godot-docs/tutorials/best_practices/godot_interfaces.rst

.. _doc_godot_interfaces:

Godot interfaces
================

Often one needs scripts that rely on other objects for features. There
are 2 parts to this process:

1. Acquiring a reference to the object that presumably has the features.

2. Accessing the data or logic from the object.

The rest of this tutorial outlines the various ways of doing all this.

Acquiring object references
---------------------------

For all :ref:`Object <class_Object>`\s, the most basic way of referencing them
is to get a reference to an existing object from another acquired instance.

.. tabs::
  .. code-tab:: gdscript GDScript

    var obj = node.object # Property access.
    var obj = node.get_object() # Method access.

  .. code-tab:: csharp

    GodotObject obj = node.Object; // Property access.
    GodotObject obj = node.GetObject(); // Method access.

The same principle applies for :ref:`RefCounted <class_RefCounted>` objects.
While users often access :ref:`Node <class_Node>` and
:ref:`Resource <class_Resource>` this way, alternative measures are available.

Instead of property or method access, one can get Resources by load
access.

.. tabs::
  .. code-tab:: gdscript GDScript

    # If you need an "export const var" (which doesn't exist), use a conditional
    # setter for a tool script that checks if it's executing in the editor.
    # The `@tool` annotation must be placed at the top of the script.
    @tool

    # Load resource during scene load.
    var preres = preload(path)
    # Load resource when program reaches statement.
    var res = load(path)

    # Note that users load scenes and scripts, by convention, with PascalCase
    # names (like typenames), often into constants.
    const MyScene = preload("my_scene.tscn") # Static load
    const MyScript = preload("my_script.gd")

    # This type's value varies, i.e. it is a variable, so it uses snake_case.
    @export var script_type: Script

    # Must configure from the editor, defaults to null.
    @export var const_script: Script:
        set(value):
            if Engine.is_editor_hint():
                const_script = value

    # Warn users if the value hasn't been set.
    func _get_configuration_warnings():
        if not const_script:
            return ["Must initialize property 'const_script'."]

        return []

  .. code-tab:: csharp

    // Tool script added for the sake of the "const [Export]" example.
    [Tool]
    public MyType
    {
        // Property initializations load during Script instancing, i.e. .new().
        // No "preload" loads during scene load exists in C#.

        // Initialize with a value. Editable at runtime.
        public Script MyScript = GD.Load<Script>("res://Path/To/MyScript.cs");

        // Initialize with same value. Value cannot be changed.
        public readonly Script MyConstScript = GD.Load<Script>("res://Path/To/MyScript.cs");

        // Like 'readonly' due to inaccessible setter.
        // But, value can be set during constructor, i.e. MyType().
        public Script MyNoSetScript { get; } = GD.Load<Script>("res://Path/To/MyScript.cs");

        // If need a "const [Export]" (which doesn't exist), use a
        // conditional setter for a tool script that checks if it's executing
        // in the editor.
        private PackedScene _enemyScn;

        [Export]
        public PackedScene EnemyScn
        {
            get { return _enemyScn; }
            set
            {
                if (Engine.IsEditorHint())
                {
                    _enemyScn = value;
                }
            }
        };

        // Warn users if the value hasn't been set.
        public string[] _GetConfigurationWarnings()
        {
            if (EnemyScn == null)
            {
                return new string[] { "Must initialize property 'EnemyScn'." };
            }
            return Array.Empty<string>();
        }
    }

Note the following:

1. There are many ways in which a language can load such resources.

2. When designing how objects will access data, don't forget
   that one can pass resources around as references as well.

3. Keep in mind that loading a resource fetches the cached resource
   instance maintained by the engine. To get a new object, one must
   :ref:`duplicate <class_Resource_method_duplicate>` an existing reference
   or instantiate one from scratch with ``new()``.

Nodes likewise have an alternative access point: the SceneTree.

.. tabs::
  .. code-tab:: gdscript GDScript

    extends Node

    # Slow.
    func dynamic_lookup_with_dynamic_nodepath():
        print(get_node("Child"))

    # Faster. GDScript only.
    func dynamic_lookup_with_cached_nodepath():
        print($Child)

    # Fastest. Doesn't break if node moves later.
    # Note that `@onready` annotation is GDScript-only.
    # Other languages must do...
    #     var child
    #     func _ready():
    #         child = get_node("Child")
    @onready var child = $Child
    func lookup_and_cache_for_future_access():
        print(child)

    # Fastest. Doesn't break if node is moved in the Scene tree dock.
    # Node must be selected in the inspector as it's an exported property.
    @export var child: Node
    func lookup_and_cache_for_future_access():
        print(child)

    # Delegate reference assignment to an external source.
    # Con: need to perform a validation check.
    # Pro: node makes no requirements of its external structure.
    #      'prop' can come from anywhere.
    var prop
    func call_me_after_prop_is_initialized_by_parent():
        # Validate prop in one of three ways.

