javascript - 如何处理 Node.js 模块中的异步回调？

Question

这是我第一次尝试将 Node 模块放在一起，我仍在努力思考如何构建异步回调。这是一个很好的例子。现在我正在尝试使用 featureService.getCount() 并且没有得到任何响应。使用断点，我知道 featureService.getUniqueIds() 正在工作。

由于回调在那里，我假设我没有得到长度的原因是 getCount 中的回调尚未响应。在看了一个下午的大部分时间之后，除了递归循环检查要用超时填充的值之外，并没有真正提出一个很好的解决方案，我正在寻求建议如何更好地构建我的代码来完成任务就在眼前。

我读过一些关于 promises 的内容。这是一个适用的实例，甚至是一个可行的解决方案？我真的不知道如何实现 promise ，但在这种情况下它是合乎逻辑的。

显然我在这里迷路了。感谢您提供的任何帮助。

var Client = require('node-rest-client').Client;
var client = new Client();

var featureService = function(endpoint){

    var uniqueIds;
    var count;

    // get list of unique id's
    this.getUniqueIds = function(){
        if (!uniqueIds) {
            var options = {
                parameters: {
                    f: 'json',
                    where: "OBJECTID LIKE '%'",
                    returnIdsOnly: 'true'
                }
            };
            client.get(endpoint + '/query', options, function(data, res){
                var dataObject = JSON.parse(data);
                var uniqueIds = dataObject.objectIds;
                return uniqueIds;
            });
        } else {
            return uniqueIds;
        }
    };

    // get feature count
    this.getCount = function(){

        // get uniqueIds
        uniqueIds = this.getUniqueIds();

        // get length of uniqueIds
        count = uniqueIds.length;
    };

    // get list of unique attribute values in a single field for typeahead
    this.getTypeaheadJson = function(field){};

    // find features in a field with a specific value
    this.find = function(field, value){};
};

var endpoint = 'http://services.arcgis.com/SgB3dZDkkUxpEHxu/arcgis/rest/services/aw_accesses_20140712b/FeatureServer/1';
var afs = new featureService(endpoint);
console.log(afs.getCount());

exports.featureService = featureService;

现在，在进一步研究并使用 bluebird 文档中的 request 之后(我无法让上面的模块工作)，我有这个工作，但无法弄清楚如何获得计算值，即迭代次数。

var Promise = require("bluebird"),
    request = Promise.promisifyAll(require("request"));

var FeatureService = function(){

    // get count from rest endpoint
    var getCount = function(){
        var optionsCount = {
            url: endpoint + '/query',
            qs: {
                f: 'json',
                where: "OBJECTID LIKE '%'",
                returnCountOnly: 'true'
            }
        };
        return request.getAsync(optionsCount)
            .get(1)
            .then(JSON.parse)
            .get('count');
    };

    // get max record count for each call to rest endpoint
    var getMaxRecordCount = function(){
        var optionsCount = {
            url: endpoint,
            qs: {
                f: 'json'
            }
        };
        return request.getAsync(optionsCount)
            .get(1)
            .then(JSON.parse)
            .get('maxRecordCount');
    };

    // divide the total record count by the number of records returned per query to get the number of query iterations
    this.getQueryIterations = function(){
        getCount().then(function(count){
            getMaxRecordCount().then(function(maxCount){
                return  Math.ceil(count/maxCount);
            });
        });
    };

};

// url to test against
var endpoint = 'http://services.arcgis.com/SgB3dZDkkUxpEHxu/arcgis/rest/services/aw_accesses_20140712b/FeatureServer/1';

// create new feature service object instance
afs = new FeatureService();

// This seems like it should work, but only returns undefined
console.log(afs.getQueryIterations());

// this throws an error telling me "TypeError: Cannot call method 'then' of undefined"
//afs.getQueryIterations().then(function(iterCount){
//    console.log(iterCount);
//});

Answer 1

是的，使用 promise !他们是powerful tool ，正是为此目的而制作的，并带有 decent library它们易于使用。在你的情况下:

var Promise = require('bluebird'); // for example, the Bluebird libary
var Client = Promise.promisifyAll(require('node-rest-client').Client);
var client = new Client();

function FeatureService(endpoint) {

    var uniqueIds;
    var count;

    // get list of unique id's
    this.getUniqueIds = function(){
        if (!uniqueIds) { // by caching the promise itself, you won't do multiple requests
                          // even if the method is called again before the first returns
            uniqueIds = client.getAsync(endpoint + '/query', {
                parameters: {
                    f: 'json',
                    where: "OBJECTID LIKE '%'",
                    returnIdsOnly: 'true'
                }
            })
            .then(JSON.parse)
            .get("objectIds");
        }
        return uniqueIds;
    };

    // get feature count
    this.getCount = function(){
        if (!count)
            count = this.getUniqueIds() // returns a promise now!
                    .get("length");
        return count; // return a promise for the length
    };

    // get list of unique attribute values in a single field for typeahead
    this.getTypeaheadJson = function(field){};

    // find features in a field with a specific value
    this.find = function(field, value){};
};

var endpoint = 'http://services.arcgis.com/SgB3dZDkkUxpEHxu/arcgis/rest/services/aw_accesses_20140712b/FeatureServer/1';
var afs = new FeatureService(endpoint);
afs.getCount().then(function(count) {
    console.log(count);
}); // you will need to use a callback to do something with async results (always!)

exports.FeatureService = FeatureService;

在这里，使用 Bluebird's Promise.promisifyAll ，您可以只使用 .getAsync() 而不是 .get()，并且会得到对结果的 promise 。

---

// divide the total record count by the number of records returned per query to get the number of query iterations
this.getQueryIterations = function(){
    getCount().then(function(count){
        getMaxRecordCount().then(function(maxCount){
            return  Math.ceil(count/maxCount);
        });
    });
};

这是正确的想法!只有你总是想从.then处理程序返回一些东西，这样.then()返回的promise > 调用将以该值解析。

// divide the total record count by the number of records returned per query to get the number of query iterations
this.getQueryIterations = function(){
    return getCount().then(function(count){
//  ^^^^^^ return the promise from the `getQueryIterations` method
        return getMaxRecordCount().then(function(maxCount){
//      ^^^^^^ return the promise for the iteration number
            return  Math.ceil(count/maxCount);
        });
    });
};

现在，你得到一个 promise for the innermost result ，这现在可以工作了:

afs.getQueryIterations().then(function(iterCount){
    console.log(iterCount);
});