8

We've got a Web App where we have a lot (>50) of little WebComponents that interact with each other.

To keep everything decoupled, we have as a rule that no component can directly reference another. Instead, components fire events which are then (in the "main" app) wired to call another component's methods.

As time went by more and more components where added and the "main" app file became littered with code chunks looking like this:

buttonsToolbar.addEventListener('request-toggle-contact-form-modal', () => {
  contactForm.toggle()
})

buttonsToolbar.addEventListener('request-toggle-bug-reporter-modal', () => {
  bugReporter.toggle()
})

// ... etc

To ameliorate this we grouped similar functionality together, in a Class, name it something relevant, pass the participating elements when instantiating and handle the "wiring" within the Class, like so:

class Contact {
  constructor(contactForm, bugReporter, buttonsToolbar) {
    this.contactForm = contactForm
    this.bugReporterForm = bugReporterForm
    this.buttonsToolbar = buttonsToolbar

    this.buttonsToolbar
      .addEventListener('request-toggle-contact-form-modal', () => {
        this.toggleContactForm()
      })

    this.buttonsToolbar
      .addEventListener('request-toggle-bug-reporter-modal', () => {
        this.toggleBugReporterForm()
      })
  }

  toggleContactForm() {
    this.contactForm.toggle()
  }

  toggleBugReporterForm() {
    this.bugReporterForm.toggle()
  }
}

and we instantiate like so:

<html>
  <contact-form></contact-form>
  <bug-reporter></bug-reporter>

  <script>
    const contact = new Contact(
      document.querySelector('contact-form'),
      document.querySelector('bug-form')
    )
  </script>
</html>

I'm really weary of introducing patterns of my own, especially ones that aren't really OOP-y since I'm using Classes as mere initialisation containers, for lack of a better word.

Is there a better/more well known defined pattern for handling this type of tasks that I'm missing?

  • 2
    This actually looks mildly awesome. – Robert Harvey Apr 24 '18 at 19:42
  • When I remember correctly, in this former question of yours you already called it a mediator, which is also the pattern name from the GoF book, so this is definitely an OOP pattern. – Doc Brown Apr 24 '18 at 19:47
  • @RobertHarvey Well your word has a lot of weight to me; Would you do it any differently? I'm not sure I'm overthinking this. – Nik Kyriakides Apr 24 '18 at 19:53
  • 2
    Don't overthink this. Your "wiring" class looks SOLID to me, if it works and you are pleased with the name, it should not matter how it is called. – Doc Brown Apr 24 '18 at 20:28
  • 1
    @NicholasKyriakides: better ask one of your coworkers (who surely knows the system better than me) for a good name, not a stranger like me from the internet. – Doc Brown Apr 24 '18 at 21:23
6
+50

The code you have is pretty good. The thing that seems a bit off-putting is the initialization code is not part of the object itself. That is, you can instantiate an object, but if you forget to call its wiring class, it's useless.

Consider a Notification Center (aka Event Bus) defined something like this:

class NotificationCenter(){
    constructor(){
        this.dictionary = {}
    }
    register(message, callback){
        if not this.dictionary.contains(message){
            this.dictionary[message] = []
        }
        this.dictionary[message].append(callback)
    }
    notify(message, payload){
        if this.dictionary.contains(message){
            for each callback in this.dictionary[message]{
                callback(payload)
            }
        }
    }
}

This is a DIY multi-dispatch event handler. You would then be able to do your own wiring by simply requiring a NotificationCenter as a constructor argument. Sending messages into it and waiting for it to pass you payloads is the only contact you have with the system, so it's very SOLID.

class Toolbar{
    constructor(notificationCenter){
        this.NC = notificationCenter
        this.NC.register('request-toggle-contact-form-modal', (payload) => {
            this.toggleContactForm(payload)
          }
    }
    toolbarButtonClicked(e){
        this.NC.notify('toolbar-button-click-event', e)
    }
}

Note: I used in-place string literals for keys to be consistent with the style used in the question and for simplicity. This is not advisable due to risk of typos. Instead, consider using an enumeration or string constants.

In the above code, the Toolbar is responsible for letting the NotificationCenter know what type of events it's interested in, and publishing all of its external interactions via the notify method. Any other class interested in the toolbar-button-click-event would simply register for it in its constructor.

Interesting variations on this pattern include:

  • Using multiple NCs to handle different parts of the system
  • Having the Notify method spawn off a thread for each notification, rather than blocking serially
  • Using a priority list rather than a regular list inside the NC to guarantee a partial ordering on which components get notified first
  • Register returning an ID which can be used to Unregister later
  • Skip the message argument and just dispatch based on the message's class/type

Interesting features include:

  • Instrumenting the NC is as easy as registering loggers to print payloads
  • Testing one or more components interacting is simply a matter of instantiating them, adding listeners for the expected results, and sending in messages
  • Adding new components listening for old messages is trivial
  • Adding new components sending messages to old ones is trivial

Interesting gotchas and possible remedies include:

