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Script properties

Script properties provide a simple and powerful way of defining and exposing custom properties for a specific game object instance. Script properties can be edited on specific instances directly in the editor and their settings can be used in code to alter the behavior of a game object. There are many cases where script properties are very useful:

  • When you want to override values for specific instances in the editor, and thereby increase script re-usability.
  • When you want to spawn a game object with initial values.
  • When you want to animate the values of a property.
  • When you want to access state data in one script from another. (Note that if you access properties frequently between objects, it may be better to move the data to a shared storage.)

Common use cases are to set the health or speed of a specific enemy AI, the tint color of a pickup object, or what message a button object should send when pressed—and/or where to send it.

Defining a script property

Script properties are added to a script component by defining them with the go.property() special function. The function has to be used at the top level—outside any callback-functions like init() and update(). The default value provided for the property governs the type of the property: number, boolean, hash, msg.url, vmath.vector3, vmath.vector4 or vmath.quaternion.

-- Define script properties for health and an attack target
go.property("health", 100)
go.property("target", msg.url())

function init(self)
  -- store initial position of target.
  -- self.target is an url referencing another objects.
  self.target_pos = go.get_position(self.target)
  ...
end

function on_message(self, message_id, message, sender)
  if message_id == hash("take_damage") then
    -- decrease the health property
    self.health = self.health - message.damage
    if self.health <= 0 then
      go.delete()
    end
  end
end

Any script component instance created from this script can then set the property values.

Component with properties

Select the script component in the Outline view in the editor and the properties appear in the Properties view allowing you to edit them:

Properties

Any property that is overridden with a new instance specific value is marked blue. Click the reset button by the property name to revert the value to the default (as set in the script).

Accessing script properties

Any defined script property is available as a stored member in self, the script instance reference:

-- my_script.script
go.property("my_property", 1)

function update(self, dt)
  -- Read and write the property
  if self.my_property == 1 then
      self.my_property = 3
  end
end

User-defined script properties can also be accessed through the getting, setting and animation functions, the same way as any other property:

-- another.script

-- increase "my_property" in "myobject#script" by 1
local val = go.get("myobject#my_script", "my_property")
go.set("myobject#my_script", "my_property", val + 1)

-- animate "my_property" in "myobject#my_script"
go.animate("myobject#my_script", "my_property", go.PLAYBACK_LOOP_PINGPONG, 100, go.EASING_LINEAR, 2.0)

Factory created objects

If you use a factory to create the game object, it is possible to set script properties at creation time:

local props = { health = 50, target = msg.url("player") }
factory.create("#can_factory", nil, nil, props)

When spawning a hierarchy of game objects through collectionfactory.create() you need to pair object id:s with property tables. These are put together in a table and passed to the create() function:

local props = {}
props[hash("/can1")] = { health = 150 }
props[hash("/can2")] = { health = 250, target = msg.url("player") }
props[hash("/can3")] = { health = 200 }

local ids = collectionfactory.create("#cangang_factory", nil, nil, props)

The property values provided via factory.create() and collectionfactory.create() will override any value set in the prototype file as well as the default values in the script.

If several script components attached to a game object defines the same property, each component will get initialized with the value provided to factory.create() or collectionfactory.create().