如何在自定义函数中使用jQuery promise / deferred？

Question

我有一个通过navigator.geolocation获取位置的函数：

var getLocation = function( callback ){

    navigator.geolocation.getCurrentPosition( callback || function( position ){

        // Stuff with geolocation

    });

};

我想这样做，以便我可以使用jQuerys'Deffered对象链接此函数，但我仍然没有设法掌握Deffered的概念和用法。

我正在寻找类似于伪代码：

的内容

getLocation().then(function(){
    drawMarkerOnMap();
});

这种语法是否可行，而不会在代码中向后翻转并淹没？

Answer 1

您必须实例化一个新的延迟对象并从该函数返回它（或它的promise）。获得响应后调用其.resolve方法：

var getLocation = function() {
    var deferred = new $.Deferred();

    navigator.geolocation.getCurrentPosition(function( position ){
        // Stuff with geolocation
        deferred.resolve(position);
    });

    // return promise so that outside code cannot reject/resolve the deferred
    return deferred.promise();
};

用法：

getLocation().then(drawMarkerOnMap);

---

参考：jQuery.Deferred

---

<强>附录：

我建议不要使用这两种方法，延迟对象并将回调传递给函数，以保持界面简单。但是如果你必须保持向后兼容，你可以简单地在延迟对象上注册传递的回调：

var getLocation = function(callback) {
    var deferred = new $.Deferred();

    if ($.isFunction(callback)) {
        deferred.then(callback);
    }

    navigator.geolocation.getCurrentPosition(function( position ){
        // Stuff with geolocation
        deferred.resolve(position);
    });

    // return promise so that outside code cannot reject/resolve the deferred
    return deferred.promise();
};

Answer 2

尽管上面的例子对我有所帮助，但我还是需要做更多的阅读才能理解这个概念。

下面是基于我的代码的示例，其中包含的注释可以帮助我回到它并希望有人阅读此Stackoverflow问题：

/* promise based getFilter to accommodate getting surrounding suburbs */
oSearchResult.fPromiseOfFilterSetting = function fPromiseOfFilterSetting(sId) {
    var self = this;
    self.oPromiseCache = self.oPromiseCache || {}; // creates a persistent cache 
                                                   // across function calls
    var oDeferred = $.Deferred(); // `new` keyword is optional
    var oPromise = oDeferred.promise();

    // leverage the cache (it's ok if promise is still pending), you can key
    if (self.oPromiseCache[sId] !== undefined) {
        return self.oPromiseCache[sId];
    }
    else {
        self.oPromiseCache[sId] = oPromise;
    }

    // do our asynchronous action below which at some point calls
    // defered.resolve(...) and hence complete our promise
    $.cmsRestProxy.doAjaxServiceRequest('ocms_searchProperties_Extension', {
        action : 'getSurroundingSuburbs',
        sSuburbIds : 'a0RO0000003BwWeMAK'
    }, function(result, json) {
        console.log("doAjaxServiceRequest(
                       'ocms_searchProperties_Extension')", json);
        oDeferred.resolve(json); // `json` is our result and `.resolve(json)` 
                                 // passes the value as first argument to 
                                 // the `oPromise.done`, `oPromise.fail` 
                                 // and `oPromise.always` callback functions
    })

    // We can now return the promise or attach optional `oPromise.done`,
    // `oPromise.fail`, and `oPromise.always` callbacks which will execute first
    // in the chain.
    //
    // Note that `oPromise.then(doneCallback, failCallback, alwaysCallback)`
    // is short form for the below
    oPromise.done(function(value) { // returned by promise.resolve(...); call
        console.log('will run if this Promise is resolved.', value);
    })
    oPromise.fail(function(value) {
        console.log("will run if this Promise is rejected.", value);
    });
    oPromise.always(function(value) {
        console.log("this will run either way.", value);
    });

    // return a promise instead of deferred object so that
    // outside code cannot reject/resolve it
    return oPromise;
}

// then to use one would do
oSearchResult.fPromiseOfFilterSetting().done(function(value) {alert(value)});

// or using $.when chaining
$.when(
    oSearchResult.fPromiseOfFilterSetting()
)
.done(
      function fDoneCallback(arg1, arg2, argN) {
          console.debug(arguments) // `arguments` is an array of all args collected
      }
);

Answer 3

我知道它的标题中说的是jQuery，但是当我问这个问题时，promise是Web上的新手，而jQuery是事实上的库。这是没有jQuery的更现代的答案。

使用本地Promise

所有modern browsers（除了IE11及以下版本； use a polyfill if needed）都使您可以使用本机Promise构造。

let getLocation = () => {

  return new Promise( ( resolve, reject ) => {

    try {
      navigator.geolocation.getCurrentPosition( position => {
        resolve( position )
      })
    } catch ( err ) {
      reject( err )
    }

  })

};

用法：

let runGetLocation = () => { getLocation().then( position => console.log( position ) ) }

您还可以使用ES2016 async / await代替.then()：

let runGetLocation = async () => {

  try {
    let position = await getLocation()
    console.log( position )
  } catch ( err ) { console.log( err ) }

}