Support custom Action Options

As a follow-up to [hotwired#535][] and
[hotwired#546][], add support for declaring custom action
modifiers in the same style as `:prevent`, `:stop`, and `:self`.

Take, for example, the [toggle][] event. It's dispatched whenever a
`<details>` element toggles either open or closed. If an application
were able to declare a custom `open` modifier, it could choose to route
`toggle` events denoted with `:open` _only_ when the `<details open>`.
Inversely, they could choose to route `toggle` events denoted with
`:!open` _only_ when the `<details>` does not have `[open]`.

Similarly, the same kind of customization could apply to custom events.
For example, the [turbo:submit-end][turbo-events] fires after a `<form>`
element submits, but does not distinguish between success or failure. A
`:success` modifier could skip events with an unsuccessful HTTP response
code.

[hotwired#535]: hotwired#535
[hotwired#546]: hotwired#546
[toggle]: https://developer.mozilla.org/en-US/docs/Web/API/HTMLDetailsElement/toggle_event
[turbo-events]: https://turbo.hotwired.dev/reference/events

docs/reference/actions.md

@@ -109,6 +109,61 @@ Custom action option | Description
 `:prevent`           | calls `.preventDefault()` on the event before invoking the method
 `:self`              | only invokes the method if the event was fired by the element itself
 
+You can register your own action options with the `Application.registerActionOption` method.
+
+For example, consider that a `<details>` element will dispatch a [toggle][]
+event whenever it's toggled. A custom `:open` action option would help
+to route events whenever the element is toggled _open_:
+
+```javascript
+import { Application } from "@hotwired/stimulus"
+
+const application = Application.start()
+
+application.registerActionOption("open", ({ event }) => {
+  if (event.type == "toggle") {
+    return event.target.open == true
+  } else {
+    return true
+  }
+})
+```
+
+Similarly, a custom `:!open` action option could route events whenever the
+element is toggled _closed_. Declaring the action descriptor option with a `!`
+prefix will yield a `value` argument set to `false` in the callback:
+
+```javascript
+import { Application } from "@hotwired/stimulus"
+
+const application = Application.start()
+
+application.registerActionOption("open", ({ event, value }) => {
+  if (event.type == "toggle") {
+    return event.target.open == value
+  } else {
+    return true
+  }
+})
+```
+
+In order to prevent the event from being routed to the controller action, the
+`registerActionOption` callback function must return `false`. Otherwise, to
+route the event to the controller action, return `true`.
+
+The callback accepts a single object argument with the following keys:
+
+Name    | Description
+--------|------------
+name    | String: The option's name (`"open"` in the example above)
+value   | Boolean: The value of the option (`:open` would yield `true`, `:!open` would yield `false`)
+event   | [Event][]: The event instance
+element | [Element]: The element where the action descriptor is declared
+
+[toggle]: https://developer.mozilla.org/en-US/docs/Web/API/HTMLDetailsElement/toggle_event
+[Event]: https://developer.mozilla.org/en-US/docs/web/api/event
+[Element]: https://developer.mozilla.org/en-US/docs/Web/API/element
+
 ## Event Objects
 
 An _action method_ is the method in a controller which serves as an action's event listener.

src/core/action.ts

@@ -1,14 +1,12 @@
 import { ActionDescriptor, parseActionDescriptorString, stringifyEventTarget } from "./action_descriptor"
 import { Token } from "../mutation-observers"
 import { camelize } from "./string_helpers"
-import { EventModifiers } from "./event_modifiers"
-
 export class Action {
   readonly element: Element
   readonly index: number
   readonly eventTarget: EventTarget
   readonly eventName: string
-  readonly eventOptions: EventModifiers
+  readonly eventOptions: AddEventListenerOptions
   readonly identifier: string
   readonly methodName: string
 
@@ -78,4 +76,3 @@ function typecast(value: any): any {
     return value
   }
 }
-

src/core/action_descriptor.ts

@@ -1,13 +1,42 @@
-import { EventModifiers } from "./event_modifiers"
-
 export interface ActionDescriptor {
   eventTarget: EventTarget
-  eventOptions: EventModifiers
+  eventOptions: AddEventListenerOptions
   eventName: string
   identifier: string
   methodName: string
 }
 
