Populate ImageBlock alt text from default_alt_text when an image is chosen

Fixes #12660

client/src/components/ChooserWidget/index.js

@@ -1,11 +1,13 @@
+import EventEmitter from 'events';
 import { ChooserModal } from '../../includes/chooserModal';
 
-export class Chooser {
+export class Chooser extends EventEmitter {
   chooserModalClass = ChooserModal;
   titleStateKey = 'title'; // key used in the 'state' dictionary to hold the human-readable title
   editUrlStateKey = 'edit_url'; // key used in the 'state' dictionary to hold the URL of the edit page
 
   constructor(id, opts = {}) {
+    super();
     this.opts = opts;
     this.initHTMLElements(id);
     this.state = this.getStateFromHTML();
@@ -83,6 +85,7 @@ export class Chooser {
 
   setStateFromModalData(data) {
     this.setState(data);
+    this.emit('chosen', data);
   }
 
   clear() {

client/src/entrypoints/images/image-block.js

@@ -15,6 +15,11 @@ class ImageBlockDefinition extends window.wagtailStreamField.blocks
     updateStateInput();
     isDecorativeField.addEventListener('change', updateStateInput);
 
+    const imageChooserWidget = block.childBlocks.image.widget;
+    imageChooserWidget.on('chosen', (data) => {
+      altTextField.value = data.default_alt_text;
+    });
+
     return block;
   }
 }