我对使用promises有点新意,并且在返回对象时遇到问题。我有以下函数调用SPServices从列表中检索一个项目。我知道SPService调用正在返回列表项,但返回的数据是未定义的:
$('.support-View').on('click', function () {
var thisID = $(this).data("id");
var message = getOneDetails(thisID);
console.log("This Data : " + message); //<-- Comes back undefined
$.when(message).done(function() {
thisMessage = message;
viewRequestDialog(thisID, message);
});
});
getOneDetails函数如下所示:
function getOneDetails (thisID, viewtype) {
var CAML = '<Query><Where><Eq><FieldRef Name="Title" /><Value Type="Text">' + thisID + '</Value></Eq></Where></Query>';
var list = 'Support Requests';
var requestsPromise = $().SPServices.SPGetListItemsJson({
listName: list,
CAMLQuery: CAML,
includeAllAttrs: true
});
$.when(requestsPromise).done(function() {
requestData = this.data;
console.log(this.data); //<---- Data is defined here
})
.then(function(requestData) {
console.log(requestData); //<---- But undefined here
return requestData;
});
}
我确定我错过了一些简单的东西,但是我如何简单地从promise函数返回对象? TIA
答案 0 :(得分:5)
你永远不能return
来自承诺的价值,因为它可能还没有到达。对于根据第一个promise的结果计算的值,您只能返回另一个promise。您的第一个代码段需要如下所示:
$('.support-View').on('click', function () {
var thisID = $(this).data("id");
var messagePromise = getOneDetails(thisID);
messagePromise.done(function(message) {
console.log("This Data : " + message); // <-- Comes back as an argument
// to the callback
// thisMessage = message; // Don't ever do this. Global variables are
// useless when you don't know *when* they
// will contain a value. If you need to store
// something, store the `messagePromise`.
viewRequestDialog(thisID, message);
});
});
您的getOneDetails
函数需要return
一个承诺,它目前不会返回任何内容。为什么this.data
被定义,但没有作为参数传递给回调我不确定;但即使您将其分配给全局requestData
变量,该值也会被requestData
回调的本地then
变量(命名参数)遮蔽。
理想情况下,它应该看起来
return requestsPromise.then(function(requestData) { /*
^^^^^^ notice the return! */
console.log(requestData); // <---- Data is given as an argument
return requestData;
});
但你可能需要做
return requestsPromise.then(function() { // again, return the result of the call
var requestData = this.data; // take it from wherever (unsure what `this` is)
// ^^^ local variable
console.log(requestData); // <-- Data is defined here
return requestData; // and can be returned to resolve the promise with
});