如何使用 webclient 异步下载多个文件，但一次一个?

问题描述

很难找到使用 webclient 类异步方法下载多个文件，但一次下载一个的代码示例.

It has been surprisingly hard to find a code example of downloading multiple files using the webclient class asynchronous method, but downloading one at a time.

如何启动异步下载，但要等到第一个完成，直到第二个，等等.基本上是一个问题.

How can I initiate a async download, but wait until the first is finished until the second, etc. Basically a que.

(注意我不想使用同步方法，因为异步方法的功能增加了.)

(note I do not want to use the sync method, because of the increased functionality of the async method.)

下面的代码一次开始我所有的下载.(进度条到处都是)

The below code starts all my downloads at once. (the progress bar is all over the place)

private void downloadFile(string url)
        {
            WebClient client = new WebClient();

            client.DownloadProgressChanged += new DownloadProgressChangedEventHandler(client_DownloadProgressChanged);
            client.DownloadFileCompleted += new AsyncCompletedEventHandler(client_DownloadFileCompleted);

            // Starts the download
            btnGetDownload.Text = "Downloading...";
            btnGetDownload.Enabled = false;
            progressBar1.Visible = true;
            lblFileName.Text = url;
            lblFileName.Visible = true;
            string FileName = url.Substring(url.LastIndexOf("/") + 1,
                            (url.Length - url.LastIndexOf("/") - 1));
             client.DownloadFileAsync(new Uri(url), "C:\Test4\" + FileName);

        }

        void client_DownloadFileCompleted(object sender, AsyncCompletedEventArgs e)
        {

        }

        void client_DownloadProgressChanged(object sender, DownloadProgressChangedEventArgs e)
        {
            double bytesIn = double.Parse(e.BytesReceived.ToString());
            double totalBytes = double.Parse(e.TotalBytesToReceive.ToString());
            double percentage = bytesIn / totalBytes * 100;
            progressBar1.Value = int.Parse(Math.Truncate(percentage).ToString());
        }

推荐答案

我所做的是填充一个包含我所有 url 的队列，然后我下载队列中的每个项目.当没有物品剩余时，我可以处理所有物品.我在下面模拟了一些代码.请记住，下面的代码用于下载字符串而不是文件.修改下面的代码应该不会太难.

What I have done is populate a Queue containing all my urls, then I download each item in the queue. When there are no items left, I can then process all the items. I've mocked some code up below. Keep in mind, the code below is for downloading strings and not files. It shouldn't be too difficult to modify the below code.

    private Queue<string> _items = new Queue<string>();
    private List<string> _results = new List<string>();

    private void PopulateItemsQueue()
    {
        _items.Enqueue("some_url_here");
        _items.Enqueue("perhaps_another_here");
        _items.Enqueue("and_a_third_item_as_well");

        DownloadItem();
    }

    private void DownloadItem()
    {
        if (_items.Any())
        {
            var nextItem = _items.Dequeue();

            var webClient = new WebClient();
            webClient.DownloadStringCompleted += OnGetDownloadedStringCompleted;
            webClient.DownloadStringAsync(new Uri(nextItem));
            return;
        }

        ProcessResults(_results);
    }

    private void OnGetDownloadedStringCompleted(object sender, DownloadStringCompletedEventArgs e)
    {
        if (e.Error == null && !string.IsNullOrEmpty(e.Result))
        {
            // do something with e.Result string.
            _results.Add(e.Result);
        }
        DownloadItem();
    }

我已经修改了您的代码以使用队列.不完全确定您希望如何取得进展.我敢肯定，如果您希望进度满足所有下载，那么您可以将项目计数存储在PopulateItemsQueue()"方法中，并在进度更改方法中使用该字段.

I've modified your code to use a Queue. Not entirely sure how you wanted progress to work. I'm sure if you wanted the progress to cater for all downloads, then you could store the item count in the 'PopulateItemsQueue()' method and use that field in the progress changed method.

    private Queue<string> _downloadUrls = new Queue<string>();

    private void downloadFile(IEnumerable<string> urls)
    {
        foreach (var url in urls)
        {
            _downloadUrls.Enqueue(url);
        }

        // Starts the download
        btnGetDownload.Text = "Downloading...";
        btnGetDownload.Enabled = false;
        progressBar1.Visible = true;
        lblFileName.Visible = true;

        DownloadFile();
    }

    private void DownloadFile()
    {
        if (_downloadUrls.Any())
        {
            WebClient client = new WebClient();
            client.DownloadProgressChanged += client_DownloadProgressChanged;
            client.DownloadFileCompleted += client_DownloadFileCompleted;

            var url = _downloadUrls.Dequeue();
            string FileName = url.Substring(url.LastIndexOf("/") + 1,
                        (url.Length - url.LastIndexOf("/") - 1));

            client.DownloadFileAsync(new Uri(url), "C:\Test4\" + FileName);
            lblFileName.Text = url;
            return;
        }

        // End of the download
        btnGetDownload.Text = "Download Complete";
    }

    private void client_DownloadFileCompleted(object sender, AsyncCompletedEventArgs e)
    {
        if (e.Error != null)
        {
            // handle error scenario
            throw e.Error;
        }
        if (e.Cancelled)
        {
            // handle cancelled scenario
        }
        DownloadFile();
    }

    void client_DownloadProgressChanged(object sender, DownloadProgressChangedEventArgs e)
    {
        double bytesIn = double.Parse(e.BytesReceived.ToString());
        double totalBytes = double.Parse(e.TotalBytesToReceive.ToString());
        double percentage = bytesIn / totalBytes * 100;
        progressBar1.Value = int.Parse(Math.Truncate(percentage).ToString());
    }

这篇关于如何使用 webclient 异步下载多个文件，但一次一个?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！