feat(if): add static Observable.if creation operator.

package.json

@@ -73,7 +73,7 @@
     "commit": "git-cz",
     "compile_dist_amd": "tsc ./typings/main.d.ts ./dist/amd/src/Rx.ts ./dist/amd/src/Rx.KitchenSink.ts ./dist/amd/src/Rx.DOM.ts ./dist/amd/src/add/observable/of.ts -m amd      --sourceMap --outDir ./dist/amd --target ES5    --diagnostics --pretty --noImplicitAny --suppressImplicitAnyIndexErrors",
     "compile_dist_cjs": "tsc ./typings/main.d.ts ./dist/cjs/src/Rx.ts ./dist/cjs/src/Rx.KitchenSink.ts ./dist/cjs/src/Rx.DOM.ts ./dist/cjs/src/add/observable/of.ts -m commonjs --sourceMap --outDir ./dist/cjs --target ES5 -d --diagnostics --pretty --noImplicitAny --suppressImplicitAnyIndexErrors",
-    "compile_dist_es6": "tsc                                  ./dist/es6/src/Rx.ts ./dist/es6/src/Rx.KitchenSink.ts ./dist/es6/src/Rx.DOM.ts ./dist/es6/src/add/observable/of.ts -m es2015   --sourceMap --outDir ./dist/es6 --target ES6 -d --diagnostics --pretty --noImplicitAny --suppressImplicitAnyIndexErrors",
+    "compile_dist_es6": "tsc                     ./dist/es6/src/Rx.ts ./dist/es6/src/Rx.KitchenSink.ts ./dist/es6/src/Rx.DOM.ts ./dist/es6/src/add/observable/of.ts -m es2015   --sourceMap --outDir ./dist/es6 --target ES6 -d --diagnostics --pretty --noImplicitAny --suppressImplicitAnyIndexErrors",
     "cover": "npm run cover_test && npm run cover_remapping",
     "cover_test": "istanbul cover -x \"*-spec.js index.js *-helper.js spec-js/helpers/*\" ./node_modules/jasmine/bin/jasmine.js",
     "cover_remapping": "remap-istanbul -i coverage/coverage.json -o coverage/coverage-remapped.json && remap-istanbul -i coverage/coverage.json -o coverage/coverage-remapped.lcov -t lcovonly && remap-istanbul -i coverage/coverage.json -o coverage/coverage-remapped -t html",

spec/observables/if-spec.ts

@@ -0,0 +1,28 @@
+import * as Rx from '../../dist/cjs/Rx.KitchenSink';
+import {expectObservable} from '../helpers/marble-testing';
+import {it} from '../helpers/test-helper';
+
+const Observable = Rx.Observable;
+
+describe('Observable.if', () => {
+  it('should subscribe to thenSource when the conditional returns true', () => {
+    const e1 = Observable.if(() => true, Observable.of('a'));
+    const expected = '(a|)';
+
+    expectObservable(e1).toBe(expected);
+  });
+
+  it('should subscribe to elseSource when the conditional returns false', () => {
+    const e1 = Observable.if(() => false, Observable.of('a'), Observable.of('b'));
+    const expected = '(b|)';
+
+    expectObservable(e1).toBe(expected);
+  });
+
+  it('should complete without an elseSource when the conditional returns false', () => {
+    const e1 = Observable.if(() => false, Observable.of('a'));
+    const expected = '|';
+
+    expectObservable(e1).toBe(expected);
+  });
+});

src/Observable.ts

@@ -6,6 +6,7 @@ import {root} from './util/root';
 import {SymbolShim} from './util/SymbolShim';
 import {toSubscriber} from './util/toSubscriber';
 
+import {IfObservable} from './observable/IfObservable';
 import {ErrorObservable} from './observable/ErrorObservable';
 
 export type ObservableOrPromise<T> = Observable<T> | Promise<T>;
@@ -150,7 +151,8 @@ export class Observable<T> {
     return this.source.subscribe(subscriber);
   }
 
-  // Throw is the special snow flake, the compiler sees it as a reserved word
+  // `if` and `throw` are special snow flakes, the compiler sees them as reserved words
+  static if: typeof IfObservable.create;
   static throw: typeof ErrorObservable.create;
 
   /**
@@ -161,4 +163,4 @@ export class Observable<T> {
   [SymbolShim.observable]() {
     return this;
   }
-}
+}

src/Rx.KitchenSink.ts

@@ -1,6 +1,7 @@
 export * from './Rx';
 
 // statics
+import './add/observable/if';
 import './add/observable/using';
 
 // Operators

src/add/observable/if.ts

@@ -0,0 +1,4 @@
+import {Observable} from '../../Observable';
+import {IfObservable} from '../../observable/IfObservable';
+
+Observable.if = IfObservable.create;

src/observable/IfObservable.ts

@@ -0,0 +1,42 @@
+import {Observable} from '../Observable';
+import {Subscriber} from '../Subscriber';
+import {Subscription} from '../Subscription';
+
+export class IfObservable<T, R> extends Observable<T> {
+
+  static create<T, R>(condition: () => boolean,
+                      thenSource?: Observable<T>,
+                      elseSource?: Observable<R>): Observable<T|R> {
+    return new IfObservable(condition, thenSource, elseSource);
+  }
+
+  constructor(private condition: () => boolean,
+              private thenSource?: Observable<T>,
+              private elseSource?: Observable<R>) {
+    super();
+  }
+
+  protected _subscribe(subscriber: Subscriber<T|R>): Subscription | Function | void {
+
+    const { condition, thenSource, elseSource } = this;
+
+    let result: boolean, error: any, errorHappened = false;
+
+    try {
+      result = condition();
+    } catch (e) {
+      error = e;
+      errorHappened = true;
+    }
+
+    if (errorHappened) {
+      subscriber.error(error);
+    } else if (result && thenSource) {
+      return thenSource.subscribe(subscriber);
+    } else if (elseSource) {
+      return elseSource.subscribe(subscriber);
+    } else {
+      subscriber.complete();
+    }
+  }
+}