Events and Callbacks

Params pass data downwards from owner to owned-by component. Data comes back upwards asynchronously via callbacks, which are simply Procs passed as params into the owned-by component.
The upwards flow of data via callbacks is triggered by some event such as a mouse click, or input change:
class ClickDemo2 < HyperComponent
render do
BUTTON { "click" }.on(:click) { |evt| puts "I was clicked"}
When the BUTTON is clicked, the event (evt) is passed to the attached click handler.
The details of the event object will be discussed below.

Firing Events from Components

You can also define events in your components to communicate back to the owner:
class Clicker < HyperComponent
param title: "click"
fires :clicked
before_mount { @clicks = 0 }
render do
BUTTON { title }.on(:click) { clicked!(@clicks += 1) }
class ClickDemo3 < HyperComponent
render(DIV) do
DIV { "I have been clicked #{pluralize(@clicks, 'times')}" } if @clicks
Clicker().on(:clicked) { |clicks| mutate @clicks = clicks }
Each time the Clicker's button is clicked it fires the clicked event, indicated by the event name followed by a bang (!).
The clicked event is received by ClickDemo3, and it updates its state. As you can see events can send arbitrary data back out.
Notice also that Clicker does not call mutate. It could, but since the change in @clicks is not used anywhere to control its display there is no need for Clicker to mutate.

Relationship between Events and Params

Notice how events (and callbacks in general as we will see) move data upwards, while params move data downwards. We can emphasize this by updating our example:
class ClickDemo4 < HyperComponent
def title
@clicks ? "Click me again!" : "Let's start clicking!"
render(DIV) do
DIV { "I have been clicked #{pluralize(@clicks, 'times')}" } if @clicks
Clicker(title: title).on(:clicked) { |clicks| mutate @clicks = clicks }
When ClickDemo4 is first rendered, the title method will return "Let's start clicking!", and will be passed to Clicker.
The user will (hopefully so we can get on with this chapter) click the button, which will fire the event. The handler in ClickDemo4 will mutate its state, causing title to change to "Click me again!". The new value of the title param will be passed to Clicker, and Clicker will re-render with the new title.
Events (and callbacks) push data up, params move data down.

Callbacks and Proc Params

Under the hood Events are simply params of type Proc, with the on and fires method using some naming conventions to clean things up:
class IntemittentButton < HyperComponent
param :frequency
param :pulse, type: Proc
before_mount { @clicks = 0 }
render do
on_click: lambda {} do
@clicks += 1
pulse(@clicks) if (@clicks % frequency).zero?
) { 'click me' }
class ClickDemo5 < HyperComponent
render do
frequency: 5,
pulse: -> (total_clicks) { alert "you are clicking a lot" }
There is really no reason not to use the fires method to declare Proc params, and no reason not use the on method to attach handlers. Both will keep your code clean and tidy.

Naming Conventions

The notation on(:click) is short for passing a proc to a param named on_click. In general on(:xxx) will pass the given block as the on_xxx parameter in a Hyperstack component and onXxx in a JS component.
All the built-in events and many React libraries follow the on_xxx (or onXxx in JS) convention. However even if a library does not use this convention you can still attach the handler using on('<name-of-param>'). Whatever string is inside the <..> brackets will be used as the param name.
Likewise the fires method is shorthand for creating a Proc param following the on_xxx naming convention:
fires :foo is short for param :on_foo, type: Proc, alias: :foo!

The Event Object

UI events like click send an object of class Event to the handler. Some of the data you can get from Event objects are:
  • target : the DOM object that was the target of the UI interaction
  • target.value : the value of the DOM object
  • key_code : the key pressed (for key_down and key_up events)

Other Sources of Events

Besides the UI there are several other sources of events:
  • Timers
  • HTTP Requests
  • Hyperstack Operations
  • Websockets
  • Web Workers
The way you receive events from these sources depends on the event. Typically though the method will either take a block, or callback proc, or in many cases will return a Promise. Regardless, the event handler will do one of three things: mutate some state within the component, fire an event to a higher level component, or update some shared store.
For details on updating shared stores, which is often the best answer see the chapter on HyperState...
You have seen the every method used to create events throughout this chapter, here is an example with an HTTP post (which returns a promise.)
class SaveButton < HyperComponent
fires :saved
fires :failed
render do
BUTTON { "Save" }
.on(:click) do
# Posting to some non-hyperstack endpoint for example
# Data is our class holding some data
END_POINT, payload: Data.to_payload
).then do |response|
saved!(response.json) do |response|