MICROSOFT_PRESS_EBOOK_PROGRAMMING_WINDOWS_8_APPS_WITH_HTML_CSS_AND_JAVASCRIPT_PDF

Note that Task.Run does not support progress reporting and the AsAsyncAction and
AsAsyncOperation extension methods don’t support cancellation. In these cases you need to use the
System.Runtime.InteropServices.WindowsRuntime.AsyncInfo class and one of its Run methods as
appropriate to the chosen async behavior. The Task.AsAsyncOperation call at the end is unnecessary
here because AsyncInfo.Run already provides the right interface:
public IAsyncOperation SomeMethodAsync(int id)
{
return AsyncInfo.Run(async (token) =>
{
var idString = await GetMyStringAsync(id);
token.ThrowIfCancellationRequested();
return idString;
});
}
In this code, AsyncInfo.Run provides the delegate with an argument of type
System.-Threading.CancellationToken. To support cancellation, you must periodically call the
token’s ThrowIfCancellationRequested method. This will pick up whether the original caller of the async
method has canceled it (for example, calling a promise’s cancel method). Because cancelation is
typically a user-initiated action, there’s no need to call ThrowIfCancellationRequested inside a very
tight loop; calling it every 50 milliseconds or so will keep the app fully responsive.
Alternately, if a method like GetMyStringAsync accepted the CancellationToken, you could just pass
the token to it. One strength of the CancellationToken model is that it’s highly composable: if you
receive a token in your own async call, you can hand it off to any number of other functions you call
that also accept a token. If cancellation happens, the request will automatically be propagated to all
those operations.
Note that WinRT methods can accept a token because of an AsTask overload. Instead of this:
await SomeWinRTMethodAsync();
you can use this:
await SomeWinRTMethodAsync().AsTask(token);
Anyway, given these examples, here’s a noncancellable async version of CountFromZero:
public static IAsyncOperation CountFromZeroAsync(double max, double increment)
{
var task = Task.Run(() =>
{
double sum = 0;
for (double x = 0; x < max; x += increment)
{
sum += x;
}
return sum;
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