ListView Xamarin 表单中的下载进度答案

【问题标题】：Downloadprogress in ListView Xamarin FormsListView Xamarin 表单中的下载进度
【发布时间】：2017-11-26 19:09:24
【问题描述】：

我有一个 ListView（如 Xamarin Forms MasterDetails-Template），我想在其中显示项目的标题和下载进度（以百分比表示）。

我创建了一个从 url 下载数据的类。在此类中，我使用 WebClient 下载文件并使用 downloadProgress-EventHandler 更改全局变量以获取百分比进度。

如何刷新我的 ListView 以百分比显示进度？

谢谢

    public class DownloadHelper
{
    string url;
    string saveToPath;
    string filename;

    private string downloadError = "";
    private int downloadProgress = -1;
    private long totalBytesToReceive = -1;
    private long bytesReceived = -1;
    private long bytesThatRest = -1;
    private bool downloadfinished = false;

    public string DownloadError { get => downloadError;}
    public int DownloadProgress { get => downloadProgress;}
    public long TotalBytesToReceive { get => totalBytesToReceive;}
    public long BytesReceived { get => bytesReceived;}
    public long BytesThatRest { get => bytesThatRest;}
    public bool Downloadfinished { get => downloadfinished;}

    public DownloadHelper(string url, string saveToPath, string filename)
    {
        ///Prepare a download.
        this.url = url;
        this.saveToPath = saveToPath;
        this.filename = filename;
    }

    async public void DownloadZip()
    {
        ///Download an Unzip a Zip-File in the saveToDirektory.
        var filenameAndPath = Path.Combine(saveToPath, filename);
        string foldernameAndPath = saveToPath.Replace(".zip", "");

        if (Directory.Exists(foldernameAndPath))
        {
            Directory.Delete(foldernameAndPath, true);
        }
        Directory.CreateDirectory(foldernameAndPath);

        var webClient = new WebClient();
        var urlen = new Uri(url);
        await webClient.DownloadFileTaskAsync(urlen, filenameAndPath);

        webClient.DownloadProgressChanged += (s, e) =>
        {
            downloadProgress = e.ProgressPercentage;
            totalBytesToReceive = e.TotalBytesToReceive;
            bytesReceived = e.BytesReceived;
            bytesThatRest = TotalBytesToReceive - BytesReceived;
        };

        webClient.DownloadFileCompleted += (s, e) =>
        {
            downloadfinished = true;

            if (downloadProgress != 100)
            {
                downloadError = e.Error.Message.ToString();
            }
            else
            {
                //Unzip downloaded Zipfile
                ZipFile.ExtractToDirectory(filenameAndPath, foldernameAndPath);

                //Delete unused Zip-File
                File.Delete(filenameAndPath);
            }
        };
    } 
}

public partial class DownloadedView : ContentPage, INotifyPropertyChanged
{

    ItemsViewModel viewModel;
    public DownloadedView(List<Item> items)
    {
        InitializeComponent();
        BindingContext = viewModel = new ItemsViewModel(items);
    }

    void Handle_ItemTapped(object sender, ItemTappedEventArgs e)
    {
        var item = (Item)((ListView)sender).SelectedItem;
        string selectedItemName = item.Text;

        switch (selectedItemName)
        {
            case "download1":
                string downloadAddress = "";
                string destinationFolder = "";
                string filename = "";
                break;

            case "download2":
                string downloadAddress = "";
                string destinationFolder = "";
                string filename = "";
                break;

            case "download3":
                string downloadAddress = "";
                string destinationFolder = "";
                string filename = "";
                break;
        }

        DownloadHelper dh = download(downloadAddress, destinationFolder, filename);

        //UPDATE DOWNLOAD PROGRESS HERE.
        //WITH: dh.DownloadProgress

        //Deselect Item
        ((ListView)sender).SelectedItem = null;
    }

    DownloadHelper download(string downloadAddress, string destinationFolder, string filename)
    {
        DownloadHelper dh = new DownloadHelper(downloadAddress, destinationFolder, filename);

        dh.DownloadZip();
        return dh;
    }
}

【问题讨论】：

如果数据源中的每个项目都有 PerecentComplete 属性，则只需更新该属性并使用 INotifyPropertyChanged 更新 UI
我试过了，但没用。你能给我举个例子吗？此外，我注意到，如果我按下后退按钮然后重新打开视图，视图将会更新。

标签： c# download xamarin.forms

【解决方案1】：

现在我解决了。我必须在 MainThread 上更新我的视图模型（因为 GUI 正在其上运行）。

我做了一个更新功能

void UpdateGUI(Item item, string newItemText){
item.Description = newItemText;
viewModel.DataStore.UpdateItemAsync(item);
viewModel.LoadItemsCommand.Execute(null);}

在我必须更新 GUI 的地方这样调用它：

Device.BeginInvokeOnMainThread(() => UpdateGUI(item, "newItemText..."));

【讨论】：