Design Patterns
---------------

There are several design patterns that are repeated throughout the
repository. This guide is meant to supplement the `TypeScript Style
Guide <https://github.com/jupyterlab/jupyterlab/wiki/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 enables options to be later added
while preserving backward compatibility.

ContentFactory Option
~~~~~~~~~~~~~~~~~~~~~

| A common option for a widget is a ``IContentFactory``, which is used
  to customize the child content in the widget.
| If not given, a ``defaultRenderer`` instance is used if no arguments
  are required. 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 enables 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.
| To mirror the pattern of construction, ``super.dispose()`` should be
  called last in the ``dispose()`` method if there is a parent class.
  Make sure any signal connections are cleared in either the local or
  parent ``dispose()`` method. Use a sentinel value to guard against
  reentry, typically by checking if an internal value is null, and then
  immediately setting the value to null. A subclass should never
  override the ``isDisposed`` getter, because it short-circuits the
  parent class getter. The object should not be considered disposed
  until the base class ``dispose()`` method is called.

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 enable 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.
Wherever possible as signal connection should be made with the pattern
``.connect(this._onFoo, this)``. Providing the ``this`` context enables
the connection to be properly cleared by ``clearSignalData(this)``.
Using a private method avoids allocating a closure for each connection.

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 enable
consumers to handle a change in model. The reason to enable 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.

.. _getters-vs-methods:

Getters vs. Methods
~~~~~~~~~~~~~~~~~~~

Prefer a method when the return value must be computed each time. Prefer
a getter for simple attribute lookup. A getter should yield the same
value every time.

Data Structures
~~~~~~~~~~~~~~~

For public API, we have three options: JavaScript ``Array``,
``IIterator``, and ``ReadonlyArray`` (an interface defined by
TypeScript).

Prefer an ``Array`` for:

-  A value that is meant to be mutable.

Prefer a ``ReadonlyArray``

-  A return value is the result of a newly allocated array, to avoid the
   extra allocation of an iterator.
-  A signal payload - since it will be consumed by multiple listeners.
-  The values may need to be accessed randomly.
-  A public attribute that is inherently static.

Prefer an ``IIterator`` for:

-  A return value where the value is based on an internal data structure
   but the value should not need to be accessed randomly.
-  A set of return values that can be computed lazily.

DOM Events
~~~~~~~~~~

If an object instance should respond to DOM events, create a
``handleEvent`` method for the class and register the object instance as
the event handler. The ``handleEvent`` method should switch on the event
type and could call private methods to carry out the actions. Often a
widget class will add itself as an event listener to its own node in the
``onAfterAttach`` method with something like
``this.node.addEventListener('mousedown', this)`` and unregister itself
in the ``onBeforeDetach`` method with
``this.node.removeEventListener('mousedown', this)`` Dispatching events
from the ``handleEvent`` method makes it easier to trace, log, and debug
event handling. For more information about the ``handleEvent`` method,
see the
`EventListener <https://developer.mozilla.org/en-US/docs/Web/API/EventListener>`__
API.

Promises
~~~~~~~~

We use Promises for asynchronous function calls, and a shim for browsers
that do not support them. When handling a resolved or rejected Promise,
make sure to check for the current state (typically by checking an
``.isDisposed`` property) before proceeding.

Command Names
~~~~~~~~~~~~~

Commands used in the application command registry should be formatted as
follows: ``package-name:verb-noun``. They are typically grouped into a
``CommandIDs`` namespace in the extension that is not exported.
