function wait with return until $.getJSON is finis

问题:

I am writing a function which has to get the thumbnail information from a given video using the embed.ly API, however currently the function returns a value before it even got the JSON result from the API.

I am using the following code:

function getThumbnail(vUrl) {
    var thumbnail   = '';
    var title       = '';
    var caption     = '';
    var content     = '';

    $.when( $.getJSON("http://api.embed.ly/1/oembed?key=:key&url="+vurl) ).then(function(data){
        var thumbnail = data.thumbnail_url;
            console.log(thumbnail);

        return {
            thumbnail:thumbnail,
            vurl:vurl
        }
    });
}

However when using the Chrome Javascript console I can see that:

1. the function is called
2. undefined is returned
3. XHR request is finished
4. variable thumbnail content is shown in console

This is obviously the wrong order.

Any help is greatly appreciated!

回答1:

Updated answer

getJSON returns a promise (a read-only deferred), so you can listen to it. But since you need some post-processing, you'd want to chain a then which allows you to alter the resolved value.

// Now using `then`
function getThumbnail(vUrl){
  return $.getJSON("http://api.embed.ly/1/oembed?key=:key&url="+vurl).then(function(data){
    return {
      thumbnail:data.thumbnail_url,
      vurl:vurl
    }
  });
}

//and in your call will listen for the custom deferred's done
getThumbnail('the_vurl_').then(function(returndata){
  //received data!
});

---

Original answer

You can use a deferred object, and listen for the done().

function getThumbnail(vUrl) {
    //create our deferred object
    var def = $.Deferred();

    //get our JSON and listen for done
    $.getJSON("http://api.embed.ly/1/oembed?key=:key&url="+vurl)
        .done(function(data){

            //resolve the deferred, passing it our custom data
            def.resolve({
                thumbnail:data.thumbnail_url,
                vurl:vurl
            });
        });

    //return the deferred for listening
    return def;
}

//and in your call will listen for the custom deferred's done
getThumbnail('the_vurl_')
    .done(function(returndata){
        //received data!
    });

You could return $.getJSON's deferred to get the raw data. But because of "post-processing" into an object, the custom deferred is needed. You could also pass a callback to getThumbnail():

function getThumbnail(vUrl,callback) {
    $.getJSON("http://api.embed.ly/1/oembed?key=:key&url="+vurl,function(returndata){
        callback(returndata);
    });
}

getThumbnail('the_vurl_',function(returndata){
    //received data!
})

回答2:

you can simple use $.getJSON's callback like following:

function result(res) {
  console.log(res);
}

function getThumbnail(vUrl) {
   var thumbnail   = '';
   var title       = '';
   var caption     = '';
   var content     = '';

   $.getJSON("http://api.embed.ly/1/oembed?key=:key&url="+vurl, function(data) {
     var thumbnail = data.thumbnail_url;
     console.log(thumbnail);

     var result = {
        thumbnail:thumbnail,
        vurl:vurl
      };

     // passing the result to a function
     getResult(result);

   });
}

NOTE:

You see that I'm calling a function to pass the result, where you are trying to return, but you can't return result to caller function. Because, $.getJSON is asynchronous.