ngRx 官方示例分析 - 6

@ngrx/effect

前面我们提到，在 Book 的 reducer 中，并没有 Search 这个 Action 的处理，由于它需要发出一个异步的请求，等到请求返回前端，我们需要根据返回的结果来操作 store。所以，真正操作 store 的应该是 Search_Complete 这个 Action。我们在 recducer 已经看到了。

对于 Search 来说，我们需要见到这个 Action 就发出一个异步的请求，等到异步处理完毕，根据返回的结果，构造一个 Search_Complete 来将处理的结果派发给 store 进行处理。

这个解耦是通过 @ngrx/effect 来处理的。

@ngrx/effect 提供了装饰器 @Effect 和 Actions 来帮助我们检查 store 派发出来的 Action，将特定类型的 Action 过滤出来进行处理。监听特定的 Action, 当发现特定的 Action 发出之后，自动执行某些操作，然后将处理的结果重新发送回 store 中。

Book 搜索处理

以 Book 为例，我们需要监控 Search 这个 Action, 见到这个 Action 就发出异步的请求，然后接收请求返回的数据，如果在接收完成之前，又遇到了下一个请求，那么，直接结束上一个请求的返回数据。如何找到下一个请求呢？使用 skip 来获取下一个 Search Action。

源码

/src/effects/book.ts

import 'rxjs/add/operator/catch';

import 'rxjs/add/operator/map';

import 'rxjs/add/operator/switchMap';

import 'rxjs/add/operator/debounceTime';

import 'rxjs/add/operator/skip';

import 'rxjs/add/operator/takeUntil';

import { Injectable } from '@angular/core';

import { Effect, Actions, toPayload } from '@ngrx/effects';

import { Action } from '@ngrx/store';

import { Observable } from 'rxjs/Observable';

import { empty } from 'rxjs/observable/empty';

import { of } from 'rxjs/observable/of';

import { GoogleBooksService } from '../services/google-books';

import * as book from '../actions/book';

/**

 * Effects offer a way to isolate and easily test side-effects within your

 * application.

 * The `toPayload` helper function returns just

 * the payload of the currently dispatched action, useful in

 * instances where the current state is not necessary.

 *

 * Documentation on `toPayload` can be found here:

 * https://github.com/ngrx/effects/blob/master/docs/api.md#topayload

 *

 * If you are unfamiliar with the operators being used in these examples, please

 * check out the sources below:

 *

 * Official Docs: http://reactivex.io/rxjs/manual/overview.html#categories-of-operators

 * RxJS 5 Operators By Example: https://gist.github.com/btroncone/d6cf141d6f2c00dc6b35

 */

@Injectable()

export class BookEffects {

  @Effect()

  search$: Observable<Action> = this.actions$

    .ofType(book.ActionTypes.SEARCH)

    .debounceTime(300)

    .map(toPayload)

    .switchMap(query => {

      if (query === '') {

        return empty();

      }

      const nextSearch$ = this.actions$.ofType(book.ActionTypes.SEARCH).skip(1);

      return this.googleBooks.searchBooks(query)

        .takeUntil(nextSearch$)

        .map(books => new book.SearchCompleteAction(books))

        .catch(() => of(new book.SearchCompleteAction([])));

    });

    constructor(private actions$: Actions, private googleBooks: GoogleBooksService) { }

}

collection.ts

import 'rxjs/add/operator/map';

import 'rxjs/add/operator/catch';

import 'rxjs/add/operator/startWith';

import 'rxjs/add/operator/switchMap';

import 'rxjs/add/operator/mergeMap';

import 'rxjs/add/operator/toArray';

import { Injectable } from '@angular/core';

import { Action } from '@ngrx/store';

import { Effect, Actions } from '@ngrx/effects';

import { Database } from '@ngrx/db';

import { Observable } from 'rxjs/Observable';

import { defer } from 'rxjs/observable/defer';

import { of } from 'rxjs/observable/of';

import * as collection from '../actions/collection';

import { Book } from '../models/book';

@Injectable()

export class CollectionEffects {

  /**

   * This effect does not yield any actions back to the store. Set

   * `dispatch` to false to hint to @ngrx/effects that it should

   * ignore any elements of this effect stream.

   *

   * The `defer` observable accepts an observable factory function

   * that is called when the observable is subscribed to.

   * Wrapping the database open call in `defer` makes

   * effect easier to test.

   */

  @Effect({ dispatch: false })

  openDB$: Observable<any> = defer(() => {

    return this.db.open('books_app');

  });

  /**

   * This effect makes use of the `startWith` operator to trigger

   * the effect immediately on startup.

   */

  @Effect()

  loadCollection$: Observable<Action> = this.actions$

    .ofType(collection.ActionTypes.LOAD)

    .startWith(new collection.LoadAction())

    .switchMap(() =>

      this.db.query('books')

        .toArray()

        .map((books: Book[]) => new collection.LoadSuccessAction(books))

        .catch(error => of(new collection.LoadFailAction(error)))

    );

  @Effect()

  addBookToCollection$: Observable<Action> = this.actions$

    .ofType(collection.ActionTypes.ADD_BOOK)

    .map((action: collection.AddBookAction) => action.payload)

    .mergeMap(book =>

      this.db.insert('books', [ book ])

        .map(() => new collection.AddBookSuccessAction(book))

        .catch(() => of(new collection.AddBookFailAction(book)))

    );

  @Effect()

  removeBookFromCollection$: Observable<Action> = this.actions$

    .ofType(collection.ActionTypes.REMOVE_BOOK)

    .map((action: collection.RemoveBookAction) => action.payload)

    .mergeMap(book =>

      this.db.executeWrite('books', 'delete', [ book.id ])

        .map(() => new collection.RemoveBookSuccessAction(book))

        .catch(() => of(new collection.RemoveBookFailAction(book)))

    );

    constructor(private actions$: Actions, private db: Database) { }

}