feat(PairsObservable): add PairsObservable creation method

closes ReactiveX#1804

spec/observables/pairs-spec.ts

@@ -0,0 +1,43 @@
+import {expect} from 'chai';
+import * as Rx from '../../dist/cjs/Rx';
+
+declare const {hot, asDiagram, expectObservable, expectSubscriptions};
+declare const rxTestScheduler: Rx.TestScheduler;
+const Observable = Rx.Observable;
+
+describe('Observable.pairs', () => {
+  asDiagram('pairs({a: 1, b:2})')('should create an observable emits key-value pair', () => {
+    const e1 = Observable.pairs({a: 1, b: 2}, rxTestScheduler);
+    const expected = '(ab|)';
+    const values = {
+      a: ['a', 1],
+      b: ['b', 2]
+    };
+
+    expectObservable(e1).toBe(expected, values);
+  });
+
+  it('should create an observable without scheduler', (done: MochaDone) => {
+    let expected = [
+      ['a', 1],
+      ['b', 2],
+      ['c', 3]
+    ];
+
+    Observable.pairs({a: 1, b: 2, c: 3}).subscribe(x => {
+      expect(x).to.deep.equal(expected.shift());
+    }, x => {
+      done(new Error('should not be called'));
+    }, () => {
+      expect(expected).to.be.empty;
+      done();
+    });
+  });
+
+  it('should work with empty object', () => {
+    const e1 = Observable.pairs({}, rxTestScheduler);
+    const expected = '|';
+
+    expectObservable(e1).toBe(expected);
+  });
+});

src/Rx.ts

@@ -27,6 +27,7 @@ import './add/observable/race';
 import './add/observable/never';
 import './add/observable/of';
 import './add/observable/onErrorResumeNext';
+import './add/observable/pairs';
 import './add/observable/range';
 import './add/observable/using';
 import './add/observable/throw';

src/add/observable/pairs.ts

@@ -0,0 +1,10 @@
+import {Observable} from '../../Observable';
+import {pairs as staticPairs} from '../../observable/pairs';
+
+Observable.pairs = staticPairs;
+
+declare module '../../Observable' {
+  namespace Observable {
+    export let pairs: typeof staticPairs;
+  }
+}

src/observable/PairsObservable.ts

@@ -0,0 +1,74 @@
+import {Scheduler} from '../Scheduler';
+import {Observable} from '../Observable';
+import {Subscriber} from '../Subscriber';
+import {TeardownLogic} from '../Subscription';
+
+interface PairsContext<T> {
+  obj: Object;
+  keys: Array<string>;
+  length: number;
+  index: number;
+  subscriber: Subscriber<Array<string | T>>;
+}
+
+function dispatch<T>(state: PairsContext<T>) {
+  const {obj, keys, length, index, subscriber} = state;
+
+  if (index === length) {
+    subscriber.complete();
+    return;
+  }
+
+  const key = keys[index];
+  subscriber.next([key, obj[key]]);
+
+  state.index = index + 1;
+
+  (<any> this).schedule(state);
+}
+
+/**
+ * We need this JSDoc comment for affecting ESDoc.
+ * @extends {Ignored}
+ * @hide true
+ */
+export class PairsObservable<T> extends Observable<Array<string | T>> {
+  private keys: Array<string>;
+
+  /**
+   * Convert an object into an observable sequence of [key, value] pairs
+   * using an optional Scheduler to enumerate the object.
+   *
+   * @param {Object} obj The object to inspect and turn into an
+   * Observable sequence.
+   * @param {Scheduler} [scheduler] An optional Scheduler to run the
+   * enumeration of the input sequence on.
+   * @returns {(Observable<Array<string | T>>)} An observable sequence of
+   * [key, value] pairs from the object.
+   */
+  static create<T>(obj: Object, scheduler?: Scheduler): Observable<Array<string | T>> {
+    return new PairsObservable<T>(obj, scheduler);
+  }
+
+  constructor(private obj: Object, private scheduler?: Scheduler) {
+    super();
+    this.keys = Object.keys(obj);
+  }
+
+  protected _subscribe(subscriber: Subscriber<Array<string | T>>): TeardownLogic {
+    const {keys, scheduler} = this;
+    const length = keys.length;
+
+    if (scheduler) {
+      return scheduler.schedule(dispatch, 0, {
+        obj: this.obj, keys, length, index: 0, subscriber
+      });
+    } else {
+      for (let idx = 0; idx < length; idx++) {
+        const key = keys[idx];
+        subscriber.next([key, this.obj[key]]);
+      }
+      subscriber.complete();
+    }
+  }
+}

src/observable/pairs.ts

@@ -0,0 +1,3 @@
+import { PairsObservable } from './PairsObservable';
+
+export const pairs = PairsObservable.create;