es6/operators/distinctUntilKeyChanged.js

import { distinctUntilChanged } from './distinctUntilChanged';
/* 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(key, compare) {
    return distinctUntilChanged((x, y) => compare ? compare(x[key], y[key]) : x[key] === y[key]);
}
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