在循环和嵌套函数中使用 Promises

Question

我正在尝试将承诺与循环和其中一些循环中的嵌套函数一起使用。我有一系列函数应该从 REST 调用中带回 SharePoint 列表项 - 一旦这些函数执行完毕，就会调用另一个函数，该函数使用带回的数据。

由于有多个列表，随后每个列表中有多个列表项，我使用了一个 while 循环来进行每个 REST 调用，并从那里将数据（列表项）放入对象中。这些对象被放入一个数组中，这就是第二个函数用来继续的对象。

我无法收到 promise 的回复。我在考虑将多个承诺推送到一个数组，然后最终使用 Promise.all 查看是否在使用 then 之前一切都已解决。问题是所有的承诺都保持 pending 因为我没有正确返回 resolve。请看下面。

    function onQuerySuccess(sender, args) {
        var itemEnumerator = items.getEnumerator();

        while (itemEnumerator.moveNext()) {
            var promise = new Promise(function (resolve, reject) {
                var item = itemEnumerator.get_current();
                item = item.get_item('URL');
                var itemUrl = item.get_description();

                getRequestItemsFromList(itemUrl);
            });

            promises.push(promise); // all promises are present, but their status is pending
        }

        console.log(promises);

        Promise.all(promises).then(function (val) {
            console.log(val);
            execFuncs(); // function to execute once all the above are done
        }).catch(function (response) {
            console.log(response);
        });
    }

因为涉及的函数比较多，所以执行顺序是这样的：

getRequestItemsFromList //gets url for each list
execCrossDomainRequest (on success call) // makes REST call to get list and its items
cleanData // trims data and puts it in objects

最后一个是我想调用 Promise.resolve() 的地方，因为那是该行的结尾。

不管怎样，这都行不通。我检查了其他线程，但我试图在不使用任何库的情况下执行此操作。提前致谢。

编辑：

完整相关代码：

var promises = [];

window.requests = [];

function getRequestLists() {
    var requestsLists = hostWeb.get_lists().getByTitle('Name');  // sharepoint list with all the request list urls.
    context.load(requestsLists);

    var camlQuery = new SP.CamlQuery();
    camlQuery.set_viewXml('<View></View>');
    var items = requestsLists.getItems(camlQuery);

    context.load(items, 'Include(URL)');

    context.executeQueryAsync(onQuerySuccess, onQueryFail);

    function onQuerySuccess(sender, args) {
        var itemEnumerator = items.getEnumerator();

        while (itemEnumerator.moveNext()) {
            var promise = new Promise(function (resolve, reject) {
                var item = itemEnumerator.get_current();
                item = item.get_item('URL');
                var itemUrl = item.get_description();

                getRequestItemsFromList(itemUrl);
            });

            promises.push(promise);
        }

        console.log(promises);

        Promise.all(promises).then(function (val) {
            console.log(val);
            execFuncs(); // not shown here
        }).catch(function (response) {
            console.log(response);
        });
    }

    function onQueryFail(sender, args) {
        alert("Request to retrieve list URL items has failed. " + args.get_message());
    }
}

function getRequestItemsFromList(url) {
    var lastPos = getSubstringIndex(url, "/", 4);
    var webUrl = url.substring(0, lastPos); // truncates list url to parse out web url       

    var absListPos = getSubstringIndex(url, "AllItems.aspx", 1);
    var absListUrl = url.substring(0, absListPos); // truncates the AllItems.aspx at the end of the list url

    var relListPos = getSubstringIndex(absListUrl, "/", 3);
    var relListUrl = absListUrl.substring(relListPos, absListUrl.length); // gets the list's relative url 

    var listName = "List Name";

    console.log(webUrl);
    execCrossDomainRequest(webUrl, listName, absListUrl);
}

function execCrossDomainRequest(webUrl, listName, absListUrl) {
    var executor = new SP.RequestExecutor(appWebUrl);

    executor.executeAsync({ // to collect the list description
        url: appWebUrl + "/_api/SP.AppContextSite(@target)/web/lists/getbytitle(@name)?" +
            "@target='" + webUrl + "'&@name='" + listName + "'" +
            "&$select=Description",
        method: "GET",
        headers: { "Accept": "application/json; odata=verbose" },
        success: onCallSuccess,
        error: onCallFail
    });

    function onCallSuccess(data) {
        var json = JSON.parse(data.body);
        var description = json.d.Description;

        executor.executeAsync({ // to collect the list item information
            url: appWebUrl + "/_api/SP.AppContextSite(@target)/web/lists/getbytitle(@name)/items?" +
                "@target='" + webUrl + "'&@name='" + listName + "'" +
                "&$top=500&$select=*," +
                "Assigned_x0020_To/Title" +
                "&$expand=Assigned_x0020_To/Id",
            method: "GET",
            headers: { "Accept": "application/json; odata=verbose" },
            success: onItemsCallSuccess,
            error: onItemsCallFail
        });

        function onItemsCallSuccess(data) {
            var itemsJson = JSON.parse(data.body);
            var results = itemsJson.d.results;

            cleanData(results, description, absListUrl);
        }

        function onItemsCallFail(data, errorCode, errorMessage) {
            console.log("Could not make list items cross domain call. " + errorMessage);
        }
    }

    function onCallFail(data, errorCode, errorMessage) {
        console.log("Could not make list cross domain call. " + errorMessage);
    }
}

function cleanData(results, listDescription, absListUrl) {

    if (!results.length) {
        return;
    }

    for (var i = 0; i < results.length; i++) {
        var client = listDescription;
        var id = results[i].ID;
        ...

        }

        var request = new Request(client, id, path, title, status, estimated, assignedTo, priority, description, response);

        window.requests.push(request);
    }

    return Promise.resolve();
}

Answer 1

在我看来你的 promises 数组没有定义：do var promises = [];在你的 while 循环之前。

Answer 2

当你像这样使用 promise 构造函数时：

var promise = new Promise(function (resolve, reject) {

这意味着您正在调用 resolve and/or reject 来完成承诺。

但你永远不会这样做，让创建的承诺对象永远处于挂起状态。

见Constructing a promise。

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