  • Events triggering other events can get confusing.
    • Include a sender ID in the event to pinpoint the source of an unexpected event.
  • Each component has no idea whether any given part of the system is up and running before it receives an event, so early messages may be dropped.
    • This may be handled by the code creating the components sending a 'system ready' message , which of course interested components would need to register for.
  • The event bus creates an implied interface between components, meaning there is no way for the compiler to be sure you've implemented everything you should.
    • The standard arguments between static and dynamic apply here.
  • This approach groups together components, not necessarily behavior. Tracing events through the system may require more work here than the OP's approach. For example, OP could have all of the saving-related listeners set up together and the deleting-related listeners set up together elsewhere.
    • This can be mitigated with good event naming and documentation such as a flow chart. (Yes, the documentation is famously out of step with the code). You could also add pre- and post- catchall handler lists that get all messages and print out who sent what in which order.
  • This approach seems reminiscent of the Event Bus architecture. The only difference is that your registration is a topic string rather than a message type. The main weakness of using strings is that they are subject to mistyping them--meaning the notification or the listener could be mispelled and it would be hard to debug. – Berin Loritsch May 1 '18 at 12:33
  • As far as I can tell this is a classic example of a Mediator. A problem with this approach is that it couples a component with the Event Bus/Mediator. What if I want to move a component, e.g the buttonsToolbar to another project that does not use an Event Bus? – Nik Kyriakides May 1 '18 at 13:41
  • +1 The benefit of the mediator is it allows you to register against strings/enums and have the loose coupling within the class. If you move the wiring outside the object to your main/setup class then it knows about all the objects and could wire them up directly to events/functions without worrying about coupling. @NicholasKyriakides Pick one or the other rather than trying to use both – Ewan May 1 '18 at 16:02
  • With the classic event bus architecture the only coupling is to the message itself. The message is typically an immutable object. The object that sends messages only needs the publisher interface to send the messages. If you use the type of the message object then you only need to publish the message object. If you use a string, you have to supply both the topic string and the message payload (unless the string is the payload). Using strings means you just have to be meticulous about the values on both sides. – Berin Loritsch May 1 '18 at 17:20
  • @NicholasKyriakides What happens if you move your original code to a new solution? You have to bring along your setup class and change it for its new context. Same thing applies to this pattern. – Joel Harmon May 1 '18 at 22:32
3

I used to introduce an "event bus" of some sort, and in later years I've started relying more and more on the Document Object Model itself to communicate events for UI code.

In a browser, the DOM is the one dependency that is always present - even during page load. The key is utilizing Custom Events in JavaScript, and relying on event bubbling to communicate those events.

Before people start shouting about "waiting for the document to be ready" before attaching subscribers, the document.documentElement property references the <html> element from the moment JavaScript begins execution, no matter where the script is imported or the order in which it appears in your markup.

This is where you can start listening for events.

It's very common to have a JavaScript component (or widget) live within a certain HTML tag on the page. The "root" element of the component is where you can trigger your bubbling events. Subscribers on the <html> element will receive these notifications just like any other user generated event.

Just some example boiler plate code:

(function (window, document, html) {
    html.addEventListener("custom-event-1", function (event) {
        // ...
    });
    html.addEventListener("custom-event-2", function (event) {
        // ...
    });

    function someOperation() {
        var customData = { ... };
        var event = new CustomEvent("custom-event-3", { detail : customData });

        event.dispatchEvent(componentRootElement);
    }
})(this, this.document, this.document.documentElement);

So the pattern becomes:

  1. Use Custom Events
  2. Subscribe to these events on the document.documentElement property (no need to wait for the document to be ready)
  3. Publish events on a root element for your component, or the document.documentElement.

This should work for both functional and object oriented code bases.

0

For what it's worth, I am doing something as part of a backend project and have taken a similar approach:

  • My system does not involve widgets (web components) but abstract 'adapters,' concrete implementations of which handle different protocols.
  • A protocol is modeled as a set of possible 'conversations' . The protocol adapter triggers these conversations depending on an incoming event.
  • There is an Event Bus which is basically an Rx Subject.
  • The Event Bus subscribes to the output of all adapters, and all adapters subscribe to the output of the Event Bus.
  • The 'adapter' is modelled as the aggregate stream of all its 'conversations.' A conversation is a stream subscribed to the output of the event bus, generating messages to the event bus, driven by a State Machine.

How I handled your construction/wiring challenges:

  • A protocol (implemented by adapter) defines conversation initiating criteria as filters over the input stream it is subscribed to. In C# these are LINQ queries over streams. In ReactJS these would be .Where or .Filter operators.
  • A conversation decides what is a relevant message using its own filters.
  • In general, anything subscribed to the bus is a stream, and the bus is subscribed to those streams.

The analogy with your toolbar:

  • The toolbar class is a .Map of an input observable (the bus), which is an observable of toolbar events, to which the bus is subscribed
  • An observable of toolbars (if you multiple sub-toolbars) means that you may have multiple observables, so your toolbar is an observable of observables. These would be RxJs .Merge'd into a single output to the bus.

Issues you may face:

  • Ensuring that events are not cyclical and hang the process.
  • Concurrency (don't know if this is relevant for WebComponents): for asynchronous operations or operations that may be long running, your event handler may block the observable thread if not run as a background task. RxJS schedulers can address this (by default you can .ObserveOn a default scheduler for all bus subscriptions, for example)
  • More complex scenarios that cannot be modeled without some notion of a conversation (eg: handling an event by sending a message and waiting for a response, which is itself an event). In this case, a state machine is useful to dynamically specify which events you want to handle (conversation in my model). I do this my having the conversation stream .filter according to state (actually, the implementation is more functional - the conversation is a flat map of observables from an observable of state change events ).

So, in summary, you can look at your whole problem domain as observables, or 'functionally'/'declaratively' and consider your webcomponents as event streams, as observables, derived from the bus (an observable) , to which the bus (an observer) is also subscribed. Instantiation of observables (eg: a new toolbar) is declarative, in that the whole process can be seen as an observable of observables .map'd from the input stream.

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