为什么用这种方式更新UITableView的单元格是错误的？

Question

I am making a download manager I have been struggling with updating the cells because of the " reusable cells "....

I had problems with updating the cells after a scroll but I found a workaround that doesn't work properly, this way is leading the app to crash after the cell becomes unvisible, I don't know why I hope you guys can explain, and if you know how to fix it please let me know.

I add a download task this way

func addDownloadTask(URL: NSURL) {

    let sessionConfiguration = NSURLSessionConfiguration.defaultSessionConfiguration()
    let mainQueue = NSOperationQueue.mainQueue()
    let session = NSURLSession(configuration: sessionConfiguration, delegate: self, delegateQueue: mainQueue).downloadTaskWithURL(URL)

    let newDownload = RCHDownloadAddictModelClass(fileURL: URL)

    newDownload.downloadTask = session

    downloadingArray.append(newDownload)

    newDownload.downloadIndex = downloadingArray.indexOf(downloadingArray.last!)

    self.tableView.reloadData()

    session.resume()

}

When a NSURLSessionDownloadTask begins downloading this method get invoked

func URLSession(session: NSURLSession, downloadTask: NSURLSessionDownloadTask, didWriteData bytesWritten: Int64, totalBytesWritten: Int64, totalBytesExpectedToWrite: Int64) {

    for (index, downloadModel) in downloadingArray.enumerate() {
        if downloadTask.isEqual(downloadModel.downloadTask) {
            dispatch_async(dispatch_get_main_queue(), {

                let indexPath = NSIndexPath(forRow: index, inSection: 0)
                /* xCode stops here when the app crashes */ let cell = self.tableView.cellForRowAtIndexPath(indexPath) as! RCHDownloadAddictTableViewCell 
                let countOfBytesWritten: Double!
                if totalBytesExpectedToWrite < 0 {

                    countOfBytesWritten = 0

                } else {

                    countOfBytesWritten  = Double(totalBytesWritten) / Double(totalBytesExpectedToWrite)

                }

                downloadModel.fileName = downloadTask.response?.suggestedFilename
                downloadModel.downloadIndex = downloadingArray.indexOf(downloadModel)
                downloadModel.downloadSize = Double(totalBytesExpectedToWrite)
                downloadModel.downloadProgress = Float(countOfBytesWritten)
                downloadModel.downloadTaskIdentifier = downloadTask.taskIdentifier
                downloadModel.didFinishDownload = false

                self.updateCell(cell, forRowAt: indexPath)

            })
        }
    }


}

and this is the way the cell gets updated:

override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {

    let cell = tableView.dequeueReusableCellWithIdentifier("downloadCell", forIndexPath: indexPath) as! RCHDownloadAddictTableViewCell

    updateCell(cell, forRowAt: indexPath)

    return cell
}



func updateCell(cell: RCHDownloadAddictTableViewCell, forRowAt indexPath: NSIndexPath) {

    let arrayInfo = downloadingArray[indexPath.row]

    cell.cellProgressView.setProgress((arrayInfo.downloadProgress)!, animated: true)
    cell.cellFileName.text = arrayInfo.fileName
    cell.cellDownloadSpeed.text = String(format: "%.1fMB", (arrayInfo.downloadSize / 1000000))
    cell.cellBlock = {

        switch arrayInfo.downloadTask.state {
        case .Running:
            arrayInfo.downloadTask.suspend()
            cell.cellButton.setImage(UIImage(named: "resume.png"), forState: UIControlState.Normal)
        case .Suspended:
            arrayInfo.downloadTask.resume()
            cell.cellButton.setImage(UIImage(named: "pause.png"), forState: UIControlState.Normal)
        default:
            arrayInfo.downloadTask.suspend()
            cell.cellButton.setImage(UIImage(named: "resume.png"), forState: UIControlState.Normal)
        }

    }

    if (arrayInfo.didFinishDownload == true) {
        cell.cellButton.hidden = true
        cell.cellFinishIndicator.text = "Finished."
        cell.cellProgressView.hidden = true
        cell.cellFinishIndicator.hidden = false
    } else {
        cell.cellButton.hidden = false
        cell.cellProgressView.hidden = false
        cell.cellFinishIndicator.hidden = true
    }

}

Answer 1

you can see it at the apple doc :

public func cellForRowAtIndexPath(indexPath: NSIndexPath) -> UITableViewCell? 
// returns nil if cell is not visible or index path is out of range

//cell will be nil when this row is not visible
let cell = self.tableView.cellForRowAtIndexPath(indexPath) as! RCHDownloadAddictTableViewCell 

then you call self.updateCell(cell, forRowAt: indexPath) it will crash

Answer 2

You are on the right path with your code with one exception.

After your download has finished and you have stored the correct data into the array in the correct place you should run the code...

tableView.reloadRowsAtIndexPaths([indexPath], withRowAnimation:.Automatic)

This will then trigger the table to reload the cell at the correct index path. If that cell is on screen it will reload. If that cell is not on screen it will not reload. But the next time the cell is displayed it will now show the correct information.

In essence. Don't update the cell directly. Just tell the tableview that you have changed the data for that particular row.