Question

我想用RxJs实现Time Expiry缓存。这是＆＃34;普通＆＃34;的例子。缓存：

//let this represents "heavy duty job"
var data = Rx.Observable.return(Math.random() * 1000).delay(2000);

//and we want to cache result
var cachedData = new Rx.AsyncSubject();
data.subscribe(cachedData);

cachedData.subscribe(function(data){
    //after 2 seconds, result is here and data is cached
    //next subscribe returns immediately data
    cachedData.subscribe(function(data2){ /*this is "instant"*/ });
});

第一次调用subscribe cachedData时，＆＃34;重负荷工作＆＃34;被调用，2秒后结果保存在cachedData（AsyncSubject）中。 subscribe上的任何其他后续cachedData会立即返回已保存的结果（因此缓存实现）。

我想要达到的目标是＆＃34; spice＆＃34; cachedData以内的时间段有效，当该时间过去时，我想重新运行＆＃34;重型工作＆＃34;对于新数据并在新时间段再次缓存此等...

期望的行为：

//pseudo code
cachedData.youShouldExpireInXSeconds(10);


//let's assume that all code is sequential from here

//this is 1.st run
cachedData.subscribe(function (data) {
    //this first subscription actually runs "heavy duty job", and
    //after 2 seconds first result data is here
});

//this is 2.nd run, just after 1.st run finished
cachedData.subscribe(function (data) {
    //this result is cached
});

//15 seconds later
// cacheData should expired
cachedData.subscribe(function (data) {
    //i'm expecting same behaviour as it was 1.st run:
    // - this runs new "heavy duty job"
    // - and after 2 seconds we got new data result
});


//....
//etc

我是Rx（Js）的新手，并且无法弄清楚如何通过冷却来实现这个热观察。

Answer 1

您缺少的是安排一项任务，在一段时间后将cachedData替换为新的AsyncSubject。以下是如何将其作为新的Rx.Observable方法：

Rx.Observable.prototype.cacheWithExpiration = function(expirationMs, scheduler) {
    var source = this,
        cachedData = undefined;

    // Use timeout scheduler if scheduler not supplied
    scheduler = scheduler || Rx.Scheduler.timeout;

    return Rx.Observable.create(function (observer) {

        if (!cachedData) {
            // The data is not cached.
            // create a subject to hold the result
            cachedData = new Rx.AsyncSubject();

            // subscribe to the query
            source.subscribe(cachedData);

            // when the query completes, start a timer which will expire the cache
            cachedData.subscribe(function () {
                scheduler.scheduleWithRelative(expirationMs, function () {
                    // clear the cache
                    cachedData = undefined;
                });
            });
        }

        // subscribe the observer to the cached data
        return cachedData.subscribe(observer);
    });
};

用法：

// a *cold* observable the issues a slow query each time it is subscribed
var data = Rx.Observable.return(42).delay(5000);

// the cached query
var cachedData = data.cacheWithExpiration(15000);

// first observer must wait
cachedData.subscribe();

// wait 3 seconds

// second observer gets result instantly
cachedData.subscribe();

// wait 15 seconds

// observer must wait again
cachedData.subscribe();

Answer 2

一个简单的解决方案是创建一个自定义的管道运算符，以repeatWhen的持续时间过去。这是我想出的：

 export const refreshAfter = (duration: number) => (source: Observable<any>) =>
                                  source.pipe(
                                     repeatWhen(obs => obs.pipe(delay(duration))),
                                     publishReplay(1), 
                                     refCount());

然后我像这样使用它：

const serverTime$ = this.environmentClient.getServer().pipe(map(s => s.localTime))
const cachedServerTime$ = serverTime.pipe(refreshAfter(5000)); // 5s cache

重要说明：它使用publishReplay（1）和refCount（），因为shareReplay（1）不会从可观察到的源退订，因此它将永远打在您的服务器上。不幸的是，这样做的结果是，一旦出错，就会从publishReplay(1)，refCount()重播错误。即将推出“新的改进” shareReplay。 See notes here on a similar question。一旦有了这个“新”版本，就应该更新此答案-但是自定义运算符的优点是您可以将它们固定在一个位置。

如何在RxJS中实现时间到期热观察（或在Reactive Extensions中一般）

2 个答案: