feat(core): add observe util (#2808)

libs/core/src/utils/observe.spec.ts

@@ -0,0 +1,35 @@
+import {
+  cold,
+  hot,
+} from 'jasmine-marbles';
+
+import { observe } from './observe';
+
+describe('@daffodil/core | observe', () => {
+  describe('when the passed value is an observable', () => {
+    it('should return an equivalent observable', () => {
+      const val = 5;
+      expect(observe(hot('--a', { a: val }))).toBeObservable(cold('--a', { a: val }));
+    });
+  });
+
+  describe('when the passed value is a promise', () => {
+    it('should return an equivalent observable', (done) => {
+      const val = 5;
+      observe(Promise.resolve(val)).subscribe((res) => {
+        expect(res).toEqual(val);
+        done();
+      });
+    });
+  });
+
+  describe('when the passed value is neither a promise nor an observable', () => {
+    it('should return an observable that emits the passed value', (done) => {
+      const val = 5;
+      observe(val).subscribe((res) => {
+        expect(res).toEqual(val);
+        done();
+      });
+    });
+  });
+});

libs/core/src/utils/observe.ts

@@ -0,0 +1,15 @@
+import {
+  Observable,
+  isObservable,
+  from,
+} from 'rxjs';
+
+/**
+ * Converts a value to an observable.
+ * If the value is an observable, just returns that observable.
+ * If the value is a promise, converts it to an observable (see rxjs `from`).
+ * If the value is neither, just returns an observable that immediately emits the value.
+ */
+export function observe<T>(val: T | Promise<T> | Observable<T>): Observable<T> {
+  return isObservable(val) ? val : from(Promise.resolve(val));
+}

libs/core/src/utils/public_api.ts

@@ -13,3 +13,4 @@ export { shuffle } from './shuffle';
 export { unique } from './unique';
 export * from './long-arithmetic';
 export * from './identity';
+export * from './observe';