Swift，NSOperationQueue.mainQueue（）：在运行过程中更新运行中的数据？

Question

I use background file downloading in IOS project, and when file downloading is started, progress view's update is started by block NSOperationQueue.mainQueue().addOperationWithBlock() in method:

func URLSession(session: NSURLSession, downloadTask: NSURLSessionDownloadTask, didWriteData bytesWritten: Int64, totalBytesWritten: Int64, totalBytesExpectedToWrite: Int64) {
    if totalBytesExpectedToWrite == NSURLSessionTransferSizeUnknown {
       println("Unknown transfer size");
    }
     else{
       let data = getDBInfoWithTaskIdentifier(downloadTask.taskIdentifier)

       NSOperationQueue.mainQueue().addOperationWithBlock(){
           data.db_info.downloadProgress = Float(totalBytesWritten) / Float(totalBytesExpectedToWrite)
            let inf = data.db_info.indexPath
            let cell = self.tableView.cellForRowAtIndexPath(NSIndexPath(forRow: data.index, inSection: 0)) as SettingsTableViewCell
            cell.downloadProgress.hidden = false
            cell.downloadProgress.setProgress(data.db_info.downloadProgress, animated: true)
        }
    }
}

When view with download UI is dismissed and present once more, self.tableView is new object, but self.tableView in NSOperationQueue.mainQueue() operation, which update progress view, is old one, that what was before dismissing. Println() returns two different objects. Is there possibility update data about self.tableView in that NSOperationQueue.mainQueue() block?

Answer 1

First of all you should consider switch to GCD (Grand Central Dispatch).

Your problem arises because the reference to the tableView is captured in the closure. You can consider to put the logic in a separate class function so tableView is not exposed in your closure. It has an added benefit of cleaner, more organised code.

Here is a general idea:

// new function
func setProgressInCell (data:?) { // '?' because i do not know the type
    let inf = data.db_info.indexPath
    let cell = self.tableView.cellForRowAtIndexPath(NSIndexPath(forRow: data.index, inSection: 0)) as SettingsTableViewCell
    cell.downloadProgress.hidden = false
    cell.downloadProgress.setProgress(data.db_info.downloadProgress, animated: true)

}

func URLSession(session: NSURLSession, downloadTask: NSURLSessionDownloadTask, didWriteData bytesWritten: Int64, totalBytesWritten: Int64, totalBytesExpectedToWrite: Int64) {
    if totalBytesExpectedToWrite == NSURLSessionTransferSizeUnknown {
       println("Unknown transfer size");
    }
     else{
       let data = getDBInfoWithTaskIdentifier(downloadTask.taskIdentifier)

       dispatch_async(dispatch_get_main_queue())  {
           [weak self] in
           data.db_info.downloadProgress = Float(totalBytesWritten) / Float(totalBytesExpectedToWrite)
           self.setProgressInCell(data)

        }
    }
}