This project is intended to show how simple it is to implement an asynchronous downloader of an unlimited number of files, using the
WebClient class. The key features are implemented using
DownloadFileAsync and few callback functions which report progress information about active download.
The project does not use any additional threads. Everything is implemented using asynchronous callback methods which are called by the
WebClient object when the internal state of the downloader changes.
Some Screen Shots
First of all, look at the program window when some parallel downloads are in progress. It’s possible to add as many download tasks as you want.
Figure 1. Multiple asynchronous download in progress.
Figure 2. Multiple asynchronous downloads finished. One has been cancelled and the second is completed.
Figure 3. Interaction diagram.
Initially, a main process is created which brings up to the user the main window with a input field for URI, an add button which starts the download process, and
listview which is used to display status (in real time) of each
WebClient (Figure 3). After the user presses the Add button ([+]), the
DownloaderTask object creates a new downloader and assigns to it Progress Callback and End Callback events.
The download is started in the same turn. The resource is saved to the current folder and the name is automatically extracted from the last segment of the URI:
fileName = this.uriData.Segments[this.uriData.Segments.Length - 1];
Figure 4. Class diagram.
When the user presses the add button, a
DownloaderTask object is created and added to the
dwnTasks list (Figure 4):
dwnTasks.Add(new DownloaderTask(tbUri.Text, lvDownloads));
The first parameter represents the URI and the second is a
listview, where the download task object itself creates an informational group for the downloaded file and continuously updates it. The
DownloaderTask constructor will create a
WebClient() object, then will assign callbacks:
webClient.DownloadFileCompleted += new System.ComponentModel.AsyncCompletedEventHandler
Also the constructor of the
DownloaderTask object creates a group in passed
listview (if not
null) which will describe the current status of the downloadable file and the operation result at the end (See figure 2).
There are, of course, some TO DO items like:
- Remove incomplete downloaded files (after failure or cancellation)
- Allow a user to select destination folder and file name, etc.
- 27th May, 2008: Initial version