Flutter等待异步内容

有时候需要弹出弹窗等待用户选择某条件后再执行后续代码，可以用Completer类。 

//事例

    var c = Completer<bool>();

    Dialogs.tipsCard(title, tip,
      failMsg: '',
      actions: [i18n.ok],
      callback: (index) => c.complete(true),
      dismissCallBack: () {
        if (!c.isCompleted)
          c.complete(false);
      },
    );

    return c.future;