Fire callbacks when targets are added or removed

Closes [hotwired#336][]

---

Implements the `TargetObserver` to monitor when elements declaring
`[data-${identifier}-target]` are added or removed from a `Scope`.

In support of iterating through target tokens, export the
`TokenListObserver` module's `parseTokenString` function.

[hotwired#336]: https://3.basecamp.com/2914079/buckets/20224425/todos/3391985862

packages/@stimulus/core/src/context.ts

@@ -7,20 +7,23 @@ import { Module } from "./module"
 import { Schema } from "./schema"
 import { Scope } from "./scope"
 import { ValueObserver } from "./value_observer"
+import { TargetObserver } from "./target_observer"
 
 export class Context implements ErrorHandler {
   readonly module: Module
   readonly scope: Scope
   readonly controller: Controller
   private bindingObserver: BindingObserver
   private valueObserver: ValueObserver
+  private targetObserver: TargetObserver
 
   constructor(module: Module, scope: Scope) {
     this.module = module
     this.scope = scope
     this.controller = new module.controllerConstructor(this)
     this.bindingObserver = new BindingObserver(this, this.dispatcher)
     this.valueObserver = new ValueObserver(this, this.controller)
+    this.targetObserver = new TargetObserver(this, this.controller)
 
     try {
       this.controller.initialize()
@@ -32,6 +35,7 @@ export class Context implements ErrorHandler {
   connect() {
     this.bindingObserver.start()
     this.valueObserver.start()
+    this.targetObserver.start()
 
     try {
       this.controller.connect()
@@ -47,6 +51,7 @@ export class Context implements ErrorHandler {
       this.handleError(error, "disconnecting controller")
     }
 
+    this.targetObserver.stop()
     this.valueObserver.stop()
     this.bindingObserver.stop()
   }

packages/@stimulus/core/src/target_observer.ts

@@ -0,0 +1,73 @@
+import { Context } from "./context"
+import { ElementObserver, ElementObserverDelegate, parseTokenString } from "@stimulus/mutation-observers"
+
+export class TargetObserver implements ElementObserverDelegate {
+  readonly context: Context
+  readonly receiver: any
+  private elementObserver: ElementObserver
+
+  constructor(context: Context, receiver: any) {
+    this.context = context
+    this.receiver = receiver
+    this.elementObserver = new ElementObserver(this.element, this)
+  }
+
+  start() {
+    this.elementObserver.start()
+  }
+
+  stop() {
+    this.elementObserver.stop()
+  }
+
+  matchElement(element: Element): boolean {
+    return element.matches(this.selector)
+  }
+
+  matchElementsInTree(tree: Element): Element[] {
+    const match = this.matchElement(tree) ? [tree] : []
+    const matches = Array.from(tree.querySelectorAll(this.selector))
+    return match.concat(matches)
+  }
+
+  elementMatched(element: Element): void {
+    const value = element.getAttribute(this.attributeName) || ""
+    const tokens = parseTokenString(value, element, this.attributeName)
+
+    tokens.forEach((token) => this.dispatchCallback(`${token.content}TargetAdded`, element))
+  }
+
+  elementUnmatched(element: Element): void {
+    const value = element.getAttribute(this.attributeName) || ""
+    const tokens = parseTokenString(value, element, this.attributeName)
+
+    tokens.forEach((token) => this.dispatchCallback(`${token.content}TargetRemoved`, element))
+  }
+
+  private dispatchCallback(method: string, element: Element) {
+    const callback = (this.controller as any)[method]
+    if (typeof callback == "function") {
+      callback.call(this.controller, element)
+    }
+  }
+
+  private get selector() {
+    return `[${this.attributeName}]`
+  }
+
+  private get attributeName() {
+    return `data-${this.identifier}-target`
+  }
+
+  private get controller() {
+    return this.context.controller
+  }
+
+  private get identifier() {
+    return this.controller.identifier
+  }
+
+  private get element() {
+    return this.context.element
+  }
+}

packages/@stimulus/core/src/tests/controllers/target_controller.ts

@@ -18,4 +18,12 @@ export class TargetController extends BaseTargetController {
   inputTarget!: Element | null
   inputTargets!: Element[]
   hasInputTarget!: boolean
+
+  inputTargetAdded(element: Element) {
+    element.classList.add("added")
+  }
+
+  inputTargetRemoved(element: Element) {
+    element.classList.add("removed")
+  }
 }

packages/@stimulus/core/src/tests/modules/target_tests.ts

@@ -62,4 +62,20 @@ export default class TargetTests extends ControllerTestCase(TargetController) {
     this.assert.equal(this.controller.betaTargets.length, 0)
     this.assert.throws(() => this.controller.betaTarget)
   }
+
+  "test target added callback"() {
+    this.controller.element.insertAdjacentHTML("beforeend", `<input id="added-input" data-${this.controller.identifier}-target="input">`)
+
+    const addedInput = this.controller.element.querySelector("#input-added")
+
+    this.assert.ok(addedInput && addedInput.classList.contains("added"), "inputTargetAdded callback fired")
+  }
+
+  "test target removed callback"() {
+    const removedInput = this.findElement("#input1")
+
+    removedInput.remove()
+
+    this.assert.ok(removedInput && removedInput.classList.contains("removed"), "inputTargetRemoved callback fired")
+  }
 }

packages/@stimulus/mutation-observers/src/token_list_observer.ts

@@ -102,7 +102,7 @@ export class TokenListObserver implements AttributeObserverDelegate {
   }
 }
 
-function parseTokenString(tokenString: string, element: Element, attributeName: string): Token[] {
+export function parseTokenString(tokenString: string, element: Element, attributeName: string): Token[] {
   return tokenString.trim().split(/\s+/).filter(content => content.length)
     .map((content, index) => ({ element, attributeName, content, index }))
 }