使用promise異步遍歷一組文件/ defer

Question

我的目標是遍歷文件目錄，對每個文件運行一些操作，並返回包含目錄子集的json對象。

我得到它在Node的fs庫中使用同步版本的調用，但我想找出最好的異步解決方案。我在異步版本上的失敗嘗試在下面使用Q庫推遲。無論我做什麼，我都無法推遲最後一步，直到迭代完成。

迭代成功完成，但不是在調用sendResponse（）之前。

任何人都可以幫助我理解我做錯了什麼嗎？

router.get('/mediaTree', function(req, res){ 
    var mediaTree = { "identifier" : "id", "label" : "name", "items" : []}; 
    var idCounter = 1; 

    var fs_readdir = q.denodeify(fs.readdir); 

    fs_readdir(MEDIAPATH) 
    .then(function(files) { 
     files.forEach(function(dir) { 
      fs.stat(MEDIAPATH + dir, function(err, stats) { 
      if(err) { console.log(err); return; } 

      var thisFile = {}; 
      if(stats.isDirectory()) { 
      thisFile.id = idCounter++; 
      thisFile.type = "branch"; 
      thisFile.name = dir; 
      thisFile.path = MEDIAPATH + dir; 
      mediaTree.items.push(thisFile); 
      } 
      }); 
     }); 
    }) 
    .then(sendResponse); 


    function sendResponse() { 
    res.json(mediaTree); 
    } 
}); 

Answer 1

爲了讓您的上述代碼正常工作，您必須將promise用於其完整擴展。請參閱MDN或有關承諾如何詳細工作的類似來源。

鑑於此，您應該將fs.stat包裝在承諾中。這樣的承諾管理等待你的結果，並給你一個選擇，以更多的同步問題運行你的大部分代碼。

var q = require('q'); // implied from your code sample 
var path = require('path'); // joining paths with "+" might fail 

router.get('/mediaTree', function(req, res){ 
    var qreaddir = q.denodeify(fs.readdir); 
    var qstat = q.denodeify(fs.stat); 

    qreaddir(MEDIAPATH) 
    .then(function(files) { 
     // from inside out 
     // - a promise for the fs.stat of a single file, 
     // EDIT: Containing an object ob both the result and the dir 
     // - an array of these promises created via Array.prototype.map from the files 
     // - a promise from this array 
     return q.all(files.map(function(dir) { 
      return qstat(path.join(MEDIAPATH, dir)) 
       .then(function(stat) { // EDIT: extending the original answer 
        return { 
         dir: dir, // the dir from the outer outer scope 
         stat: stat, // the stats result from the qstat promise 
        }; 
       }); 
     })); 
    }) 
    .then(function(results) { 
     // Promises should have no side effects, declare vars in function scope 
     var mediaTree = { 
      identifier: "id", 
      label: "name", 
      items: [] 
     }; 
     var idCounter = 1; 

     // since we now have a sync array, we can continue as needed. 
     results.forEach(function(result) { 
      // EDIT: The original answer had the stats as immediate result 
      // Now we get an object with both the dir and it's stat result. 

      var stats = result.stats; 
      var dir = result.dir; // Use this as needed. 

      if (stats.isDirectory()) { 
       var thisFile = {}; 

       thisFile.id = idCounter++; 
       thisFile.type = "branch"; 
       thisFile.name = dir; 
       thisFile.path = path.join(MEDIAPATH, dir); 

       mediaTree.items.push(thisFile); 
      } 
     }); 

     return res.json(mediaTree); 
    }) 
    .catch(function(err) { 
     // Failsafe. Will log errors from all promises above. 
     console.log(err); 
    }); 
});