RxJS Observable - 每次满足条件时订阅一次

Question

Observables 对我来说是新的，所以我什至不确定它是否可能（但我猜它是可能的）。我想要实现的是拥有一个可观察的（Angular 2 http.get 调用），并在变量更改时订阅它一次。示例流程如下所示：

let x = false;
// ... somewhere later I change "x" to true
subscribe gets called once, x => false
// stuffs happening, 10 minutes later x => true again
subscribe gets called once, x => false

更大的概念是我将拥有一个UserService，并且在构造函数中我将订阅/api/user/me 一次，只要在localStorage 中发生令牌更改。这是一个有效的用例吗？如果是，我该如何使用 observables 来做到这一点？

Answer 1

这样的事情应该可以解决问题：

// Observable that on subscription will result in an api call. It would 
// naturaly have another type in an acaual use case.
let apiCallObservable : Rx.Observable< { someResult: any }>;

let localStorageChangedObservable = new Rx.Subject<boolean>()

localStorageChangedObservable
    // only care about local storage change if not null
    // This is just an example the type of localStorageChangedObservable
    // could be anything you want, and you could check anything you want in the 
    // "where" 
    .where(v => v != null)
    .selectMany(() =>
        // Take one is to make sure we terminate apiCallObservable after recieving
        // one result from it. 
        apiCallObservable.take(1))
    .subscribe(
        (result: { someResult: any}) => {

            // here we would have the result from the call to apiCallObservable
        });

// To execute the chain abouve we would pass values to the localStorageChangedObservable 
// subject like this:

localStorageChangedObservable.onNext(true); // results in api call and result in subscribe
localStorageChangedObservable.onNext(null); // results in nothing.
localStorageChangedObservable.onNext(false); // results in api call and result in subscribe

所以当你想触发一个 api 调用时，基本上传递一些东西给localStorageChangedObservable.onNext(...)。