等待javascript中的回调

Question

我正在尝试创建一个返回带有回调信息的对象的函数:

var geoloc;

var successful = function (position) {
    geoloc = {
        longitude: position.coords.longitude,
        latitude: position.coords.latitude
    };
};

var getLocation = function () {
    navigator.geolocation.getCurrentPosition(successful, function () {
        alert("fail");
    });

    return geoloc;
};

我怎样才能做到这一点？该函数getLocation在successful执行之前返回null值.

谢谢!

Answer 1

使用回调是因为该函数是异步的.回调在未来的某个时刻运行.

因此,yes getLocation在触发回调之前返回.这就是异步方法的工作原理.

你不能等待回调,这不是它的工作方式.您可以添加一个回调函数getLocation,该函数在完成后运行.

var getLocation = function(callback){
    navigator.geolocation.getCurrentPosition(function(pos){
        succesfull(pos);
        typeof callback === 'function' && callback(geoloc);
    }, function(){
        alert("fail");
    });
};

现在不是做var x = getLocation()和期望返回值,而是这样称呼它:

getLocation(function(pos){
    console.log(pos.longitude, pos.latitude);
});

Answer 2

我会在Rocket的答案中推荐这种方法.但是,如果您真的想要,则可以在getLocation完成后使用jQuery延迟对象触发其余代码.这将为您提供比仅使用提供的回调更细粒度的控制getCurrentPosition.

// create a new deferred object
var deferred = $.Deferred();

var success = function (position) {
    // resolve the deferred with your object as the data
    deferred.resolve({
        longitude: position.coords.longitude,
        latitude: position.coords.latitude
    });
};

var fail = function () {
    // reject the deferred with an error message
    deferred.reject('failed!');
};

var getLocation = function () {
    navigator.geolocation.getCurrentPosition(success, fail); 

    return deferred.promise(); // return a promise
};

// then you would use it like this:
getLocation().then(
    function (location) {
         // success, location is the object you passed to resolve
    }, 
    function (errorMessage) {
         // fail, errorMessage is the string you passed to reject
    });