jQuery推迟了ajax缓存

Question

我阅读了关于使用jQuery Deferred的这个问题的最佳答案.

我正在遍历一系列ID.对于每个ID,我需要从ajax请求获取与之相关的数据,或者如果ajax请求之前已成功返回数据,则需要从缓存中获取数据.

在每个循环中,我使用$ .when()来观察getData()是否在处理该ID之前从缓存或成功的ajax调用返回一些内容.目前的问题是ID处理无论如何都要继续进行,而不必等待getData()的ajax成功.

一些伪代码:

var IDs = ["1", "2", "1", "3", "1"]; 
//ID "1" is repeated
//data for "1" should should require ajax get the first time
//subsequent processing should get data for "1" from dataCache

var dataCache = [];

function getData(ID){
    if (/*data for ID in dataCache*/){
        //return data pertaining to ID from dataCache
    } else {
        return $.getJSON("returnJSONDataByID/" + ID, function(resp){
            //push resp data to dataCache
        })
    }
}

for (/*each item i in IDs*/){
    $.when(getData(IDs[i])).then(function(){
        //process IDs[i] data

        //this is the resolved handler, which should be executed
        //when either getData() returns data from the dataCache,
        //or $.getJSON succeeds
        //PROBLEM: this is currently executing every loop and
        //and doesn't wait for the ajax to return resp
    })
}

Answer 1

问题是你的循环会getData立即触发所有调用,但是只有在JSON调用返回后,结果才会存储在缓存中.因此,对于循环中的每个调用,缓存仍然为空,并且每个调用都将执行新的JSON请求.

解决方案:而不是结果存储Deferred缓存中的对象.

var IDs = ["1", "2", "1", "3", "1"];

var dataCache = {};

function getData(id) {
    if (id in dataCache) {
        console.log("Cache hit for ID " + id);
        return dataCache[id];
    } else {
        console.log("Retrieving data for ID " + id);
        var deferred = $.getJSON("http://jsfiddle.net/echo/jsonp/?callback=?", {
            id: id
        }, function(response) {
            console.log("Retrieved data for ID " + id);
        });
        dataCache[id] = deferred;
        return deferred;
    }
}

for (var i=0; i<IDs.length; i++) {
    $.when(getData(IDs[i])).then(function(result) {
        console.log("result: " + result.id);
    });
}

注意:这是工作代码,你可以在jsFiddle中使用它.