patterns.md 3.6 KB
Design Patterns
There are several design patterns that are repeated throughout the repository. This guide is meant to supplement the TypeScript Style Guide.
TypeScript
TypeScript is used in all of the source code. TypeScript is used because it provides features from the most recent EMCAScript 6 standards, while providing type safety. The TypeScript compiler eliminates an entire class of bugs, while making it much easier to refactor code.
Initialization Options
Objects will typically have an IOptions interface for
initializing the widget. The use of this interface allows options
to be later added while preserving backward compatibility.
Renderer Option
A common option for a widget is a IRenderer, which is used to customize the
rendering of content in the widget. If not given, a defaultRenderer instance
is typically used. In this way, widgets can be customized
without subclassing them, and widgets can support customization
of their nested content.
Static Namespace
An object class will typically have an exported static namespace sharing the same name as the object. The namespace is used to declutter the class definition.
Private Module Namespace
The "Private" module namespace is used to group variables and
functions that are not intended to be exported and may have
otherwise existed as module-level variables and functions.
The use of the namespace also makes it clear when a variable access
is to an imported name or from the module itself. Finally,
the namespace allows the entire section to be collapsed in
an editor if desired.
Disposables
JavaScript does not support "destructors", so the IDisposable
pattern is used to ensure resources are freed and can be claimed by the
Garbage Collector when no longer needed. It should always be safe to
dispose() of an object more than once. Typically the object that
creates another object is responsible for calling the dispose method
of that object unless explicitly stated otherwise.
Messages
Messages are intended for many-to-one communication where outside objects influence another object. Messages can be conflated and processed as a single message. They can be posted and handled on the next animation frame.
Signals
Signals are intended for one-to-many communication where outside objects react to changes on another object. Signals are always emitted with the sender as the first argument, and contain a single second argument with the payload. Signals should generally not be used to trigger the "default" behavior for an action, but to allow others to trigger additional behavior. If a "default" behavior is intended to be provided by another object, then a callback should be provided by that object.
Models
Some of the more advanced widgets have a model associated with them.
The common pattern used is that the model is settable and must be set
outside of the constructor. This means that any consumer of the widget
must account for a model that may be null, and may change at any time.
The widget should emit a modelChanged signal to allow consumers to
handle a change in model. The reason to allow a model to swap is that
the same widget could be used to display different model content
while preserving the widget's location in the application. The reason
the model cannot be provided in the constructor is the initialization
required for a model may have to call methods that are subclassed.
The subclassed methods would be called before the subclass constructor has
finished evaluating, resulting in undefined state.