arrays - 仅在发出多个请求后调用函数

Question

我正在使用 for-in 循环为数组中的每一项使用 Alamofire 发出 HTTP 请求。我想在收到所有回复后调用一个函数:

for product in products {
    let requestURL = "http://api.com/" + product
    let parameters = ["apiKey" : "myApiKey"]
    Alamofire.request(.GET, requestURL, parameters: parameters)                  
        .responseJSON { response in
             // do stuff here
        }
}

为了在完成时调用一个函数，我想我可以检查 product 是否是数组的最后一个元素，然后如果是这样就调用该函数(因为请求是异步的).我该怎么做？

Answer 1

您应该使用 GCD 在所有请求完成时得到通知。使用 dispatch_group_create 和 dispatch_group_notify。有关实现细节，请查看此 thread .

来自链接线程的示例代码:

func downloadAllData(allDataDownloadedCompletionHandler:()->Void) {
    let dispatchGroup: dispatch_group_t = dispatch_group_create()
    let types = ["one", "two", "three"]  // there are actually about 10 requests called, but to make it simple I set it to 3
    for type in types {
        // enter group and run request
        dispatch_group_enter(dispatchGroup)
        self.downloadDataForType(type, group: dispatchGroup)
    }

    dispatch_group_notify(dispatchGroup, dispatch_get_main_queue(), {
        allDataDownloadedCompletionHandler()
    });
}

func downloadDataForType(type:String, group: dispatch_group_t) {
    Alamofire.request(Router.TypeData(type: type)).response({ (request, response, xmlResponse, error) -> Void in
        // request finished
        println("Data for type \(type) downloaded")

        // let's parse response in different queue, because we don't want to hold main UI queue 
        var db_queue = dispatch_queue_create("db_queue", nil)
            dispatch_async(db_queue, {
            if response?.statusCode == 200 {
               saveToDatabase(xmlResponse)
           }

            // leave group
            dispatch_group_leave(group)
        })
    })
}