jupyterlab/tutorial/patterns.md

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.