create Observable from result

问题:

I am trying Angular2.

I noticed that the http service use Observable object instead of Promise (I don't like much that choice.. async/await are arriving).

In my service, I download a list of Plants from the webservice. Clicking on a plant I show the details using the routing. But in this way when I go back, the plants are downloaded again (because the constructor is called again).

To avoid this I want to do something like:

public getPlants(): Observable<Plants[]>
{   
    if (this._plants != null)
        return Observable.fromResult (this._plants); //This method does not exists 

    return this._http.get('../../res/heroes.json')...
}

Is there a way to do that? How can I import the Observable class in my ts file?

Thanks!

回答1:

The method in TypeScript (or JavaScript for that matter) is called of. Learn rxjs has a nice tutorial as well

If you are using rxjs6 you get everything from rxjs

import { Observable, of } from 'rxjs';

public getPlants(): Observable<Plant[]> {
  const mocked: Plant[] = [
    { id: 1, image: 'hello.png' }
  ];
  // returns an Observable that emits one value, mocked; which in this case is an array,
  // and then a complete notification
  // You can easily just add more arguments to emit a list of values instead
  return of(mocked);
}

---

In previous version you imported the operator of from a different location

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

public getPlants(): Observable<Plant[]> {
  const mocked: Plant[] = [
    { id: 1, image: 'hello.png' }
  ];
  return of(mocked);
}

---

And before that you imported it as an extension for the Observable class

import { Observable } from "rxjs/Observable";
import 'rxjs/add/observable/of';

public getPlants(): Observable<Plants[]> {
    // this can be changed to a member variable of course
    let mocked: Plants[] = [{
        id: 1,
        image: "hello.png"
    }];
    return Observable.of(mocked);
}

回答2:

This is my working solution:

if (this._heroes != null && this._heroes !== undefined) {
    return Observable.create(observer => {
        observer.next(this._heroes);
        observer.complete();
    });
}

I hope that this is the best solution.