When switching the main scene of your game (e.g. 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
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 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.
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.
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)¶
Use this method if you need to load the entire resource in the current frame, without any more steps.
Obtaining the resource¶
If everything goes well, use this method to retrieve your loaded resource.
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
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)
goto_scene is called from the game when the scene
needs to be switched. It requests an interactive loader, and calls
set_process(true) to start polling the loader in the
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 we use
OS.get_ticks_msec to control how long we block the
thread. Some stages might load 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 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
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 length = 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 * length, 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
You can find an example class for loading resources in threads here:
resource_queue.gd. Usage is as follows:
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.
Remove a resource from the queue, discarding any loading done.
Returns true if a resource is done loading and ready to be retrieved.
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
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.
# initialize queue = preload("res://resource_queue.gd").new() queue.start() # suppose your game starts with a 10 second cutscene, 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.tres") start_cutscene() # later when the user presses the pause button for the first time: pause_menu = queue.get_resource("res://pause_menu.tres").instance() pause_menu.show() # when you need a new scene: queue.queue_resource("res://level_1.tscn", 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.tscn"): show_new_level(queue.get_resource("res://level_1.tscn")) else: update_progress(queue.get_process("res://level_1.tscn")) # when the user walks away from the trigger zone in your Metroidvania game: queue.cancel_resource("res://zone_2.tscn")
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.