在Phonegap DB函數中的函數外返回一個變量

Question

我有一個phonegap數據庫，其功能如下。我想在整個函數之外返回最後的字符串變量「feeds」。這隻返回「未定義」。請幫我所需的更改返回「飼料」 variable.`

function getProviders() { 
    var feeds = ""; 
    var db = window.openDatabase("db", "1.0", "desc", 1000000); 

    db.transaction(function(tx) { 
     var db = window.openDatabase("db", "1.0", "desc", 1000000); 
     tx.executeSql("SELECT * FROM `feed_provider`", [], function(tx, results) { 
      var len = results.rows.length; 

      for (var i = 0; i < len; i++) { 
       feeds += results.rows.item(i).id + "|" + results.rows.item(i).name + "|" + results.rows.item(i).status + "|" + results.rows.item(i).feed_url + ","; 
      } 
     }, sqlerror); 
    }, sqlerror2); 
    return feeds; 
} 

Answer 1

我要去承擔任何db.transaction或tx.executeSql是異步，在這種情況下，我會用一個deferred：

function getProviders() { 
    var feeds = ""; 
    var def = $.Deferred(); 
    var db = window.openDatabase("db", "1.0", "desc", 1000000); 

    db.transaction(function(tx) { 
     var db = window.openDatabase("db", "1.0", "desc", 1000000); 
     tx.executeSql("SELECT * FROM `feed_provider`", [], function(tx, results) { 
      var len = results.rows.length; 

      for (var i = 0; i < len; i++) { 
       feeds += results.rows.item(i).id + "|" + results.rows.item(i).name + "|" + results.rows.item(i).status + "|" + results.rows.item(i).feed_url + ","; 
      } 
      def.resolve(feeds); 
     }, sqlerror); 
    }, sqlerror2); 
    return def.promise(); 
} 

這樣稱呼：

getProviders().done(function(feeds) { 
    // do something with feeds 
});