WebClient - wait until file has downloaded

问题

I'm developing a function to return a collection, generated from an xml file.

Initially, I was using a local xml file for testing, but now I'm ready to have the app download the real xml file from a server. I'm struggling to see how I could do this due to the fact a WebClient object needs to be given an OpenReadCompleted event handler - I cannot return the collection data from this, and also by the time this handler executes, the original function has ended.

My original code is as follows:

public static ObservableCollection<OutletViewModel> GetNear(GeoCoordinate location)
{
    ObservableCollection<OutletViewModel> Items = new ObservableCollection<OutletViewModel>();

    // Load a local XML doc to simulate server response for location
    XDocument xDoc = XDocument.Load("SampleRemoteServer/outlet_list.xml");

    foreach (XElement outlet in xDoc.Descendants("outlet"))
    {
        Items.Add(new OutletViewModel()
        {
            Name = outlet.Attribute("name").Value,
            Cuisine = outlet.Attribute("cuisine").Value
        });
    }

    return Items;
}

How can I load the file in this function, have the event handler run, and then continue the function?

The only was I can think of is to add a loop to keep checking a variable, which is updated by the event handler code... and that doesn't sound like a good solution.

Thanks, Josh

回答1:

You should start to take a look at async programming. One (old school) way would be to implement a public event and subscribe to that event in the calling class.

However, using callbacks is more elegant. I whipped up a simple (useless, but still conceptually valid) example that you can build upon:

public static void Main(string[] args)
{
  List<string> list = new List<string>();

  GetData(data =>
  {
    foreach (var item in data)
    {
      list.Add(item);
      Console.WriteLine(item);
    }
    Console.WriteLine("Done");
  });
  Console.ReadLine();
}

public static void GetData(Action<IEnumerable<string>> callback)
{
  WebClient webClient = new WebClient();
  webClient.DownloadStringCompleted += (s, e) =>
    {
      List<string> data = new List<string>();
      for (int i = 0; i < 5; i++)
      {
        data.Add(e.Result);
      }
      callback(e.Error == null ? data : Enumerable.Empty<string>());
    };

  webClient.DownloadStringAsync(new Uri("http://www.google.com"));
}

If you want to jump onto the C# async bandwagon (link for WP7 implementation), you can implement it using the new async and await keywords:

public static async void DoSomeThing()
{
  List<string> list = new List<string>();
  list = await GetDataAsync();

  foreach (var item in list)
  {
    Console.WriteLine(item);
  }
}

public static async Task<List<string>> GetDataAsync()
{
  WebClient webClient = new WebClient();
  string result = await webClient.DownloadStringTaskAsync(new Uri("http://www.google.com"));

  List<string> data = new List<string>();
  for (int i = 0; i < 5; i++)
  {
    data.Add(result);
  }
  return data;
}

回答2:

You move the foreach() loop to the completed event.

And that indeed means you cannot return anything from the original method. Make it a void.

This is how async I/O works, better get used to it. You will need to rethink your design.

来源：https://stackoverflow.com/questions/9727928/webclient-wait-until-file-has-downloaded

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