Question

I have an application that has a class named: UploadItem. The application creates uploading tasks based on information it has, for example, an upload needs to be created to upload a file to sitex.com with this the application creates a new UploadItem and adds that to an ObservableCollection, the collection is bound to a listview.

Now comes the part that I cannot solve.. I decided to change the structure so that people can create their own plugins that can upload a file, the problem lies with the fact that the UploadItem class has properties such as:

    string _PercentagedDone;
    public string PercentageDone
    {
        get { return _PercentagedDone; }
        set { _PercentagedDone = value + "%"; NotifyPropertyChanged("PercentageDone"); }
    }

But the plugin controls on how a file is uploaded, so how would the plugin edit the PercentageDone property that is located in the UploadItem class? If there is no way to do such a thing, then is there another way to achieve the same, i.e. showing the progress on the main GUI?

Answer 1

You’ll want to define an interface for the plugins. Something like:

public interface IUploadPlugin
{
   Task<bool> Upload(IEnumerable<Stream> files);

   int Progress { get; }
}

The plugins then need to implement this interface and export themselves:

[Export(typeof(IUploadPlugin))]
public class MyUploader : IUploadPlugin, INotifyPropertyChanged
{
   // ...
}

Notice that this plugin implements INotifyPropertyChanged. This is an easy way to handle updating the progress. Fire PropertyChanged on the Progress property and then databind your ProgressBar control in the main view to this property. Make sure that you fire PropertyChanged on the UI thread.

Another option would be to fire a custom event when the property changes. You could handle this event in the main view logic and update the progress.

Notice that I’m using Task for the return. This allows the caller to wait until the upload task finishes. You could use a callback instead, but with the CTP of the next version of .NET, using Task<> will allow you to use the await keyword for your async programming. Check it out here and here.