+export type ActionDescriptorFilters = Record<string, ActionDescriptorFilter>
+export type ActionDescriptorFilter = (options: ActionDescriptorFilterOptions) => boolean
+type ActionDescriptorFilterOptions = {
+  name: string
+  value: boolean
+  event: Event
+  element: Element
+}
+
+export const defaultActionDescriptorFilters: ActionDescriptorFilters = {
+  stop: ({ event, value }: ActionDescriptorFilterOptions): boolean => {
+    if (value) event.stopPropagation()
+
+    return true
+  },
+
+  prevent: ({ event, value }: ActionDescriptorFilterOptions): boolean => {
+    if (value) event.preventDefault()
+
+    return true
+  },
+
+  self: ({ event, value, element }: ActionDescriptorFilterOptions): boolean => {
+    if (value) {
+      return element === event.target
+    } else {
+      return true
+    }
+  }
+}
+
 // capture nos.:            12   23 4               43   1 5   56 7      768 9  98
 const descriptorPattern = /^((.+?)(@(window|document))?->)?(.+?)(#([^:]+?))(:(.+))?$/
 
@@ -31,7 +60,7 @@ function parseEventTarget(eventTargetName: string): EventTarget | undefined {
   }
 }
 
-function parseEventOptions(eventOptions: string): EventModifiers {
+function parseEventOptions(eventOptions: string): AddEventListenerOptions {
   return eventOptions.split(":").reduce((options, token) =>
     Object.assign(options, { [token.replace(/^!/, "")]: !/^!/.test(token) })
   , {})

src/core/application.ts

@@ -5,12 +5,14 @@ import { ErrorHandler } from "./error_handler"
 import { Logger } from "./logger"
 import { Router } from "./router"
 import { Schema, defaultSchema } from "./schema"
+import { ActionDescriptorFilter, ActionDescriptorFilters, defaultActionDescriptorFilters } from "./action_descriptor"
 
 export class Application implements ErrorHandler {
   readonly element: Element
   readonly schema: Schema
   readonly dispatcher: Dispatcher
   readonly router: Router
+  readonly actionDescriptorFilters: ActionDescriptorFilters
   logger: Logger = console
   debug: boolean = false
 
@@ -25,6 +27,7 @@ export class Application implements ErrorHandler {
     this.schema = schema
     this.dispatcher = new Dispatcher(this)
     this.router = new Router(this)
+    this.actionDescriptorFilters = { ...defaultActionDescriptorFilters }
   }
 
   async start() {
@@ -46,6 +49,10 @@ export class Application implements ErrorHandler {
     this.load({ identifier, controllerConstructor })
   }
 
+  registerActionOption(name: string, filter: ActionDescriptorFilter) {
+    this.actionDescriptorFilters[name] = filter
+  }
+
   load(...definitions: Definition[]): void
   load(definitions: Definition[]): void
   load(head: Definition | Definition[], ...rest: Definition[]) {

src/core/binding.ts

@@ -3,8 +3,6 @@ import { ActionEvent } from "./action_event"
 import { Context } from "./context"
 import { Controller } from "./controller"
 import { Scope } from "./scope"
-import { EventModifiers } from "./event_modifiers"
-
 export class Binding {
   readonly context: Context
   readonly action: Action
@@ -22,7 +20,7 @@ export class Binding {
     return this.action.eventTarget
   }
 
-  get eventOptions(): EventModifiers {
+  get eventOptions(): AddEventListenerOptions {
     return this.action.eventOptions
   }
 
@@ -31,10 +29,7 @@ export class Binding {
   }
 
   handleEvent(event: Event) {
-    if (this.willBeInvokedByEvent(event) && this.shouldBeInvokedPerSelf(event)) {
-      this.processStopPropagation(event);
-      this.processPreventDefault(event);
-
+    if (this.willBeInvokedByEvent(event) && this.applyEventModifiers(event)) {
       this.invokeWithEvent(event)
     }
   }
@@ -51,16 +46,25 @@ export class Binding {
     throw new Error(`Action "${this.action}" references undefined method "${this.methodName}"`)
   }
 
