<?xml version="1.0" encoding="UTF-8" ?>
<class name="WorkerThreadPool" inherits="Object" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../class.xsd">
	<brief_description>
		A singleton that allocates some [Thread]s on startup, used to offload tasks to these threads.
	</brief_description>
	<description>
		The [WorkerThreadPool] singleton allocates a set of [Thread]s (called worker threads) on project startup and provides methods for offloading tasks to them. This can be used for simple multithreading without having to create [Thread]s.
		Tasks hold the [Callable] to be run by the threads. [WorkerThreadPool] can be used to create regular tasks, which will be taken by one worker thread, or group tasks, which can be distributed between multiple worker threads. Group tasks execute the [Callable] multiple times, which makes them useful for iterating over a lot of elements, such as the enemies in an arena.
		Here's a sample on how to offload an expensive function to worker threads:
		[codeblocks]
		[gdscript]
		var enemies = [] # An array to be filled with enemies.

		func process_enemy_ai(enemy_index):
		    var processed_enemy = enemies[enemy_index]
		    # Expensive logic...

		func _process(delta):
		    var task_id = WorkerThreadPool.add_group_task(process_enemy_ai, enemies.size())
		    # Other code...
		    WorkerThreadPool.wait_for_group_task_completion(task_id)
		    # Other code that depends on the enemy AI already being processed.
		[/gdscript]
		[csharp]
		private List&lt;Node&gt; _enemies = new List&lt;Node&gt;(); // A list to be filled with enemies.

		private void ProcessEnemyAI(int enemyIndex)
		{
		    Node processedEnemy = _enemies[enemyIndex];
		    // Expensive logic here.
		}

		public override void _Process(double delta)
		{
		    long taskId = WorkerThreadPool.AddGroupTask(Callable.From&lt;int&gt;(ProcessEnemyAI), _enemies.Count);
		    // Other code...
		    WorkerThreadPool.WaitForGroupTaskCompletion(taskId);
		    // Other code that depends on the enemy AI already being processed.
		}
		[/csharp]
		[/codeblocks]
		The above code relies on the number of elements in the [code]enemies[/code] array remaining constant during the multithreaded part.
		[b]Note:[/b] Using this singleton could affect performance negatively if the task being distributed between threads is not computationally expensive.
	</description>
	<tutorials>
		<link title="Using multiple threads">$DOCS_URL/tutorials/performance/using_multiple_threads.html</link>
		<link title="Thread-safe APIs">$DOCS_URL/tutorials/performance/thread_safe_apis.html</link>
	</tutorials>
	<methods>
		<method name="add_group_task">
			<return type="int" />
			<param index="0" name="action" type="Callable" />
			<param index="1" name="elements" type="int" />
			<param index="2" name="tasks_needed" type="int" default="-1" />
			<param index="3" name="high_priority" type="bool" default="false" />
			<param index="4" name="description" type="String" default="&quot;&quot;" />
			<description>
				Adds [param action] as a group task to be executed by the worker threads. The [Callable] will be called a number of times based on [param elements], with the first thread calling it with the value [code]0[/code] as a parameter, and each consecutive execution incrementing this value by 1 until it reaches [code]element - 1[/code].
				The number of threads the task is distributed to is defined by [param tasks_needed], where the default value [code]-1[/code] means it is distributed to all worker threads. [param high_priority] determines if the task has a high priority or a low priority (default). You can optionally provide a [param description] to help with debugging.
				Returns a group task ID that can be used by other methods.
			</description>
		</method>
		<method name="add_task">
			<return type="int" />
			<param index="0" name="action" type="Callable" />
			<param index="1" name="high_priority" type="bool" default="false" />
			<param index="2" name="description" type="String" default="&quot;&quot;" />
			<description>
				Adds [param action] as a task to be executed by a worker thread. [param high_priority] determines if the task has a high priority or a low priority (default). You can optionally provide a [param description] to help with debugging.
				Returns a task ID that can be used by other methods.
			</description>
		</method>
		<method name="get_group_processed_element_count" qualifiers="const">
			<return type="int" />
			<param index="0" name="group_id" type="int" />
			<description>
				Returns how many times the [Callable] of the group task with the given ID has already been executed by the worker threads.
				[b]Note:[/b] If a thread has started executing the [Callable] but is yet to finish, it won't be counted.
			</description>
		</method>
		<method name="is_group_task_completed" qualifiers="const">
			<return type="bool" />
			<param index="0" name="group_id" type="int" />
			<description>
				Returns [code]true[/code] if the group task with the given ID is completed.
			</description>
		</method>
		<method name="is_task_completed" qualifiers="const">
			<return type="bool" />
			<param index="0" name="task_id" type="int" />
			<description>
				Returns [code]true[/code] if the task with the given ID is completed.
			</description>
		</method>
		<method name="wait_for_group_task_completion">
			<return type="void" />
			<param index="0" name="group_id" type="int" />
			<description>
				Pauses the thread that calls this method until the group task with the given ID is completed.
			</description>
		</method>
		<method name="wait_for_task_completion">
			<return type="int" enum="Error" />
			<param index="0" name="task_id" type="int" />
			<description>
				Pauses the thread that calls this method until the task with the given ID is completed.
				Returns [constant @GlobalScope.OK] if the task could be successfully awaited.
				Returns [constant @GlobalScope.ERR_INVALID_PARAMETER] if a task with the passed ID does not exist (maybe because it was already awaited and disposed of).
				Returns [constant @GlobalScope.ERR_BUSY] if the call is made from another running task and, due to task scheduling, the task to await is at a lower level in the call stack and therefore can't progress. This is an advanced situation that should only matter when some tasks depend on others.
			</description>
		</method>
	</methods>
</class>
