From d06a3e781e11f5efb8d17f8c9e6fc61f8fc4e597 Mon Sep 17 00:00:00 2001
From: Elizabeth Mitchell <lizmitchell@google.com>
Date: Tue, 19 Dec 2023 16:27:52 -0800
Subject: [PATCH] chore: add event hooks for default prevention behavior

PiperOrigin-RevId: 592375327
---
 internal/events/dispatch-hooks.ts      | 176 ++++++++++++++++++++++++
 internal/events/dispatch-hooks_test.ts | 180 +++++++++++++++++++++++++
 2 files changed, 356 insertions(+)
 create mode 100644 internal/events/dispatch-hooks.ts
 create mode 100644 internal/events/dispatch-hooks_test.ts

diff --git a/internal/events/dispatch-hooks.ts b/internal/events/dispatch-hooks.ts
new file mode 100644
index 000000000..e67f491ff
--- /dev/null
+++ b/internal/events/dispatch-hooks.ts
@@ -0,0 +1,176 @@
+/**
+ * @license
+ * Copyright 2023 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * A symbol used to access dispatch hooks on an event.
+ */
+const dispatchHooks = Symbol('dispatchHooks');
+
+/**
+ * An `Event` with additional symbols for dispatch hooks.
+ */
+interface EventWithDispatchHooks extends Event {
+  [dispatchHooks]: EventTarget;
+}
+
+/**
+ * Add a hook for an event that is called after the event is dispatched and
+ * propagates to other event listeners.
+ *
+ * This is useful for behaviors that need to check if an event is canceled.
+ *
+ * The callback is invoked synchronously, which allows for better integration
+ * with synchronous platform APIs (like `<form>` or `<label>` clicking).
+ *
+ * Note: `setupDispatchHooks()` must be called on the element before adding any
+ * other event listeners. Call it in the constructor of an element or
+ * controller.
+ *
+ * @example
+ * ```ts
+ * class MyControl extends LitElement {
+ *   constructor() {
+ *     super();
+ *     setupDispatchHooks(this, 'click');
+ *     this.addEventListener('click', event => {
+ *       afterDispatch(event, () => {
+ *         if (event.defaultPrevented) {
+ *           return
+ *         }
+ *
+ *         // ... perform logic
+ *       });
+ *     });
+ *   }
+ * }
+ * ```
+ *
+ * @example
+ * ```ts
+ * class MyController implements ReactiveController {
+ *   constructor(host: ReactiveElement) {
+ *     // setupDispatchHooks() may be called multiple times for the same
+ *     // element and events, making it safe for multiple controllers to use it.
+ *     setupDispatchHooks(host, 'click');
+ *     host.addEventListener('click', event => {
+ *       afterDispatch(event, () => {
+ *         if (event.defaultPrevented) {
+ *           return;
+ *         }
+ *
+ *         // ... perform logic
+ *       });
+ *     });
+ *   }
+ * }
+ * ```
+ *
+ * @param event The event to add a hook to.
+ * @param callback A hook that is called after the event finishes dispatching.
+ */
+export function afterDispatch(event: Event, callback: () => void) {
+  const hooks = (event as EventWithDispatchHooks)[dispatchHooks];
+  if (!hooks) {
+    throw new Error(`'${event.type}' event needs setupDispatchHooks().`);
+  }
+
+  hooks.addEventListener('after', callback);
+}
+
+/**
+ * A lookup map of elements and event types that have a dispatch hook listener
+ * set up. Used to ensure we don't set up multiple hook listeners on the same
+ * element for the same event.
+ */
+const ELEMENT_DISPATCH_HOOK_TYPES = new WeakMap<Element, Set<string>>();
+
+/**
+ * Sets up an element to add dispatch hooks to given event types. This must be
+ * called before adding any event listeners that need to use dispatch hooks
+ * like `afterDispatch()`.
+ *
+ * This function is safe to call multiple times with the same element or event
+ * types. Call it in the constructor of elements, mixins, and controllers to
+ * ensure it is set up before external listeners.
+ *
+ * @example
+ * ```ts
+ * class MyControl extends LitElement {
+ *   constructor() {
+ *     super();
+ *     setupDispatchHooks(this, 'click');
+ *     this.addEventListener('click', this.listenerUsingAfterDispatch);
+ *   }
+ * }
+ * ```
+ *
+ * @param element The element to set up event dispatch hooks for.