-  private processStopPropagation(event: Event) {
-    if (this.eventOptions.stop) {
-      event.stopPropagation();
-    }
+  private get actionDescriptorFilters() {
+    return this.context.application.actionDescriptorFilters
   }
 
-  private processPreventDefault(event: Event) {
-    if (this.eventOptions.prevent) {
-      event.preventDefault();
+  private applyEventModifiers(event: Event): boolean {
+    const { element } = this.action
+    let passes = true
+
+    for (const [name, value] of Object.entries(this.eventOptions)) {
+      if (name in this.actionDescriptorFilters) {
+        const filter = this.actionDescriptorFilters[name]
+
+        passes = passes && filter({ name, value, event, element })
+      } else {
+        continue
+      }
     }
+
+    return passes
   }
 
   private invokeWithEvent(event: Event) {
@@ -77,14 +81,6 @@ export class Binding {
     }
   }
 
-  private shouldBeInvokedPerSelf(event: Event): boolean {
-    if (this.action.eventOptions.self === true) {
-      return this.action.element === event.target
-    } else {
-      return true
-    }
-  }
-
   private willBeInvokedByEvent(event: Event): boolean {
     const eventTarget = event.target
     if (this.element === eventTarget) {

src/tests/cases/application_test_case.ts

@@ -10,10 +10,11 @@ class TestApplication extends Application {
 
 export class ApplicationTestCase extends DOMTestCase {
   schema: Schema = defaultSchema
-  application: Application = new TestApplication(this.fixtureElement, this.schema)
+  application!: Application
 
   async runTest(testName: string) {
     try {
+      this.application = new TestApplication(this.fixtureElement, this.schema)
       this.setupApplication()
       this.application.start()
       await super.runTest(testName)

src/tests/modules/core/event_options_tests.ts

@@ -5,6 +5,7 @@ export default class EventOptionsTests extends LogControllerTestCase {
   fixtureHTML = `
     <div data-controller="c d">
       <button></button>
+      <details></details>
     </div>
     <div id="outside"></div>
   `
@@ -210,15 +211,58 @@ export default class EventOptionsTests extends LogControllerTestCase {
     this.assertNoActions()
   }
 
+  async "test custom option"() {
+    this.application.registerActionOption("open", ({ value, event: { type, target } }) => {
+      switch (type) {
+        case "toggle": return target instanceof HTMLDetailsElement && target.open == value
+        default: return true
+      }
+    })
+    this.setAction(this.detailsElement, "toggle->c#log:open")
+
+    await this.nextFrame
+    await this.toggleElement(this.detailsElement)
+    await this.toggleElement(this.detailsElement)
+    await this.toggleElement(this.detailsElement)
+
+    this.assertActions({ name: "log", eventType: "toggle" }, { name: "log", eventType: "toggle" })
+  }
+
+  async "test inverted custom option"() {
+    this.application.registerActionOption("open", ({ value, event: { type, target } }) => {
+      switch (type) {
+        case "toggle": return target instanceof HTMLDetailsElement && target.open == value
+        default: return true
+      }
+    })
+    this.setAction(this.detailsElement, "toggle->c#log:!open")
+
+    await this.nextFrame
+    await this.toggleElement(this.detailsElement)
+    await this.toggleElement(this.detailsElement)
+    await this.toggleElement(this.detailsElement)
+
+    this.assertActions({ name: "log", eventType: "toggle" })
+  }
+
   setAction(element: Element, value: string) {
     element.setAttribute("data-action", value)
   }
 
+  toggleElement(details: Element) {
+    details.toggleAttribute("open")
+    return this.nextFrame
+  }
+
   get element() {
     return this.findElement("div")
   }
 
   get buttonElement() {
     return this.findElement("button")
   }
+
+  get detailsElement() {
+    return this.findElement("details")
+  }
 }