godot-docs/classes/class_thread.rst

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.. _class_Thread:

Thread
======

**Inherits:** :ref:`RefCounted<class_RefCounted>` **<** :ref:`Object<class_Object>`

A unit of execution in a process.

.. rst-class:: classref-introduction-group

Description
-----------

A unit of execution in a process. Can run methods on :ref:`Object<class_Object>`\ s simultaneously. The use of synchronization via :ref:`Mutex<class_Mutex>` or :ref:`Semaphore<class_Semaphore>` is advised if working with shared objects.

\ **Note:** Breakpoints won't break on code if it's running in a thread. This is a current limitation of the GDScript debugger.

\ **Warning:**\ 

To ensure proper cleanup without crashes or deadlocks, when a **Thread**'s reference count reaches zero and it is therefore destroyed, the following conditions must be met:

- It must not have any :ref:`Mutex<class_Mutex>` objects locked.

- It must not be waiting on any :ref:`Semaphore<class_Semaphore>` objects.

- :ref:`wait_to_finish<class_Thread_method_wait_to_finish>` should have been called on it.

.. rst-class:: classref-introduction-group

Tutorials
---------

- :doc:`Using multiple threads <../tutorials/performance/using_multiple_threads>`

- :doc:`Thread-safe APIs <../tutorials/performance/thread_safe_apis>`

- `3D Voxel Demo <https://godotengine.org/asset-library/asset/676>`__

.. rst-class:: classref-reftable-group

Methods
-------

.. table::
   :widths: auto

   +---------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------+
   | :ref:`String<class_String>`           | :ref:`get_id<class_Thread_method_get_id>` **(** **)** |const|                                                                                  |
   +---------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------+
   | :ref:`bool<class_bool>`               | :ref:`is_alive<class_Thread_method_is_alive>` **(** **)** |const|                                                                              |
   +---------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------+
   | :ref:`bool<class_bool>`               | :ref:`is_started<class_Thread_method_is_started>` **(** **)** |const|                                                                          |
   +---------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------+
   | :ref:`Error<enum_@GlobalScope_Error>` | :ref:`start<class_Thread_method_start>` **(** :ref:`Callable<class_Callable>` callable, :ref:`Priority<enum_Thread_Priority>` priority=1 **)** |
   +---------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------+
   | :ref:`Variant<class_Variant>`         | :ref:`wait_to_finish<class_Thread_method_wait_to_finish>` **(** **)**                                                                          |
   +---------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------+

.. rst-class:: classref-section-separator

----

.. rst-class:: classref-descriptions-group

Enumerations
------------

.. _enum_Thread_Priority:

.. rst-class:: classref-enumeration

enum **Priority**:

.. _class_Thread_constant_PRIORITY_LOW:

.. rst-class:: classref-enumeration-constant

:ref:`Priority<enum_Thread_Priority>` **PRIORITY_LOW** = ``0``

A thread running with lower priority than normally.

.. _class_Thread_constant_PRIORITY_NORMAL:

.. rst-class:: classref-enumeration-constant

:ref:`Priority<enum_Thread_Priority>` **PRIORITY_NORMAL** = ``1``

A thread with a standard priority.

.. _class_Thread_constant_PRIORITY_HIGH:

.. rst-class:: classref-enumeration-constant

:ref:`Priority<enum_Thread_Priority>` **PRIORITY_HIGH** = ``2``

A thread running with higher priority than normally.

.. rst-class:: classref-section-separator

----

.. rst-class:: classref-descriptions-group

Method Descriptions
-------------------

.. _class_Thread_method_get_id:

.. rst-class:: classref-method

:ref:`String<class_String>` **get_id** **(** **)** |const|

Returns the current **Thread**'s ID, uniquely identifying it among all threads. If the **Thread** has not started running or if :ref:`wait_to_finish<class_Thread_method_wait_to_finish>` has been called, this returns an empty string.

.. rst-class:: classref-item-separator

----

.. _class_Thread_method_is_alive:

.. rst-class:: classref-method

:ref:`bool<class_bool>` **is_alive** **(** **)** |const|

Returns ``true`` if this **Thread** is currently running the provided function. This is useful for determining if :ref:`wait_to_finish<class_Thread_method_wait_to_finish>` can be called without blocking the calling thread.

To check if a **Thread** is joinable, use :ref:`is_started<class_Thread_method_is_started>`.

.. rst-class:: classref-item-separator

----

.. _class_Thread_method_is_started:

.. rst-class:: classref-method

:ref:`bool<class_bool>` **is_started** **(** **)** |const|

Returns ``true`` if this **Thread** has been started. Once started, this will return ``true`` until it is joined using :ref:`wait_to_finish<class_Thread_method_wait_to_finish>`. For checking if a **Thread** is still executing its task, use :ref:`is_alive<class_Thread_method_is_alive>`.

.. rst-class:: classref-item-separator

----

.. _class_Thread_method_start:

.. rst-class:: classref-method

:ref:`Error<enum_@GlobalScope_Error>` **start** **(** :ref:`Callable<class_Callable>` callable, :ref:`Priority<enum_Thread_Priority>` priority=1 **)**

Starts a new **Thread** that calls ``callable``.

If the method takes some arguments, you can pass them using :ref:`Callable.bind<class_Callable_method_bind>`.

The ``priority`` of the **Thread** can be changed by passing a value from the :ref:`Priority<enum_Thread_Priority>` enum.

Returns :ref:`@GlobalScope.OK<class_@GlobalScope_constant_OK>` on success, or :ref:`@GlobalScope.ERR_CANT_CREATE<class_@GlobalScope_constant_ERR_CANT_CREATE>` on failure.

.. rst-class:: classref-item-separator

----

.. _class_Thread_method_wait_to_finish:

.. rst-class:: classref-method

:ref:`Variant<class_Variant>` **wait_to_finish** **(** **)**

Joins the **Thread** and waits for it to finish. Returns the output of the :ref:`Callable<class_Callable>` passed to :ref:`start<class_Thread_method_start>`.

Should either be used when you want to retrieve the value returned from the method called by the **Thread** or before freeing the instance that contains the **Thread**.

To determine if this can be called without blocking the calling thread, check if :ref:`is_alive<class_Thread_method_is_alive>` is ``false``.

.. |virtual| replace:: :abbr:`virtual (This method should typically be overridden by the user to have any effect.)`
.. |const| replace:: :abbr:`const (This method has no side effects. It doesn't modify any of the instance's member variables.)`
.. |vararg| replace:: :abbr:`vararg (This method accepts any number of arguments after the ones described here.)`
.. |constructor| replace:: :abbr:`constructor (This method is used to construct a type.)`
.. |static| replace:: :abbr:`static (This method doesn't need an instance to be called, so it can be called directly using the class name.)`
.. |operator| replace:: :abbr:`operator (This method describes a valid operator to use with this type as left-hand operand.)`