Wait for Json call to be completed in javascript

问题:

I am using the below json call in my javascript method

function go123(){
    var cityName = "";
    var temp = $.getJSON("https://abc.in/api/city?callback=?", args,function (data) {
        if (data.properties.city != null){ 
            cityName = data.properties.city;
            check = true;
        } else {
            cityName = "NaN"
        }
    }); // end of my Json Call.

    // my validation is done below
    if(cityName != "NaN"){
        return false;
    } else {
    // here I except the cityName to not be "" but be some value which  is set as :cityName = data.properties.city;
        return true;
    }
} // end of my function 

Now what problem I am facing is that before my Json call is compelete the next set of statements ( in the code below the line "// my validation is done below " ) is already executed.

I want to get the values set in my json call (cityName) and only once when the call is completed then only I want the next set of statements to be executed.

Please help me on this. Any advice/ideas/suggestions will be highly appreciated ! Thanks.

回答1:

AJAX calls are asyncrhonous. They don't wait for the reply. They operate in the background and execute the code that follows it immediately after the call. Therefore, the data received in getJSON is not yet there when the operations below it are executed.

You can put the operations you want in the callback so that they get executed when the data is revceived:

function go123(callback){
    var temp = $.getJSON("https://abc.in/api/city?callback=?", args,function (data) {
        //execute the callback, passing it the data
        callback(data);
    });
}

//when you call go123, it get's back the result:
function goBefore123(){

    //get our JSON
    go123(function(data){

        //when we get our data, evaluate
        if (data.properties.city != null){

            cityName = data.properties.city;
            check = true;

            alert('executed after returned');
            afterCall();
        } else {
            cityName = "NaN"
        }
    });

    alert('i am executed before anything else');
}

function afterCall(){
    alert('im also executed after');
}

回答2:

The function you passed into $.getJSON() is the callback run when the function completes successfully. All else being equal, stick the "rest of it" inside that method. If you can't do so, what you're after is called a jQuery Deferred. See http://www.erichynds.com/jquery/using-deferreds-in-jquery/ and http://joseoncode.com/2011/09/26/a-walkthrough-jquery-deferred-and-promise/ for code that looks like so:

var req = $.getJSON('blah', 'de', 'blah');

req.success(function(response){
    // The request is done, and we can do something else
});

回答3:

Calling an external url will take too much time, wait for the result Check below

var jqxhr = $.getJSON("example.json", function() {
  alert("success");
})
.success(function() { alert("second success"); })
.error(function() { alert("error"); })
.complete(function() { alert("complete"); });

http://api.jquery.com/jQuery.getJSON/

回答4:

.success

http://api.jquery.com/jQuery.getJSON/