The Values and Classes APIs

[sstephenson]

This pull request introduces two new APIs to Stimulus: Values and Classes. These APIs are designed to improve upon, and ultimately obviate, the current Data Map API. We plan to ship them together in the upcoming Stimulus 2.0 release.

Values

Most uses of the Data Map API in Basecamp fall under the following categories:

• Storing small strings, such as URLs, dates, or color values
• Keeping track of a numeric index into a collection
• Bootstrapping a controller with a JSON object or array
• Conditioning behavior on a per-controller basis

However, the Data Map API only works with string values. That means we must manually convert to and from other types as needed. The Values API handles this type conversion work automatically.

Value properties

The Values API adds support for a static values object on controllers. The keys of this object are Data Map keys, and the values declare their data type:

export default class extends Controller {
  static values = {
    url: String,
    refreshInterval: Number,
    loadOnConnect: Boolean
  }

  connect() {
    if (this.loadOnConnectValue) {
      this.load()
    }
  }

  async load() {
    const response = await fetch(this.urlValue)
    // ...
    setTimeout(() => this.load(), this.refreshIntervalValue)
  }
}

Supported types and defaults

This pull request implements support for five built-in types:

Type	Serialized attribute value	Default value
Array	JSON.stringify(array)	[]
Boolean	boolean.toString()	false
Number	number.toString()	0
Object	JSON.stringify(object)	{}
String	Itself	""

Each type has a default value. If a value is declared in a controller but its associated data attribute is missing, the getter property will return its type's default.

Controller properties

Stimulus automatically generates three properties for each entry in the object:

Type	Kind	Property name	Effect
Boolean, Number, Object, String	Getter	this.[name]Value	Reads data-[identifier]-[name]-value
Array	Getter	this.[name]Values	Reads data-[identifier]-[name]-values
Boolean, Number, Object, String	Setter	this.[name]Value=	Writes data-[identifier]-[name]-value
Array	Setter	this.[name]Values=	Writes data-[identifier]-[name]-values
Boolean, Number, Object, String	Existential	this.has[Name]Value	Tests for presence of data-[identifier]-[name]-value
Array	Existential	this.has[Name]Values	Tests for presence of data-[identifier]-[name]-values

Note that array values are always pluralized, both as properties and as attributes.

Value changed callbacks

In addition to value properties, the Values API introduces value changed callbacks. A value changed callback is a specially named method called by Stimulus whenever a value's data attribute is modified.

To observe changes to a value, define a method named [name]ValueChanged(). For example, a slideshow controller with a numeric index property might define an indexValueChanged() method to display the specified slide:

export default class extends Controller {
  static values = { index: Number }

  indexValueChanged() {
    this.showSlide(this.indexValue)
  }

  // ...
}

Stimulus invokes each value changed callback once when the controller is initialized, and again any time the value's data attribute changes.

Even if a value's data attribute is missing when the controller is initialized, Stimulus will still invoke its value changed callback. Use the existential property to determine whether the data attribute is present.

Classes

Another common use of the Data Map API is to store CSS class names.

For example, Basecamp's copy-to-clipboard controller applies a CSS class to its element after a successful copy. To avoid inlining a long BEM string in our controller, and to keep things loosely coupled, we declare the class in a data-clipboard-success-class attribute:

<div data-controller="clipboard"
     data-clipboard-success-class="copy-to-clipboard--success">

and access it using this.data.get("successClass") in the controller:

this.element.classList.add(this.data.get("successClass"))

The Classes API formalizes and refines this pattern.

Class properties

The Classes API adds a static classes array on controllers. As with targets, Stimulus automatically adds properties for each class listed in the array:

// clipboard_controller.js
export default class extends Controller {
  static classes = [ "success", "supported" ]

  initialize() {
    if (/* ... */) {
      this.element.classList.add(this.supportedClass)
    }
  }

  copy() {
    // ...
    this.element.classList.add(this.successClass)
  }
}

Kind	Property name	Effect
Getter	this.[name]Class	Reads the data-[identifier]-[name]-class attribute
Existential	this.has[Name]Class	Tests whether the data-[identifier]-[name]-class attribute is present

Declarations are assumed to be present

When you access a class property in a controller, such as this.supportedClass, you assert that the corresponding data attribute is present on the controller element. If the declaration is missing, Stimulus throws a descriptive error:

If a class is optional, you must first use the existential property (e.g. this.hasSupportedClass) to determine whether its declaration is present.

---

Unifying target attributes

We've made a change to the target attribute syntax to align them with values and classes, and also to make the controller identifier more prominent by moving it into the attribute name.

The original syntax is:

<div data-target="[identifier].[name]">

and the updated syntax is:

<div data-[identifier]-target="[name]">

The original syntax is supported but deprecated

Stimulus 2.0 will support both syntaxes, but using the original syntax will display a deprecation message in the developer console. We intend to remove the original syntax in Stimulus 3.0.

---

Try it out in your application

Update the Stimulus entry in package.json to point to the latest development build:

"stimulus": "https://github.com/stimulusjs/dev-builds/archive/b8cc8c4/stimulus.tar.gz"

[jaredcwhite]

Wait a minute…why is CRUNCH in all caps. Could it be… a new code name?? 😆

Just teasing. Good luck on the impending launch!

[panda-madness]

It looks like this API is all but confirmed, if HEY source code is of any indication.

P.S. Gratz on the launch.

[leastbad]

@panda-madness This API was confirmed when the guy who makes the library made the PR. 🤷

If you felt there was any ambiguity on the subject, were you not convinced by repeated comments from Basecamp folks that this is coming?

I'm just very much hoping that there's still time and opportunity for default values to make it in before v2 officially launches.

[panda-madness]

@leastbad there is no need to be toxic about it.

Scrolling through this thread I cannot see a solid confirmation from any of the Basecamp guys, I asked a direct question some time ago that was left unanswered. Hence my perceived ambiguity about this PR's future.

[panda-madness]

@sstephenson In the current alpha build the lifecycle method invokation order seems to be initialize -> {valueChanged callbacks} -> connect. Is that intentional? Intuitively I would have thought that valueChanged callbacks would run after connect.

[msp-matt]

Is there any timeline on when this might get released?

[rupasix]

https://twitter.com/dhh/status/1291114354482524160
Maybe we're actually waiting for something different than this PR...

[ssaunier]

[nfilzi]