        # Fail with no notification.
        if not prop:
            return

        # Fail with an error message.
        if not prop:
            printerr("'prop' wasn't initialized")
            return

        # Fail and terminate.
        # NOTE: Scripts run from a release export template don't run `assert`s.
        assert(prop, "'prop' wasn't initialized")

    # Use an autoload.
    # Dangerous for typical nodes, but useful for true singleton nodes
    # that manage their own data and don't interfere with other objects.
    func reference_a_global_autoloaded_variable():
        print(globals)
        print(globals.prop)
        print(globals.my_getter())

  .. code-tab:: csharp

    using Godot;
    using System;
    using System.Diagnostics;

    public class MyNode : Node
    {
        // Slow
        public void DynamicLookupWithDynamicNodePath()
        {
            GD.Print(GetNode("Child"));
        }

        // Fastest. Lookup node and cache for future access.
        // Doesn't break if node moves later.
        private Node _child;
        public void _Ready()
        {
            _child = GetNode("Child");
        }
        public void LookupAndCacheForFutureAccess()
        {
            GD.Print(_child);
        }

        // Delegate reference assignment to an external source.
        // Con: need to perform a validation check.
        // Pro: node makes no requirements of its external structure.
        //      'prop' can come from anywhere.
        public object Prop { get; set; }
        public void CallMeAfterPropIsInitializedByParent()
        {
            // Validate prop in one of three ways.

            // Fail with no notification.
            if (prop == null)
            {
                return;
            }

            // Fail with an error message.
            if (prop == null)
            {
                GD.PrintErr("'Prop' wasn't initialized");
                return;
            }

            // Fail with an exception.
            if (prop == null)
            {
                throw new InvalidOperationException("'Prop' wasn't initialized.");
            }

            // Fail and terminate.
            // Note: Scripts run from a release export template don't run `Debug.Assert`s.
            Debug.Assert(Prop, "'Prop' wasn't initialized");
        }

        // Use an autoload.
        // Dangerous for typical nodes, but useful for true singleton nodes
        // that manage their own data and don't interfere with other objects.
        public void ReferenceAGlobalAutoloadedVariable()
        {
            MyNode globals = GetNode<MyNode>("/root/Globals");
            GD.Print(globals);
            GD.Print(globals.Prop);
            GD.Print(globals.MyGetter());
        }
    };

.. _doc_accessing_data_or_logic_from_object:

Accessing data or logic from an object
--------------------------------------

Godot's scripting API is duck-typed. This means that if a script executes an
operation, Godot doesn't validate that it supports the operation by **type**.
It instead checks that the object **implements** the individual method.

For example, the :ref:`CanvasItem <class_CanvasItem>` class has a ``visible``
property. All properties exposed to the scripting API are in fact a setter and
getter pair bound to a name. If one tried to access
:ref:`CanvasItem.visible <class_CanvasItem_property_visible>`, then Godot would do the
following checks, in order:

- If the object has a script attached, it will attempt to set the property
  through the script. This leaves open the opportunity for scripts to override
  a property defined on a base object by overriding the setter method for the
  property.

- If the script does not have the property, it performs a HashMap lookup in
  the ClassDB for the "visible" property against the CanvasItem class and all
  of its inherited types. If found, it will call the bound setter or getter.
  For more information about HashMaps, see the
  :ref:`data preferences <doc_data_preferences>` docs.

- If not found, it does an explicit check to see if the user wants to access
  the "script" or "meta" properties.

- If not, it checks for a ``_set``/``_get`` implementation (depending on type
  of access) in the CanvasItem and its inherited types. These methods can
  execute logic that gives the impression that the Object has a property. This
  is also the case with the ``_get_property_list`` method.

  - Note that this happens even for non-legal symbol names, such as names
    starting with a digit or containing a slash.

As a result, this duck-typed system can locate a property either in the script,
the object's class, or any class that object inherits, but only for things
which extend Object.

Godot provides a variety of options for performing runtime checks on these
accesses:

- A duck-typed property access. These will be property checks (as described above).
  If the operation isn't supported by the object, execution will halt.

  .. tabs::
    .. code-tab:: gdscript GDScript

      # All Objects have duck-typed get, set, and call wrapper methods.
      get_parent().set("visible", false)

      # Using a symbol accessor, rather than a string in the method call,
      # will implicitly call the `set` method which, in turn, calls the
      # setter method bound to the property through the property lookup
      # sequence.
      get_parent().visible = false

      # Note that if one defines a _set and _get that describe a property's
      # existence, but the property isn't recognized in any _get_property_list
      # method, then the set() and get() methods will work, but the symbol
      # access will claim it can't find the property.

    .. code-tab:: csharp

      // All Objects have duck-typed Get, Set, and Call wrapper methods.
      GetParent().Set("visible", false);

      // C# is a static language, so it has no dynamic symbol access, e.g.
      // `GetParent().Visible = false` won't work.

- A method check. In the case of
  :ref:`CanvasItem.visible <class_CanvasItem_property_visible>`, one can
  access the methods, ``set_visible`` and ``is_visible`` like any other method.

