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？

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) 
    } 
}