threads. A couple of things to keep in mind if you attempt it:

Use a Semaphore

While your thread waits for the main thread to request a new resource, use a Semaphore to sleep (instead of a busy loop or anything similar).

Not blocking main thread during the polling

If you have a mutex to allow calls from the main thread to your loader class, don’t lock it while you call poll on the loader. When a resource is finished loading, it might require some resources from the low level APIs (VisualServer, etc), which might need to lock the main thread to acquire them. This might cause a deadlock if the main thread is waiting for your mutex while your thread is waiting to load a resource.

Example class

You can find an example class for loading resources in threads here: resource_queue.gd. Usage is as follows:

func start()

Call after you instance the class to start the thread.

func queue_resource(path, p_in_front = false)

Queue a resource. Use optional parameter “p_in_front” to put it in front of the queue.

func cancel_resource(path)

Remove a resource from the queue, discarding any loading done.

func is_ready(path)

Returns true if a resource is done loading and ready to be retrieved.

func get_progress(path)

Get the progress of a resource. Returns -1 on error (for example if the resource is not on the queue), or a number between 0.0 and 1.0 with the progress of the load. Use mostly for cosmetic purposes (updating progress bars, etc), use is_ready to find out if a resource is actually ready.

func get_resource(path)

Returns the fully loaded resource, or null on error. If the resource is not done loading (is_ready returns false), it will block your thread and finish the load. If the resource is not on the queue, it will call ResourceLoader::load to load it normally and return it.

Example:

# initialize
queue = preload("res://resource_queue.gd").new()
queue.start()

# suppose your game starts with a 10 second custscene, during which the user can't interact with the game.
# For that time we know they won't use the pause menu, so we can queue it to load during the cutscene:
queue.queue_resource("res://pause_menu.xml")
start_curscene()

# later when the user presses the pause button for the first time:
pause_menu = queue.get_resource("res://pause_menu.xml").instance()
pause_menu.show()

# when you need a new scene:
queue.queue_resource("res://level_1.xml", true) # use "true" as the second parameter to put it at the front
                                                # of the queue, pausing the load of any other resource

# to check progress
if queue.is_ready("res://level_1.xml"):
    show_new_level(queue.get_resource("res://level_1.xml"))
else:
    update_progress(queue.get_process("res://level_1.xml"))

# when the user walks away from the trigger zone in your Metroidvania game:
queue.cancel_resource("res://zone_2.xml")

Note: this code in its current form is not tested in real world scenarios. Ask punto on IRC (#godotengine on irc.freenode.net) for help.