Question

在使用必要的值更新消息数组之前，我无法找到解决此函数返回的原因。

var calculateDistance = function (message, cLongitude, cLatitude, cSessionID) {
  return new Promise(function (resolve, reject) {

    distance.key = options.apiKey;
    distance.units('metric');

    var origins = [];
    origins.push(cLatitude + ',' + cLongitude);

    message.forEach(function (obj) {
      obj.sessionId = cSessionID;
      var destinations = [];
      destinations.push(obj.geoLocation.latitude + ',' + obj.geoLocation.longitude);

      distance.matrix(origins, destinations, function (err, distances) {
        if (err) {
          return console.log(err);
        }
        if (!distances) {
          return console.log('no distances');
        }
        if (distances.status == 'OK') {
          for (var i = 0; i < origins.length; i++) {
            for (var j = 0; j < destinations.length; j++) {
              var origin = distances.origin_addresses[i];
              var destination = distances.destination_addresses[j];
              if (distances.rows[0].elements[j].status == 'OK') {
                var distance = distances.rows[i].elements[j].distance.text;
                console.log('Distance from ' + origin + ' to ' + destination + ' is ' + distance);
                obj.distance = distance;
              } else {
                console.log(destination + ' is not reachable by land from ' + origin);
                obj.distance = 'N/A';
              }
            }
          }
        }
      });

    });

    return resolve(message);
  });
}

有人能指出我在这里做错了什么。

此致麦

Answer 1

 var async = require('async');
 var calculateDistance = function (message, cLongitude, cLatitude, cSessionID) {
    return new Promise(function (resolve, reject) {

        distance.key = options.apiKey;
        distance.units('metric');

        var origins = [];
        origins.push(cLatitude + ',' + cLongitude);

        async.each(message, function(obj, callback) {
          obj.sessionId = cSessionID;
          var destinations = [];
          destinations.push(obj.geoLocation.latitude + ',' + obj.geoLocation.longitude);

          distance.matrix(origins, destinations, function (err, distances) {
            if (err) {
              callback(err);
            }
            if (!distances) {
              callback('no distances');
            }
            if (distances.status == 'OK') {
              for (var i = 0; i < origins.length; i++) {
                for (var j = 0; j < destinations.length; j++) {
                  var origin = distances.origin_addresses[i];
                  var destination = distances.destination_addresses[j];
                  if (distances.rows[0].elements[j].status == 'OK') {
                    var distance = distances.rows[i].elements[j].distance.text;
                    console.log('Distance from ' + origin + ' to ' + destination + ' is ' + distance);
                    obj.distance = distance;
                  } else {
                    console.log(destination + ' is not reachable by land from ' + origin);
                    obj.distance = 'N/A';
                  }
                }
              }
              callback(null);
            }
            });
          },function(err){
             if(err){
               return reject(err);
          }else{
              return resolve(message);
                }
         });
    });
};

Answer 2

这种情况正在发生，因为您的distance.matrix(origins, destinations, callback )是异步的。在上面的代码中，distance.matrix方法被推送到事件循环并继续执行，然后返回该方法回调被执行resolve(message)。

Answer 3

你需要阅读承诺。在我看来，你好像在考虑将承诺作为设置回调的神奇方式。 “魔术”往往意味着“我不需要理解的东西”。在这种情况下，这不是真的。

你的执行函数（即以'function（resolve，reject）'开头的函数）应该设置一个异步请求。如果正常情况下请求有回调，则在回调中放置'resolve'和'reject'。结果将是一个promise对象，它具有'then'和'catch'方法，用于请求后处理。

由于您希望用大量异步请求的结果填充矩阵，因此您需要阅读“Promise.all”以便在所有这些请求都已解决时做出反应。

Node JS的问题承诺

3 个答案: