Support Constraint validation

Add support for integrating with [Constraint validation][] through the
support for the `[required]` attribute and the
`setCustomValidity(message)` method.

[Constraint validation]: https://developer.mozilla.org/en-US/docs/Web/HTML/Constraint_validation

README.md

@@ -148,6 +148,68 @@ To populate a `<trix-editor>` with stored content, include that content in the a
 
 Always use an associated input element to safely populate an editor. Trix won’t load any HTML content inside a `<trix-editor>…</trix-editor>` tag.
 
+## Validating the Editor
+
+Out of the box, `<trix-editor>` elements support browsers' built-in [Constraint
+validation][]. When rendered with the [required][] attribute, editors will be
+invalid when they're completely empty. For example, consider the following HTML:
+
+```html
+<input id="x" value="" type="hidden" name="content">
+<trix-editor input="x" required></trix-editor>
+```
+
+Since the `<trix-editor>` element is `[required]`, it is invalid when its value
+is empty:
+
+```js
+const editor = document.querySelector("trix-editor")
+
+editor.validity.valid        // => false
+editor.validity.valueMissing // => true
+editor.matches(":valid")     // => false
+editor.matches(":invalid")   // => true
+
+editor.value = "A value that isn't empty"
+
+editor.validity.valid         // => true
+editor.validity.valueMissing  // => false
+editor.matches(":valid")      // => true
+editor.matches(":invalid")    // => false
+```
+
+In addition to the built-in `[required]` attribute, `<trix-editor>`
+elements support custom validation through their [setCustomValidity][] method.
+For example, consider the following HTML:
+
+```js
+<input id="x" value="" type="hidden" name="content">
+<trix-editor input="x"></trix-editor>
+```
+
+Custom validation can occur at any time. For example, validation can occur after
+a `trix-change` event fired after the editor's contents change:
+
+```js
+addEventListener("trix-change", (event) => {
+  const editorElement = event.target
+  const trixDocument = editorElement.editor.getDocument()
+  const isValid = (trixDocument) => {
+    // determine the validity based on your custom criteria
+  }
+
+  if (isValid(trixDocument)) {
+    editorElement.setCustomValidity("The document is not valid.")
+  } else {
+    editorElement.setCustomValidity("")
+  }
+}
+```
+
+[Constraint validation]: https://developer.mozilla.org/en-US/docs/Web/HTML/Constraint_validation
+[required]: https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/required
+[setCustomValidity]: https://developer.mozilla.org/en-US/docs/Web/API/HTMLObjectElement/setCustomValidity
+
 ## Disabling the Editor
 
 To disable the `<trix-editor>`, render it with the `[disabled]` attribute:

assets/index.html

@@ -17,6 +17,10 @@
         max-width: 700px;
       }
 
+      trix-editor:invalid {
+        border: solid 1px red;
+      }
+
       #output {
         margin: 1rem 0 0;
       }

src/test/system/custom_element_test.js

@@ -630,4 +630,59 @@ testGroup("integrates with its <form>", { template: "editors_with_forms", contai
 
