如果另一个可观察对象在RxJS中有数据，如何忽略所有可观察对象? [英] How to ignore all from an observable if the other observable has data in RxJS?
问题描述
我有两个观察对象，一个是从浏览器 localstorage 接收数据，另一个是从数据库通过 WebAPI 接收的数据.
I have two observables, one receives data from the browser localstorage and the other is from the database through WebAPI.
- 我要订阅它们，因此，如果 localstorage 中的可观察对象具有数据，请不要启动该对象以从数据库中获取数据.
- 如果 localstorage 中的可观测对象没有任何内容 数据，请调用 ajax 调用以从 WebAPI 获取数据.
- I want to subscribe to them so if the observable from the localstorage has data, don't initiate the one to get the data from the database.
- If the observable from the localstorage does not have any data, invoke the ajax call to get the data from the WebAPI.
在下面的示例中，我应该只获取20, 40, 60, 80, 100，因为第一个可观察对象具有数据.第二个可观察对象未运行，因为第一个可观察对象开始发出数据.
In the following example, I should get only 20, 40, 60, 80, 100 because the first observable has data. The second observable did not run because the first observable started emitting data.
推荐答案
可观察到的本地存储需要某种方式来表示没有数据.如果只是挂起"而从未完成，那么您可以使用计时器来完成它:
The local storage observable needs some way to signal that there is no data. If it just "hangs" and never completes, then you might use a timer to complete it:
// Use .amb() instead of .race() if your rxjs version is old
const timer = Observable.timer(1000).ignoreElements();
const lsObservable2 = Observable.race(lsObservable, timer);
这将启动一个计时器，如果可观察到的本地存储在1秒内未产生值，则它将结束流.
This will start a timer and if the local storage observable does not produce a value within 1s, it will end the stream.
如果没有数据，则可观察的本地存储将自行完成，那么您可以按原样使用它:
If your local storage observable will complete on its own if there is no data, then you can use it as is:
const lsObservable2 = lsObservable;
在这一点上，我们真的很想使用 Observable.defer .我们使用defer，以便每次有人订阅时，我们都可以创建一个新的闭包变量(hasValue)，并监视可观察到的源是否为此订阅生成了一个值
At this point, we'd really like to use defaultIfEmpty, because that has the semantics you want. Unfortunately it only supports a scalar value for the default, when instead you want to yield a different observable stream. So lets write our own version of defaultIfEmpty which produces a new stream using Observable.defer. We use defer so that each time someone subscribes, we can create a new closure variable (hasValue) and monitor whether or not the source observable produces a value for this subscription
Observable.prototype.defaultObservableIfEmpty = function(defaultObservable) {
const source = this;
return Observable.defer(() => {
let hasValue = false;
// create a deferred observable that will evaluate to
// defaultObservable if we have not seen any values, or
// empty observable if we have seen any values.
const next = Observable.defer(() => hasValue ? Observable.empty() : defaultObservable);
// now use do() to set hasValue to true if we see a value from
// the source observable
const sourceSetsValue = source.do(v => hasValue = true);
// now we can can just concat this sourceSetsValue
// with out "next" observable. When the first observable
// finishes, it will subscribe to "next", which will then
// either produce the defaultObservable or an empty observable
return sourceSetsValue.concat(next);
});
}
接下来，让我们假设您已将数据库Observable设置为在实际订阅之前不发出ajax调用.这是重要的一步.同样，您可以使用defer:
Next, let's assume you've setup your db Observable to not issue the ajax call until it is actually subscribed. This is an important step. Again you can use something like defer:
const dbObservable = Observable.defer(() => makeDbCall());
然后我们可以像这样使用您的新运算符:
Then we can use your new operator like so:
const data = lsObservable2.defaultObservableIfEmpty(dbObservable);
因此，您的应用程序代码如下所示(将新运算符添加到库中之后):
So your application code looks like this (once you've added the new operator to your library):
const timer = Observable.timer(1000).ignoreElements();
const lsObservable2 = Observable.race(lsObservable, timer);
const dbObservable = Observable.defer(() => makeDbCall());
const data = lsObservable2.defaultObservableIfEmpty(dbObservable);
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