RXJS - 如果对来自observable的值满足特定条件，则执行函数

Question

I have built a search field with autocomplete. I'm listening to a observable that detects changes from a text field.

this.term.valueChanges
        .debounceTime(300)
        .distinctUntilChanged()
        .filter(query=>query.toString().length>1)
        .subscribe(query => { 

         //Execute search function and populate dropdown menu

        })

When a user search for a string the dropdown autocomplete list is populated with suggestions. If the user then erases the search string I want the dropdown list to be removed. Is there a RxJS function that makes it possible to execute a function before subscribe if a specific condition is met on the value generated from the observable stream?

Answer 1

Try something like this:

this.term.valueChanges
    .debounce(100)
    .distinctUntilChanged()
    .do(() => /* close list here */)
    .where(val => val.length > 1)
    .select(query => ({ 
        query: query, 
        completions: completions
            .filter(w => w.indexOf(query.toLowerCase()) !== -1)
    }))
    .subscribe(r => {
        /*
         * r.query          contains the current search word
         * r.completions    contain the completions for that word
         */
    });

Working basic example on JSFiddle