Angular-多个订阅引发错误

Question

I would like to create this functionality:

1. call API for return items
2. items.length > 0 -> call next API
3. items.length == 0 -> call another API

Here is my code:

let a = this.http.get("URL");
a.subscribe(response => {
  if(response.data.length == 0)
    Observable.throw({});
});

a.subscribe(x => {console.log("CALL NEXT API")}, error => { console.log("ANOTHER API")}

This code allways displayed "CALL NEXT API", never call error function in subscribe. What is the best solution for call multiple requests consecutively and with condition?

Answer 1

If this is your code

let a = this.http.get("URL");
a.subscribe(response => {
  if(response.data.length == 0)
    Observable.throw({});
});

a.subscribe(x => {console.log("CALL NEXT API")}, error => { console.log("ANOTHER API")}

Stop wondering why it doesn't work.

Let me break it up to you.

First, you make an HTTP call :

let a = this.http.get("URL");

Nothing bad here. Then, you subscribe to it once :

a.subscribe(response => {
  if(response.data.length == 0)
    Observable.throw({});
});

Here, put into words, you do this :

(nothing)

That's right, you do nothing . All you do is creating an error observable, that you don't bind to a variable. Basically, you do nothing.

Then, you do a second subscription :

a.subscribe(x => {console.log("CALL NEXT API")}, error => { console.log("ANOTHER API")}

Put into words, this gives

If the call is successful, log "CALL NEXT API", and if there is an error, log "ANOTHER API".

If you always see CALL NEXT API , then I assume you only make successful calls.

If your goal is to make successive API calls, you should use the mergeMap operator like so :

this.http.get('URL')
  .pipe(mergeMap(response1 => {
    this.http.get(response1.data.length ? 'SUCCESS URL' : 'EMPTY ARRAY URL').subscribe(response2 => {
      /* ... */
    });
  }));

Answer 2

I may have misunderstood the question, but couldnt you just call other APIs in the subscribe callback?

this.http('URL').subscribe(response => {
    if (response.data.length == 0) {
        console.log('Call another API');
    }
    else {
        console.log('Call next API');
    }
}, error => {
    console.log('Could not return result');
}

If the api is called successfully, it will return a result even if the length is 0. Then you can call the relevant API.

It should only hit the error callback if there was a problem accessing the URL or bad data formatting etc.