how to create a synchronous news feed in UWP - asynchronous

Question

I want to create a simple news feed, I use web API to get the news updates, users can use combox select category (world news & sports news),and the news will be auto updated every 5 seconds, if I only select once, the news feed can auto updated and repeat, But if I change the selection, it start to show me both categories. here is my code
public async void NewsRepeat()
{
RootObject2 myNews = await NewsProxy.GetNews();
RootObject3 mySportNews = await sportsNewsProxy.GetSportNews();
if (newsTpye.SelectedIndex==0)
{
for ( k = 0; k <= 8; k++)
{
newsImage.Source = new BitmapImage(new Uri(myNews.articles[k].urlToImage, UriKind.Absolute));
showTime.Text = myNews.articles[k].publishedAt.ToString();
showDescription.Text = "(" + myNews.source + "): " + myNews.articles[k].description;
await Task.Delay(5000);
}
}
else if (newsTpye.SelectedIndex==1)
{
for (k = 0; k <= 8; k++)
{
newsImage.Source = new BitmapImage(new Uri(mySportNews.articles[k].urlToImage, UriKind.Absolute));
showTime.Text = mySportNews.articles[k].publishedAt;
showDescription.Text = "(" + mySportNews.source + "): " + mySportNews.articles[k].description;
await Task.Delay(5000);
}
}
NewsRepeat();
}
private void newsType_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
NewsRepeat();
}

Answer 1

Your code runs continuously / infinite loops. NewsRepeat never finishes - so when you change selection your now running two instances of NewsRepeat side by side. Change it again and you're running three, and so on.
On SelectionChanged you want to somehow stop the previous instance of NewsRepeat from running.
(Also, creating BitmapImages in the ViewModel is a bad idea generally - bind the directly in XAML to the URL property - Windows will carry out some performance and memory enhancements for you)
One possible solution is to use a CancellationTokenSource, which is a very simple object you can use to manually throw OperationCanceledException's when you deem it necessary (
frequently used as a pattern to cancel async Tasks). Keep it mind it does not work automatically - it's something you have to handle.
CancellationTokenSource cts = null;
public async void NewsRepeat()
{
cts?.Cancel();
try
{
var localCts = cts = new CancellationTokenSource();
RootObject2 myNews = await NewsProxy.GetNews();
RootObject3 mySportNews = await sportsNewsProxy.GetSportNews();
localCts.Token.ThrowIfCancellationRequested();
if (newsTpye.SelectedIndex == 0)
{
for (k = 0; k <= 8; k++)
{
newsImage.Source = new BitmapImage(new Uri(myNews.articles[k].urlToImage, UriKind.Absolute));
showTime.Text = myNews.articles[k].publishedAt.ToString();
showDescription.Text = "(" + myNews.source + "): " + myNews.articles[k].description;
await Task.Delay(5000);
localCts.Token.ThrowIfCancellationRequested();
}
}
else if (newsTpye.SelectedIndex == 1)
{
for (k = 0; k <= 8; k++)
{
newsImage.Source = new BitmapImage(new Uri(mySportNews.articles[k].urlToImage, UriKind.Absolute));
showTime.Text = mySportNews.articles[k].publishedAt;
showDescription.Text = "(" + mySportNews.source + "): " + mySportNews.articles[k].description;
await Task.Delay(5000);
localCts.Token.ThrowIfCancellationRequested();
}
}
NewsRepeat();
}
catch (OperationCanceledException)
{
// Swallow this exception only - this is probably
// the one we've thrown ourselves
}
}
private void newsType_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
NewsRepeat();
}