There are several design patterns that are repeated throughout the repository. This guide is meant to supplement the TypeScript Style Guide.
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.
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.
IContentFactory, which is used to customize the child content in the widget.
defaultRendererinstance 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.
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.
IDisposablepattern 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.
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
isDisposedgetter, because it short-circuits the parent class getter. The object should not be considered disposed until the base class
dispose()method is called.
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 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
Using a private method avoids allocating a closure for each connection.
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¶
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.
ReadonlyArray (an interface defined by
- A value that is meant to be mutable.
- 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.
- 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.
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
onBeforeDetach method with
this.node.removeEventListener('mousedown', this) Dispatching events
handleEvent method makes it easier to trace, log, and debug
event handling. For more information about the
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.
Commands used in the application command registry should be formatted as
package-name:verb-noun. They are typically grouped into a
CommandIDs namespace in the extension that is not exported.