  .. tabs::
    .. code-tab:: gdscript GDScript

      var child = get_child(0)

      # Dynamic lookup.
      child.call("set_visible", false)

      # Symbol-based dynamic lookup.
      # GDScript aliases this into a 'call' method behind the scenes.
      child.set_visible(false)

      # Dynamic lookup, checks for method existence first.
      if child.has_method("set_visible"):
          child.set_visible(false)

      # Cast check, followed by dynamic lookup.
      # Useful when you make multiple "safe" calls knowing that the class
      # implements them all. No need for repeated checks.
      # Tricky if one executes a cast check for a user-defined type as it
      # forces more dependencies.
      if child is CanvasItem:
          child.set_visible(false)
          child.show_on_top = true

      # If one does not wish to fail these checks without notifying users,
      # one can use an assert instead. These will trigger runtime errors
      # immediately if not true.
      assert(child.has_method("set_visible"))
      assert(child.is_in_group("offer"))
      assert(child is CanvasItem)

      # Can also use object labels to imply an interface, i.e. assume it
      # implements certain methods.
      # There are two types, both of which only exist for Nodes: Names and
      # Groups.

      # Assuming...
      # A "Quest" object exists and 1) that it can "complete" or "fail" and
      # that it will have text available before and after each state...

      # 1. Use a name.
      var quest = $Quest
      print(quest.text)
      quest.complete() # or quest.fail()
      print(quest.text) # implied new text content

      # 2. Use a group.
      for a_child in get_children():
          if a_child.is_in_group("quest"):
              print(quest.text)
              quest.complete() # or quest.fail()
              print(quest.text) # implied new text content

      # Note that these interfaces are project-specific conventions the team
      # defines (which means documentation! But maybe worth it?).
      # Any script that conforms to the documented "interface" of the name or
      # group can fill in for it.

    .. code-tab:: csharp

      Node child = GetChild(0);

      // Dynamic lookup.
      child.Call("SetVisible", false);

      // Dynamic lookup, checks for method existence first.
      if (child.HasMethod("SetVisible"))
      {
          child.Call("SetVisible", false);
      }

      // Use a group as if it were an "interface", i.e. assume it implements
      // certain methods.
      // Requires good documentation for the project to keep it reliable
      // (unless you make editor tools to enforce it at editor time).
      // Note, this is generally not as good as using an actual interface in
      // C#, but you can't set C# interfaces from the editor since they are
      // language-level features.
      if (child.IsInGroup("Offer"))
      {
          child.Call("Accept");
          child.Call("Reject");
      }

      // Cast check, followed by static lookup.
      CanvasItem ci = GetParent() as CanvasItem;
      if (ci != null)
      {
          ci.SetVisible(false);

          // useful when you need to make multiple safe calls to the class
          ci.ShowOnTop = true;
      }

      // If one does not wish to fail these checks without notifying users,
      // one can use an assert instead. These will trigger runtime errors
      // immediately if not true.
      Debug.Assert(child.HasMethod("set_visible"));
      Debug.Assert(child.IsInGroup("offer"));
      Debug.Assert(CanvasItem.InstanceHas(child));

      // Can also use object labels to imply an interface, i.e. assume it
      // implements certain methods.
      // There are two types, both of which only exist for Nodes: Names and
      // Groups.

      // Assuming...
      // A "Quest" object exists and 1) that it can "Complete" or "Fail" and
      // that it will have Text available before and after each state...

      // 1. Use a name.
      Node quest = GetNode("Quest");
      GD.Print(quest.Get("Text"));
      quest.Call("Complete"); // or "Fail".
      GD.Print(quest.Get("Text")); // Implied new text content.

      // 2. Use a group.
      foreach (Node AChild in GetChildren())
      {
          if (AChild.IsInGroup("quest"))
          {
            GD.Print(quest.Get("Text"));
            quest.Call("Complete"); // or "Fail".
            GD.Print(quest.Get("Text")); // Implied new text content.
          }
      }

      // Note that these interfaces are project-specific conventions the team
      // defines (which means documentation! But maybe worth it?).
      // Any script that conforms to the documented "interface" of the
      // name or group can fill in for it. Also note that in C#, these methods
      // will be slower than static accesses with traditional interfaces.

- Outsource the access to a :ref:`Callable <class_Callable>`. These may be useful
  in cases where one needs the max level of freedom from dependencies. In
  this case, one relies on an external context to setup the method.

.. tabs::
  .. code-tab:: gdscript GDScript

    # child.gd
    extends Node
    var fn = null

    func my_method():
        if fn:
            fn.call()

    # parent.gd
    extends Node

    @onready var child = $Child

    func _ready():
        child.fn = print_me
        child.my_method()

    func print_me():
        print(name)

  .. code-tab:: csharp

    // Child.cs
    using Godot;

    public partial class Child : Node
    {
        public Callable? Callable { get; set; }

        public void MyMethod()
        {
            Callable?.Call();
        }
    }

    // Parent.cs
    using Godot;

    public partial class Parent : Node
    {
        private Child _child;

        public void _Ready()
        {
            _child = GetNode<Child>("Child");
            _child.Callable = Callable.From(PrintMe);
            _child.MyMethod();
        }

        public void PrintMe()
        {
            GD.Print(Name);
        }
    }

These strategies contribute to Godot's flexible design. Between them, users
have a breadth of tools to meet their specific needs.