+ * @param eventTypes The event types to add dispatch hooks to.
+ */
+export function setupDispatchHooks(
+  element: Element,
+  ...eventTypes: [string, ...string[]]
+) {
+  let typesAlreadySetUp = ELEMENT_DISPATCH_HOOK_TYPES.get(element);
+  if (!typesAlreadySetUp) {
+    typesAlreadySetUp = new Set();
+    ELEMENT_DISPATCH_HOOK_TYPES.set(element, typesAlreadySetUp);
+  }
+
+  for (const eventType of eventTypes) {
+    // Don't register multiple dispatch hook listeners. A second registration
+    // would lead to the second listener re-dispatching a re-dispatched event,
+    // which can cause an infinite loop inside the other one.
+    if (typesAlreadySetUp.has(eventType)) {
+      continue;
+    }
+
+    // When we re-dispatch the event, it's going to immediately trigger this
+    // listener again. Use a flag to ignore it.
+    let isRedispatching = false;
+    element.addEventListener(
+      eventType,
+      (event: Event) => {
+        if (isRedispatching) {
+          return;
+        }
+
+        // Do not let the event propagate to any other listener (not just
+        // bubbling listeners with `stopPropagation()`).
+        event.stopImmediatePropagation();
+        // Make a copy.
+        const eventCopy = Reflect.construct(event.constructor, [
+          event.type,
+          event,
+        ]);
+
+        // Add hooks onto the event.
+        const hooks = new EventTarget();
+        (eventCopy as EventWithDispatchHooks)[dispatchHooks] = hooks;
+
+        // Re-dispatch the event. We can't reuse `redispatchEvent()` since we
+        // need to add the hooks to the copy before it's dispatched.
+        isRedispatching = true;
+        const dispatched = element.dispatchEvent(eventCopy);
+        isRedispatching = false;
+        if (!dispatched) {
+          event.preventDefault();
+        }
+
+        // Synchronously call afterDispatch() hooks.
+        hooks.dispatchEvent(new Event('after'));
+      },
+      {
+        // Ensure this listener runs before other listeners.
+        // `setupDispatchHooks()` should be called in constructors to also
+        // ensure they run before any other externally-added capture listeners.
+        capture: true,
+      },
+    );
+
+    typesAlreadySetUp.add(eventType);
+  }
+}
diff --git a/internal/events/dispatch-hooks_test.ts b/internal/events/dispatch-hooks_test.ts
new file mode 100644
index 000000000..efe9c18d9
--- /dev/null
+++ b/internal/events/dispatch-hooks_test.ts
@@ -0,0 +1,180 @@
+/**
+ * @license
+ * Copyright 2023 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+// import 'jasmine'; (google3-only)
+
+import {afterDispatch, setupDispatchHooks} from './dispatch-hooks.js';
+
+describe('dispatch hooks', () => {
+  let element: HTMLDivElement;
+
+  beforeEach(() => {
+    element = document.createElement('div');
+    document.body.appendChild(element);
+  });
+
+  afterEach(() => {
+    document.body.removeChild(element);
+  });
+
+  describe('setupDispatchHooks()', () => {
+    it('does not add more than one setup listener for an event type', () => {
+      spyOn(element, 'addEventListener').and.callThrough();
+      setupDispatchHooks(element, 'foo');
+      setupDispatchHooks(element, 'foo');
+
+      expect(element.addEventListener)
+        .withContext('element.addEventListener')
+        .toHaveBeenCalledTimes(1);
+    });
+
+    it('can add setup listeners for multiple event types', () => {
+      spyOn(element, 'addEventListener').and.callThrough();
+
+      setupDispatchHooks(element, 'foo', 'bar', 'baz');
+      expect(element.addEventListener)
+        .withContext('element.addEventListener')
+        .toHaveBeenCalledTimes(3);
+    });
+  });
+
+  describe('afterDispatch()', () => {
+    it('resolves synchronously after the event is finished dispatching', () => {
+      setupDispatchHooks(element, 'click');
+
+      const afterDispatchCallback = jasmine.createSpy('afterDispatchCallback');
+      const clickListener = jasmine
+        .createSpy('clickListener')
+        .and.callFake((event: Event) => {
+          afterDispatch(event, afterDispatchCallback);
