Add a method for bridging Multicaster with promises

Preparation for using promises internally in the codebase.

Part of #1535; will remove the functional interface to Multicaster
if/when it’s no longer needed.

src/common/lib/util/multicaster.ts

@@ -5,6 +5,18 @@ import Logger from './logger';
 export interface MulticasterInstance<T> extends Function {
   (err?: ErrorInfo | null, result?: T): void;
   push: (fn: StandardCallback<T>) => void;
+  /**
+   * Creates a promise that will be resolved or rejected when this instance is called.
+   */
+  createPromise: () => Promise<T>;
+  /**
+   * Syntatic sugar for when working in a context that uses promises; equivalent to calling as a function with arguments (null, result).
+   */
+  resolveAll(result: T): void;
+  /**
+   * Syntatic sugar for when working in a context that uses promises; equivalent to calling as a function with arguments (err).
+   */
+  rejectAll(err: ErrorInfo): void;
 }
 
 class Multicaster<T> {
@@ -35,10 +47,29 @@ class Multicaster<T> {
     this.members.push(...args);
   }
 
+  createPromise(): Promise<T> {
+    return new Promise((resolve, reject) => {
+      this.push((err, result) => {
+        err ? reject(err) : resolve(result!);
+      });
+    });
+  }
+
+  resolveAll(result: T) {
+    this.call(null, result);
+  }
+
+  rejectAll(err: ErrorInfo) {
+    this.call(err);
+  }
+
   static create<T>(members?: Array<StandardCallback<T> | undefined>): MulticasterInstance<T> {
     const instance = new Multicaster(members);
     return Object.assign((err?: ErrorInfo | null, result?: T) => instance.call(err, result), {
       push: (fn: StandardCallback<T>) => instance.push(fn),
+      createPromise: () => instance.createPromise(),
+      resolveAll: (result: T) => instance.resolveAll(result),
+      rejectAll: (err: ErrorInfo) => instance.rejectAll(err),
     });
   }
 }