在Windows Phone 8上运行异步REST调用时更新UI

Question

In short, I want to display a progress indicator, make a REST service call, and hide the indicator.

My progress indicator is bound to a property which when set to true or false, show or hides it. My indicator is a little spinning wheel and I obviously don't want my UI to hang when I make a the web service call, so this is done by calling an Async function.

I found some code earlier on which works great to achieve what I want to do, however there is a but!

Here is the simplified version code I'm using:

try
{
    ThreadPool.QueueUserWorkItem(async data =>
        {
            var dispatcher = Deployment.Current.Dispatcher;
            dispatcher.BeginInvoke(() =>
            {
                this.IsRunning = true;
            });

            configModel = LoginUser(loginId, password);

            dispatcher.BeginInvoke(() =>
            {
                this.IsRunning = false;
            });
        });
}
catch (Exception ex)
{
    IDialogService dialogService = this.GetService<IDialogService>();
    dialogService.Show("Unhandled Exception", ex.Message);
    dialogService = null;
}

The problem occurs when my web service fails for whatever reason ie blocked by firewall for example. Even though I have a try catch, it doesn't get caught and crashes my app.

Is there another way to achieve this without using the threadpool? What's the cleanest way to achieve this?

I really need to call the this.IsRunning as this looks after showing and hiding my progress indicator but this obviously needs to be running on the UI thread, while the rest call needs to be running on another thread.

Answer 1

You need to try/catch the actual service call, not the QueueUserWorkItem:

try
{
    configModel = LoginUser(loginId, password);
}
catch (...)
{

}

dispatcher.BeginInvoke(() =>
{
    this.IsRunning = false;
});

Otherwise, your IsRunning = false will never execute.

However

If you want the cleanest way, then you make LoginUser asynchronous so you can await it.

this.IsRunning = true;

try
{
    configModel = await LoginUser(loginId, password);
}
catch (...)
{
    // report error here
}

this.IsRunning = false;