如何prevent HTTP调用角2使用时，可观察？ [英] How to prevent http call in Angular 2 when using observables?

问题描述

几个例子演示了如何使用观测挂钩的控制，显示从HTTP后端读取的数据，比如这个：的http://blog.thoughtram.io/angular/2016/01/06/taking-advantage-of-observables-in-angular2.html

Several examples demonstrate how to use observables to hook up a control to show data fetched from a http backend, such as this one: http://blog.thoughtram.io/angular/2016/01/06/taking-advantage-of-observables-in-angular2.html

你能prevent的HTTP在某些情况下打电话？例如，在提到后有一个自动完成场 - 是有办法prevent HTTP调用的情况下，用户清除字段

Can you prevent that http call in certain situations? For example in the mentioned post there is an autocomplete field - is there a way to prevent the http call in case the user clears the field?

我试图修改与switchMap功能，例如：

I have tried modifying the switchMap function with for example:

if(term.length < 1) {
   return Observable.empty();
}
else { // call the http service...

和它prevent电话，但在控制离开previous结果。

and it does prevent the call, but leaves the previous results at the control.

推荐答案

对不起，我在移动，但像滤波器（长期=＆GT; term.length大于0）应该做的伎俩。

Sorry, I'm on mobile but something like filter(term => term.length > 0) should do the trick.

注释后更新

有可能处理这个更优雅的方式，但这个怎么样？

There are probably more elegant ways of handling this but how about this?

this.items = this.term.valueChanges
             .debounceTime(400)
             .distinctUntilChanged()
             .switchMap(term => term.length > 0 
               ? this.wikipediaService.search(term) 
               : Observable.of([]));

工作plunkr： http://plnkr.co/edit/3p9eqiPAcqjBIEdLNkUG?p= preVIEW

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