跟踪所有正在运行的块

Question

我有对象列表。每个对象都包含可选的图像值，需要下载。我想在所有图像初始化后返回列表（如果有的话）。它与NSOPerationQueues有什么关系吗？

func getFullList(list:[AnyObject], completion:(result: [AnyObject]) -> Void {
   for item in list {
      downloadImage(item.url), completion: {(image) in
      item.image = image
   )}
   // I know this does not work here
   completion(result: list)
}

Answer 1

未经测试的代码。

func getFullList(list:[AnyObject], completion:(result: [AnyObject]) -> Void {
    let group = dispatch_group_create()

   for item in list {
        dispatch_group_enter(group)

      downloadImage(item.url), completion: {(image) in
          item.image = image

          dispatch_group_leave(group)
      }
   )}

   dispatch_group_notify(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)) {
        completion(result: list)
   }
}