Question

如何在两个嵌套的＆＃39; for＆＃39;中进行同步调用？ Node.JS中的循环：这是我的异步调用;我不知道如何拥有我的代码的同步版本，当create_db完成时，它会进入下一次迭代！

    var new_items = [];
    for (var key in config.models) {
       var selected_field = config.models[key];
       for (var item in config.models[key].fields) {
          var selected_item = config.models[key].fields[item];
          new_items.push({AttributeName: selected_item.name, AttributeType: selected_item.type});     
       }
       CreateDB(selected_field.name, new_items);       
       var selected_item = config.models[key].fields[item];

   }

EDDITED：在create_db中我使用回调函数！

function CreateDB(name, new_items) {

ddb.createTable(name, {hash: ['id', ddb.schemaTypes().number],
    range: ['time', ddb.schemaTypes().string],
    AttributeDefinitions: new_items
},
{read: 1, write: 1}, function(err, details) {
    console.log("The DB is now created!");
});

}

由于

Answer 1

您需要修改CreateDB函数，以便进行回调，然后在.createTable的回调中调用该回调：

function CreateDB(name, new_items, callback) {
    ddb.createTable(name, {hash: ['id', ddb.schemaTypes().number],
        range: ['time', ddb.schemaTypes().string],
        AttributeDefinitions: new_items
    },
    {read: 1, write: 1}, function(err, details) {
        console.log("The DB is now created!");
        callback(err, details);
    });
}

然后您可以使用async模块each method之类的内容。类似的东西：

var async = require('async');

async.each(Object.keys(config.models), function(key, callback) {
   var new_items = [];
   var selected_field = config.models[key];
   for (var item in config.models[key].fields) {
      var selected_item = config.models[key].fields[item];
      new_items.push({AttributeName: selected_item.name, AttributeType: selected_item.type});     
   }
   CreateDB(selected_field.name, new_items, callback);
}, function(err) {
    // All done with CreateDB calls
});

Answer 2

我建议使用async模块。它具有实用功能，使我认为你想要做的事情非常容易。

async.each(Object.keys(config.models), function(key, done) {
    var selected_field = config.models[key];

    // I used the 'map' function to make it a bit easier to read
    var new_items = selected_field.fields.map(function(item) {
        return {
            AttributeName: item.name
            AttributeType: item.type
        };
    });

    CreateDB(selected_field, new_items, done);

}, function(err) {
    // this gets called when all db calls are finished
    console.log("The DB is now created!");
});

function CreateDB(name, new_items, callback) {
    ddb.createTable(name, {
        hash: ['id', ddb.schemaTypes().number],
        range: ['time', ddb.schemaTypes().string],
        AttributeDefinitions: new_items
    }, {
        read: 1,
        write: 1
    }, callback);
}

Answer 3

您可以尝试使用Futures之类的内容，尤其是序列代码，例如：sequence

希望这有帮助！

编辑：我相信你也可以使用更加内置的es6-promise方法。但是，我对此并不了解...... :)

两个嵌套循环中的同步调用

3 个答案: