Question

How can you make a background web request and then update the UI, but have all the code that does the web requesting/parsing in a separate class so you can use it in multiple places? I thought I could use the classes methods as event handlers for a BackgroundWorker class, like

APIHelper mHelper = new APIHelper("http://example.com?foo=bar");
BackgroundWorker bw = new BackgroundWorker();
bw.DoWork +=new DoWorkEventHandler(mHelper.GetResponse);
bw.RunWorkerCompleted +=new RunWorkerCompletedEventHandler(mHelper.HandleResponse);
bw.RunWorkerAsync();

where APIHelper has the method

public void GetResponse(object sender, DoWorkEventArgs e)
{
    BackgroundWorker worker = (BackgroundWorker) sender;

    WebRequest request = HttpWebRequest.Create(this.URL);
    IAsyncResult result = (IAsyncResult)
                              request.BeginGetResponse(ResponseCallback, request);
}

but then I don’t know how to access the worker thread from ResponseCallback and, anyway, HandleResponse gets called first (obviously). (I tried putting in result.AsyncWaitHandle.WaitOne(); but I get a NotSupportedException error.) Yet I can’t work out how to make the web request call synchronously. I’m clearly trying to go about this the wrong way, but I have no idea what the right way is.

---

ETA:

My aim is to be able to go:

• user clicks (a) button(s) (on various pages)

• a “working” message is displayed on the UI thread (and then input is blocked)

• in a background thread my APIHelper class makes the relevant API call, gets the response, and passes it back to the UI thread; I only seem to be able to do this by starting another thread and waiting for that to return, because there’s no synchronous web requests

• the UI thread updates with the returned message (and input continues as before)

I can do the first two bits, and if I have the response, I can do the last bits, but I can’t work out how to do the middle bit. Hopefully that made it clearer!

Answer 1

It took me several tried before I found there is a Dispatcher.

During the BackgroundWorker‘s dowork and complete methods you can call:

this.Dispatcher.BeginInvoke(() =>
{
// UPDATE UI BITS
});

I think the Dispatcher is only available in the view. So I’m not sure if the methods can exist outside of the xaml.cs

Put whatever you want to update in your UI; when updating an ObservableCollection you must do the update of you items in the Dispatcher.BeginInvoke too

This link might be a good read too:
http://www.windowsphonegeek.com/articles/All-about-Splash-Screens-in-WP7-ndash-Creating-animated-Splash-Screen

Update to assist notes

This is just a rough idea mind you…

bw.DoWork +=new DoWorkEventHandler(DoWork);
bw.RunWorkerCompleted +=new RunWorkerCompletedEventHandler(Complete)

// At least I think the EA is DoWork....
public void DoWork(object sender, DoWorkEventArgs e)
{
        mHelper.GetResponse();
        this.Dispatcher.BeginInvoke(() =>
        {
              UIObject.Visibility Collapse.
        });

        // Wait and do work with response.
    });
}

public void Complete(object sender, RunWorkerCompleteEventArgs e)
{
        this.Dispatcher.BeginInvoke(() =>
        {
              UIObject.Visible ....
        });
}