feat(defer): implement cancellable callback (#3916)

src/lib/utils/__tests__/defer.ts

@@ -51,4 +51,61 @@ describe('defer', () => {
 
     expect(fn).toHaveBeenCalledTimes(2);
   });
+
+  it('cancels the deferred function', async () => {
+    const fn = jest.fn();
+    const deferred = defer(fn);
+
+    deferred();
+
+    expect(fn).toHaveBeenCalledTimes(0);
+
+    deferred.cancel();
+
+    expect(fn).toHaveBeenCalledTimes(0);
+
+    await Promise.resolve();
+
+    expect(fn).toHaveBeenCalledTimes(0);
+  });
+
+  it('cancels only the current deferred function', async () => {
+    const fn = jest.fn();
+    const deferred = defer(fn);
+
+    deferred();
+
+    expect(fn).toHaveBeenCalledTimes(0);
+
+    deferred.cancel();
+
+    expect(fn).toHaveBeenCalledTimes(0);
+
+    await Promise.resolve();
+
+    expect(fn).toHaveBeenCalledTimes(0);
+
+    deferred();
+
+    await Promise.resolve();
+
+    expect(fn).toHaveBeenCalledTimes(1);
+  });
+
+  it('cancels only the running deferred function', async () => {
+    const fn = jest.fn();
+    const deferred = defer(fn);
+
+    deferred.cancel();
+
+    expect(fn).toHaveBeenCalledTimes(0);
+
+    deferred();
+
+    expect(fn).toHaveBeenCalledTimes(0);
+
+    await Promise.resolve();
+
+    expect(fn).toHaveBeenCalledTimes(1);
+  });
 });

src/lib/utils/defer.ts

@@ -1,19 +1,38 @@
 const nextMicroTask = Promise.resolve();
 
 type Callback = (...args: any[]) => void;
+type Cancellable = Callback & {
+  cancel(): void;
+};
 
-const defer = (callback: Callback): Callback => {
+const defer = (callback: Callback): Cancellable => {
   let progress: Promise<void> | null = null;
-  return (...args) => {
+  let cancelled = false;
+
+  const fn: Cancellable = (...args) => {
     if (progress !== null) {
       return;
     }
 
     progress = nextMicroTask.then(() => {
+      if (cancelled) {
+        progress = null;
+        cancelled = false;
+        return;
+      }
+
       callback(...args);
       progress = null;
     });
   };
+
+  fn.cancel = () => {
+    if (progress !== null) {
+      cancelled = true;
+    }
+  };
+
+  return fn;
 };
 
 export default defer;