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});
}
return task.AsAsyncOperation();
The IAsyncOperation interface returned by this method, like all the async interfaces in
Windows.Foundation, gets projected into JavaScript as a promise, so we can use the usual code to call
the method and receive its results (asyncVars is just an object to hold the variables):
asyncVars.startCS = new Date();
var promiseCS = PixelCruncherCS.Tests.countFromZeroAsync(max, increment);
promiseCS.done(function (sum) {
asyncVars.timeCS = new Date() - asyncVars.startCS;
asyncVars.sumCS = sum;
});
With code like this, which is in the Image Manipulation example with this chapter, you can start the
async counting operations (using the Counting Perf (Async) button) and then immediately go open an
image and do grayscale conversions at the same time.
Async Methods in C++ To implement an async method in C++, we need to produce the same end
result as in C#: a method that returns one of the IAsync* interfaces and runs its internal code on another
thread.
The first part is straightforward—we just need to declare the method with the C++ types (shown
here in the C++ code; the class declaration in Grayscale.h is similar):
using namespace Windows::Foundation;
IAsyncOperation^ Tests::CountFromZeroAsync(double max, double increment)
The C++ analogue of the AsyncInfo class is a task found in what’s called the Parallel Patterns Library
for C++, also known as PPL or the Concurrency Runtime, whose namespace is concurrency (use a
#include and using namespace concurrency; in your C++ code you’re good to go). The
function that creates a task is called create_async. All we need to do is wrap our code in that function as
follows:
IAsyncOperation^ Tests::CountFromZeroAsync(double max, double increment)
{
return create_async([max, increment]()
{
double sum = 0;
for (double x = 0; x < max; x += increment)
{
sum += x;
}
}
return sum;
});
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