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我正在尝试将以下 javascript/jquery 代码转换为 dart，但我在理解期货如何工作时遇到问题。

function fnA() {
    fnB().then(function() {
        // do something
    }
}

function fnB() {
   var ret = $.Deferred();

   _db.open(database_name).then(function() {
       var defers = [];

       _db.keys().forEach(function(key_name) {
          var key_dfd = $.Deferred();
          defers.push(key_dfd);
          _db.getByKey(key_name).then(function(data) {
              key_dfd.resolve(data);
          });
       });
       $.when(defers).then(ret.resolve);
   })
   .fail(function() {
       ret.reject();
   });

   return ret.promise();
}

我想我可以像下面那样做，但它似乎不起作用：

（我正在使用草地飞镖作为数据库）

void fnA() {
    fnB().then((_) {
        // Here, all keys should have been loaded
    });
}

Future fnB() {
    return _db.open().then((_) {
        return _db.keys().forEach((String key_name) {
             return _db.getByKey(key_name).then((String data) {
                  // do something with data
                  return data;
             });
        });
    });
}
4

2 回答 2

1

我做了一些改进，但没有测试代码：

void fnA() {
    fnB().then((_) {
        // Here, all keys should have been loaded
    });
}

Future fnB() {
  return _db.open().then((_) {
    List<Future> futures = [];
    return _db.keys().forEach((String key_name) { 
      futures.add(_db.getByKey(key_name).then((String data) {
        // do something with data
        return data;
      }));
    }).then(() => Future.wait(futures));
  });
}
于 2014-05-09T04:57:26.157 回答
0

Sticking with JavaScript/jQuery for a moment, fnB can (assuming compatibility of Promise implementations) be written more efficiently as follows : 

function fnB() {
    return _db.open(database_name).then(function() {
        return $.when.apply(null, _db.keys().map(function(key_name) {
            return _db.getByKey(key_name);
        }));
    });
}

As far as I can tell from the Dart documentation, this should translate into Dart something like this : 

function fnB() {
    return _db.open().then(function() {
        return Future.wait(_db.keys().map(function(key_name) {
            return _db.getByKey(key_name);
        }));
    });
}

The assumption about Promise compatibility disappears on translation, but the above code assumes that _db.keys() returns an Array. If this assumption is not valid, then the code will be slightly different.

fnA will be something like this :

function fnA() {
    fnB()
        .then((List values) => doSomething(values))
        .catchError((e) => handleError(e));
}

That's the essence of it. You may need to work at the detail.

于 2014-05-09T01:14:55.540 回答