     assert.deepEqual({}, Object.fromEntries(new FormData(form).entries()), "does not write to FormData")
   })
+
+  test("validates with [required] attribute as invalid", () => {
+    const editor = document.getElementById("editor-with-ancestor-form")
+    const form = editor.form
+    let invalidEvent, submitEvent = null
+
+    editor.addEventListener("invalid", event => invalidEvent = event, { once: true })
+    form.addEventListener("submit", event => submitEvent = event, { once: true })
+
+    editor.required = true
+    form.requestSubmit()
+
+    // assert.equal(document.activeElement, editor, "editor receives focus")
+    assert.equal(editor.required, true, ".required property retrurns true")
+    assert.equal(editor.validity.valid, false, "validity.valid is false")
+    assert.equal(editor.validationMessage, "Please fill out this field.", "sets .validationMessage")
+    assert.equal(invalidEvent.target, editor, "dispatches 'invalid' event on editor")
+    assert.equal(submitEvent, null, "does not dispatch a 'submit' event")
+  })
+
+  test("does not validate with [disabled] attribute", () => {
+    const editor = document.getElementById("editor-with-ancestor-form")
+    let invalidEvent = null
+
+    editor.disabled = true
+    editor.required = true
+    editor.addEventListener("invalid", event => invalidEvent = event, { once: true })
+    editor.reportValidity()
+
+    assert.equal(invalidEvent, null, "does not dispatch an 'invalid' event")
+  })
+
+  test("re-validates when the value changes", async () => {
+    const editor = document.getElementById("editor-with-ancestor-form")
+    editor.required = true
+    editor.focus()
+
+    assert.equal(editor.validity.valid, false, "validity.valid is initially false")
+
+    await typeCharacters("a")
+
+    assert.equal(editor.validity.valid, true, "validity.valid is true after re-validating")
+    assert.equal(editor.validity.valueMissing, false, "validity.valueMissing is false")
+    assert.equal(editor.validationMessage, "", "clears the validationMessage")
+  })
+
+  test("accepts a customError validation message", () => {
+    const editor = document.getElementById("editor-with-ancestor-form")
+
+    editor.setCustomValidity("A custom validation message")
+
+    assert.equal(editor.validity.valid, false)
+    assert.equal(editor.validity.customError, true)
+    assert.equal(editor.validationMessage, "A custom validation message")
+  })
 })

src/trix/elements/trix_editor_element.js

@@ -162,6 +162,7 @@ installDefaultCSSForTagName("trix-editor", `\
 export default class TrixEditorElement extends HTMLElement {
   static formAssociated = true
 
+  #customValidationMessage
   #internals
 
   constructor() {
@@ -257,6 +258,8 @@ export default class TrixEditorElement extends HTMLElement {
     if (this.inputElement) {
       this.inputElement.value = value
     }
+
+    this.#synchronizeValidation()
   }
 
   // Element lifecycle
@@ -276,6 +279,7 @@ export default class TrixEditorElement extends HTMLElement {
         requestAnimationFrame(() => triggerEvent("trix-initialize", { onElement: this }))
       }
       this.editorController.registerSelectionManager()
+      this.#synchronizeValidation()
       autofocus(this)
     }
   }
@@ -284,6 +288,43 @@ export default class TrixEditorElement extends HTMLElement {
     this.editorController?.unregisterSelectionManager()
   }
 
+  // Constraint validation
+
+  set required(value) {
+    this.toggleAttribute("required", value)
+    this.#synchronizeValidation()
+  }
+
+  get required() {
+    return this.hasAttribute("required")
+  }
+
+  get validity() {
+    return this.#internals.validity
+  }
+
+  get validationMessage() {
+    return this.#internals.validationMessage
+  }
+
+  get willValidate() {
+    return this.#internals.willValidate
+  }
+
+  checkValidity() {
+    return this.#internals.checkValidity()
+  }
+
+  reportValidity() {
+    return this.#internals.reportValidity()
+  }
+
+  setCustomValidity(customValidationMessage) {
+    this.#customValidationMessage = customValidationMessage
+
+    this.#synchronizeValidation()
+  }
+
   // Form support
 
   formDisabledCallback(disabled) {
@@ -298,4 +339,14 @@ export default class TrixEditorElement extends HTMLElement {
   reset() {
     this.value = this.defaultValue
   }
+
+  #synchronizeValidation() {
+    const { required, value } = this
+    const valueMissing = required && !value
+    const customError = !!this.#customValidationMessage
+    const input = Object.assign(document.createElement("input"), { required })
+    const validationMessage = this.#customValidationMessage || input.validationMessage
+
+    this.#internals.setValidity({ valueMissing, customError }, validationMessage)
+  }
 }