"To watch the sun sink behind a flower clad hill.
To wander on in a huge forest without thought of return.
To stand upon the shore and gaze after a boat that
disappears behind distant islands. To contemplate the
flight of wild geese seen and lost among the clouds.
And, subtle shadows of bamboo on bamboo."
-- Zeami Motokiyo
Yugen is an Entity system. You can use it to create many different kinds of things. The things that you make can dynamically change their behavior, or even transform into other things.
Yugen uses Behaviors and Attributes to define an entity's functionality. This is different than other entity systems that are strictly component-based.
Unstable; in development.
Yugen follows the concept of a "component-based" entity system. Instead of using components of encapsulated logic and data, Yugen splits the two by providing Attributes and Behaviors.
In a strict component-based entity system, a single component encapsulates both a specific set of properties and the associated logic. Yugen makes a higher-level distinction between data and logic, which is why it separates Behaviors and Attributes.
A Yugen entity is composed of any number of attributes (data with minimal validation) and behaviors (logic). Attributes are independent from behaviors, and thus multiple behaviors can act upon the same attribute(s).
A Yugen entity is a generic container composed of the following:
- Attributes
- Behaviors
Attributes know nothing about behaviors. Behaviors may reference and act upon any number of attributes. An entity acts as an observable object and mediator for the collection of attributes and behaviors.
An Attribute is a property name/value pair. It can have a minimal set of validation logic, but any other logic should be inside of a behavior.
A Behavior is a set of logic that can reference and modify attribute values, be added or removed from an Entity, or respond to events.
+-----------+ send/receive
+----| Attribute |----+
+--------+ | +-----------+ | +------------+
| |<>---+ ^ +--->| |
| Entity | | | Entity |
| |<>---+ | +--->| Event |
+--------+ | +-----------+ | +------------+
+----| Behavior |----+
+-----------+ send/receive
|
| send/receive
v
+-----------+
| External |
| Events |
+-----------+
Yugen uses my "wabi-sabi" design philosphy, shared by its sister libraries (Kokou, Kanso, etc):
Mixin, Object, Delegate, Compose (MODC)
This style promotes several concepts:
- Create simple objects that perform a specific function.
- Provide a companion mixin, if possible, to provide flexibility to create new types of objects.
- Use differential overrides, where the non-overrides are delegated to the prototypical object.
- Use composition for more complex behavior.
By using these principles, you have extreme flexibility to create many combinations of objects and allow the end user to adapt the functionality to their own specific use case.
For example, attribute.js provides a factory function that creates a basic attribute object by using the mixin that itself defined.
In evented-attribute.js, you see that it uses a vanilla attribute object as the prototypical object. It also mixes in the emitter functionality and simply overrides a single function, setVal, to emit an event.
I use the "verb" style of mixin with an applied context (using "call") to link cached functions for performance.
For example, attribute mixin, asAttribute, is used like this: attr.asAttribute.call(myObj). Then, myObj also has attribute functionality.
Properties: Because I do not rely on constructors, due to the extensive use of mixins, each mixin defines an init function that sets up the required mixin properties. The init function should be labeled after the mixin's functionality, such as: initAttr, initEmitter, etc. This allows you to create a factory function to do a specific initialization, or allows the end user to manually init each functionality separately.