+        });
+
+      element.addEventListener('click', clickListener);
+      element.click();
+
+      expect(clickListener)
+        .withContext('clickListener')
+        .toHaveBeenCalledTimes(1);
+      expect(afterDispatchCallback)
+        .withContext('afterDispatch() callback')
+        .toHaveBeenCalledTimes(1);
+    });
+
+    it('supports multiple afterDispatch listeners', () => {
+      setupDispatchHooks(element, 'click');
+
+      const firstAfterDispatchCallback = jasmine.createSpy(
+        'firstAfterDispatchCallback',
+      );
+      element.addEventListener('click', (event) => {
+        afterDispatch(event, firstAfterDispatchCallback);
+      });
+
+      const secondAfterDispatchCallback = jasmine.createSpy(
+        'secondAfterDispatchCallback',
+      );
+      element.addEventListener('click', (event) => {
+        afterDispatch(event, secondAfterDispatchCallback);
+      });
+
+      element.click();
+
+      expect(firstAfterDispatchCallback)
+        .withContext('afterDispatch() first callback')
+        .toHaveBeenCalledTimes(1);
+      expect(secondAfterDispatchCallback)
+        .withContext('afterDispatch() second callback')
+        .toHaveBeenCalledTimes(1);
+    });
+
+    it('resolves synchronously after the event is finished dispatching', () => {
+      setupDispatchHooks(element, 'click');
+
+      const afterDispatchCallback = jasmine.createSpy('afterDispatchCallback');
+      const clickListener = jasmine
+        .createSpy('clickListener')
+        .and.callFake((event: Event) => {
+          afterDispatch(event, afterDispatchCallback);
+        });
+
+      element.addEventListener('click', clickListener);
+      element.click();
+
+      expect(clickListener)
+        .withContext('clickListener')
+        .toHaveBeenCalledTimes(1);
+      expect(afterDispatchCallback)
+        .withContext('afterDispatch() callback')
+        .toHaveBeenCalledTimes(1);
+    });
+
+    it('can be used to synchronously detect if event was canceled', () => {
+      setupDispatchHooks(element, 'click');
+
+      // element listener
+      let eventDefaultPreventedInAfterDispatch: boolean | null = null;
+      element.addEventListener('click', (event) => {
+        afterDispatch(event, () => {
+          eventDefaultPreventedInAfterDispatch = event.defaultPrevented;
+        });
+      });
+
+      // client listener
+      element.addEventListener('click', (event) => {
+        event.preventDefault();
+      });
+
+      element.click();
+
+      expect(eventDefaultPreventedInAfterDispatch)
+        .withContext('event.defaultPrevented() in afterDispatch() callback')
+        .toBeTrue();
+    });
+
+    it('throws if setupDispatchHooks() was not called for the event type', () => {
+      // Do not set up hooks
+      let errorThrown: unknown;
+      element.addEventListener('click', (event) => {
+        try {
+          afterDispatch(event, () => {});
+        } catch (error) {
+          errorThrown = error;
+        }
+      });
+
+      element.click();
+      expect(errorThrown)
+        .withContext('error thrown calling afterDispatch()')
+        .toBeInstanceOf(Error);
+
+      expect((errorThrown as Error).message)
+        .withContext('errorThrown.message')
+        .toMatch('setupDispatchHooks');
+    });
+
+    it('does not fire multiple times if setupDispatchHooks() is called multiple times for the same element', () => {
+      setupDispatchHooks(element, 'click');
+      setupDispatchHooks(element, 'click');
+
+      const afterDispatchCallback = jasmine.createSpy('afterDispatchCallback');
+      const clickListener = jasmine
+        .createSpy('clickListener')
+        .and.callFake((event: Event) => {
+          afterDispatch(event, afterDispatchCallback);
+        });
+
+      element.addEventListener('click', clickListener);
+      element.click();
+
+      expect(clickListener)
+        .withContext('clickListener')
+        .toHaveBeenCalledTimes(1);
+      expect(afterDispatchCallback)
+        .withContext('afterDispatch() callback')
+        .toHaveBeenCalledTimes(1);
+    });
+  });
+});