Proper way to wait all tasks while still updating the UI thread - async-await

Question

I'm trying to download the page source from multiple urls using tasks to download multiple sites at one time. The issue is that I want to keep the UI updated as each individual task completes. When I try to wait all tasks it stops updating the UI until they all finish. Here is the current code that I am using.
EDIT: I'm assuming I was down voted due to me not explaining well enough. I guess a better way to put this is why is the continueWith not being run before Task.WaitAll. I want the UI to update on each completion of the source being downloaded. Once that is all finished then the listbox would be updated to let the user know everything is done.
private void btnGetPages_Click(object sender, EventArgs e)
{
for (int i = 1; i < 11; i++)
{
string url = $"http://someURL/page-{i}.html";
listBoxStatus.Items.Add($"Downloading source from {url}...");
Task t = new Task(() =>
{
DownloadSource(url);
});
t.ContinueWith(prevTask => listBoxStatus.Items.Add($"Finished Downloading {url} source..."), TaskScheduler.FromCurrentSynchronizationContext());
tasks.Add(t);
t.Start();
}
Task.WaitAll(tasks.ToArray());
listBoxStatus.Items.Add("All Source files have completed...");
}
private void DownloadSource(string url)
{
var web = new HtmlWeb();
var doc = web.Load(url);
pageSource += doc.Text;
}

Answer 1

You really should use an asynchronous download method based on HttpClient instead of the synchronous method you are showing. Lacking that, I'll use this one:
private async Task DownloadSourceAsync(string url)
{
await Task.Run(() => DownloadSource(url));
listBoxStatus.Items.Add($"Finished Downloading {url} source...");
}
Then, you can make your btnGetPages_Click method something like this:
private async void btnGetPages_Click(object sender, EventArgs e)
{
var tasks = new List<Task>();
for (int i = 1; i < 11; i++)
{
string url = $"http://someURL/page-{i}.html";
listBoxStatus.Items.Add($"Downloading source from {url}...");
tasks.Add(DownloadSourceAsync(url));
}
Task.WaitAll(tasks.ToArray());
listBoxStatus.Items.Add("All Source files have completed...");
}