feat: add rxwait custom async policy

src/index.ts

@@ -1 +1,2 @@
 export * from './ui-router-rx';
+export * from './rx-async-policy';

src/rx-async-policy.ts

@@ -0,0 +1,38 @@
+import { CustomAsyncPolicy } from '@uirouter/core';
+import { Observable, of } from 'rxjs';
+import { first, shareReplay } from 'rxjs/operators';
+
+/**
+ * Determines the unwrapping behavior of asynchronous resolve values.
+ *
+ *   - When an Observable is returned from the resolveFn, wait until the Observable emits at least one item.
+ *     If any other value will be converted to an Observable that emits such value.
+ *   - The Observable item will not be unwrapped.
+ *   - The Observable stream itself will be provided when the resolve is injected or bound elsewhere.
+ *
+ * #### Example:
+ *
+ * The `Transition` will wait for the `main.home` resolve observables to emit their first value.
+ * Promises will be unwrapped and returned as observables before being provided to components.
+ * ```js
+ * var mainState = {
+ *   name: 'main',
+ *   resolve: mainResolves, // defined elsewhere
+ *   resolvePolicy: { async: RXWAIT },
+ * }
+ * ```
+ */
+export const RXWAIT: CustomAsyncPolicy = (resolveFnValue: Observable<any> | any): Promise<Observable<any>> => {
+  if (!(resolveFnValue instanceof Observable)) {
+    resolveFnValue = of(resolveFnValue);
+  }
+
+  const data$: Observable<any> = resolveFnValue.pipe(shareReplay(1));
+
+  return data$
+    .pipe(first())
+    .toPromise()
+    .then(() => {
+      return data$;
+    });
+};