feat(distinctUntilKeyChanged): add higher-order lettable version of d…

…istinctUntilKeyChanged

src/operator/distinctUntilKeyChanged.ts

@@ -1,5 +1,6 @@
-import { distinctUntilChanged } from './distinctUntilChanged';
+
 import { Observable } from '../Observable';
+import { distinctUntilKeyChanged as higherOrder } from '../operators';
 
 /* tslint:disable:max-line-length */
 export function distinctUntilKeyChanged<T>(this: Observable<T>, key: string): Observable<T>;
@@ -64,10 +65,5 @@ export function distinctUntilKeyChanged<T, K>(this: Observable<T>, key: string,
  * @owner Observable
  */
 export function distinctUntilKeyChanged<T>(this: Observable<T>, key: string, compare?: (x: T, y: T) => boolean): Observable<T> {
-  return distinctUntilChanged.call(this, function(x: T, y: T) {
-    if (compare) {
-      return compare(x[key], y[key]);
-    }
-    return x[key] === y[key];
-  });
+  return higherOrder<T, T>(key, compare)(this);
 }

src/operators/distinctUntilKeyChanged.ts

@@ -0,0 +1,68 @@
+import { distinctUntilChanged } from './distinctUntilChanged';
+import { MonoTypeOperatorFunction } from '../interfaces';
+
+/* tslint:disable:max-line-length */
+export function distinctUntilKeyChanged<T>(key: string): MonoTypeOperatorFunction<T>;
+export function distinctUntilKeyChanged<T, K>(key: string, compare: (x: K, y: K) => boolean): MonoTypeOperatorFunction<T>;
+/* tslint:enable:max-line-length */
+
+/**
+ * Returns an Observable that emits all items emitted by the source Observable that are distinct by comparison from the previous item,
+ * using a property accessed by using the key provided to check if the two items are distinct.
+ *
+ * If a comparator function is provided, then it will be called for each item to test for whether or not that value should be emitted.
+ *
+ * If a comparator function is not provided, an equality check is used by default.
+ *
+ * @example <caption>An example comparing the name of persons</caption>
+ *
+ *  interface Person {
+ *     age: number,
+ *     name: string
+ *  }
+ *
+ * Observable.of<Person>(
+ *     { age: 4, name: 'Foo'},
+ *     { age: 7, name: 'Bar'},
+ *     { age: 5, name: 'Foo'},
+ *     { age: 6, name: 'Foo'})
+ *     .distinctUntilKeyChanged('name')
+ *     .subscribe(x => console.log(x));
+ *
+ * // displays:
+ * // { age: 4, name: 'Foo' }
+ * // { age: 7, name: 'Bar' }
+ * // { age: 5, name: 'Foo' }
+ *
+ * @example <caption>An example comparing the first letters of the name</caption>
+ *
+ * interface Person {
+ *     age: number,
+ *     name: string
+ *  }
+ *
+ * Observable.of<Person>(
+ *     { age: 4, name: 'Foo1'},
+ *     { age: 7, name: 'Bar'},
+ *     { age: 5, name: 'Foo2'},
+ *     { age: 6, name: 'Foo3'})
+ *     .distinctUntilKeyChanged('name', (x: string, y: string) => x.substring(0, 3) === y.substring(0, 3))
+ *     .subscribe(x => console.log(x));
+ *
+ * // displays:
+ * // { age: 4, name: 'Foo1' }
+ * // { age: 7, name: 'Bar' }
+ * // { age: 5, name: 'Foo2' }
+ *
+ * @see {@link distinct}
+ * @see {@link distinctUntilChanged}
+ *
+ * @param {string} key String key for object property lookup on each item.
+ * @param {function} [compare] Optional comparison function called to test if an item is distinct from the previous item in the source.
+ * @return {Observable} An Observable that emits items from the source Observable with distinct values based on the key specified.
+ * @method distinctUntilKeyChanged
+ * @owner Observable
+ */
+export function distinctUntilKeyChanged<T>(key: string, compare?: (x: T, y: T) => boolean): MonoTypeOperatorFunction<T> {
+  return distinctUntilChanged((x: T, y: T) => compare ? compare(x[key], y[key]) : x[key] === y[key]);
+}

src/operators/index.ts

@@ -18,6 +18,7 @@ export { delay } from './delay';
 export { delayWhen } from './delayWhen';
 export { dematerialize } from './dematerialize';
 export { distinctUntilChanged } from './distinctUntilChanged';
+export { distinctUntilKeyChanged } from './distinctUntilKeyChanged';
 export { filter } from './filter';
 export { ignoreElements } from './ignoreElements';
 export { map } from './map';