等待其他期货筹集的期货(Waiting for Futures raised by other Futures)

 我正在使用Lawndart库来访问浏览器数据，并希望收集一组查询的结果。 这是我认为应该工作的： 
 
  numberOfRecordsPerSection(callback) {
    var map = new Map();

    db_sections.keys().forEach((_key) {
      db_sections.getByKey(_key).then((Map _section) {
        int count = _section.length;
        map[_key] = count;
      });
    }).then(callback(map));
  }
 
 但是，当调用回调时， map仍为空（它会正确填充，但稍后，在所有Futures完成后）。 我假设问题是由getByKey()调用创建的Futures不会被forEach()调用创建的Futures“捕获”。 
 
 如何更正我的代码以正确捕获结果？ 

I'm using the Lawndart library to access browser data, and want to collect the results of a set of queries. Here's what I thought should work:
 
  numberOfRecordsPerSection(callback) {
    var map = new Map();

    db_sections.keys().forEach((_key) {
      db_sections.getByKey(_key).then((Map _section) {
        int count = _section.length;
        map[_key] = count;
      });
    }).then(callback(map));
  }
 
However, when the callback is called, map is still empty (it gets populated correctly, but later, after all the Futures have completed). I assume the problem is that the Futures created by the getByKey() calls are not "captured by" the Futures created by the forEach() calls. 
 
How can I correct my code to capture the result correctly?

原文：https://stackoverflow.com/questions/23969680