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The structure of the repeating property pattern

I have a class DownloadManager

that manages multiple objects DownloadItem

. Everyone DownloadItem

has events like ProgressChanged

and DownloadCompleted

. Typically you want to use the same event handler for all loaded items, so it's a little annoying to have to set event handlers multiple times for each DownloadItem

.

Thus, I need to decide which pattern to use:

  • Use one DownloadItem

    as a template and clone it if needed

        var dm = DownloadManager();
    var di = DownloadItem();
    di.ProgressChanged += new DownloadProgressChangedEventHandler(di_ProgressChanged);
    di.DownloadCompleted += new DownloadProgressChangedEventHandler(di_DownloadCompleted);
    DownloadItem newDi;
    newDi = di.Clone();
    newDi.Uri = "http://google.com";
    dm.Enqueue(newDi);
    newDi = di.Clone();
    newDi.Uri = "http://yahoo.com";
    dm.Enqueue(newDi);

  • Set event handlers to DownloadManager

    and copy events to each DownloadItem

    one that is enqueued.

        var dm = DownloadManager();
    dm.ProgressChanged += new DownloadProgressChangedEventHandler(di_ProgressChanged);
    dm.DownloadCompleted += new DownloadProgressChangedEventHandler(di_DownloadCompleted);
    dm.Enqueue(new DownloadItem("http://google.com"));
    dm.Enqueue(new DownloadItem("http://yahoo.com"));

  • Or use some kind of factory

        var dm = DownloadManager();
    var dif = DownloadItemFactory();
    dif.ProgressChanged += new DownloadProgressChangedEventHandler(di_ProgressChanged);
    dif.DownloadCompleted += new DownloadProgressChangedEventHandler(di_DownloadCompleted);
    dm.Enqueue(dif.Create("http://google.com"));
    dm.Enqueue(dif.Create("http://yahoo.com"));

What would you recommend?

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2 answers


Why are DownloadItems responsible for reporting (in terms of API design)?

I would say the DownloadManager is responsible for downloading the DownloadItems and therefore also for reporting. (Of course, the internal implementation strategy may differ.)

I would go with the second option:



var dm = DownloadManager
{
    "http://google.com",
    new DownloadItem("http://yahoo.com") { Retries = 5 }
};

dm.ProgressChanged += (sender, e) =>
    Console.WriteLine("Download {0}: {1:P}", e.Uri, (double)e.Progress / 100.0);

dm.DownloadCompleted += (sender, e) =>
    Console.WriteLine("Download {0}: completed!", e.Uri);

dm.DownloadAllCompleted += (sender, e) =>
    Console.WriteLine("All downloads completed!");

dm.Add("http://stackoverflow.com");
dm.DownloadAllAsync();

If you have a copy of the Framework Design Guide (2nd ed.) On hand, see pages 305-33 (Event-Based Asynchronous Pattern).

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I would say that a Template template with a factory would be the right approach for this.



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