Async timeout support for C++/WinRT

[kennykerr]

Today C++/WinRT provides support for blocking wait via async.get() and cooperative wait via co_await async but there is not ability to use a timeout. This has lead some teams needing timeout support to implement various workarounds that are not very efficient or robust. Part 1 introduces support for a timeout for the blocking scenario. Part 2 introduces support for cooperative timeout for coroutines.

The get function follows the pattern established by std::future so it makes sense to adopt future's wait_for function to support a timeout as well. Note that WinRT only allows waiting once. That means you cannot use wait_for in a loop, or any other scenario where multiple waits would occur.

You can wait for an IAsyncAction as follows:

using namespace Windows::Foundation;

IAsyncAction async = ...

if (async.wait_for(5s) == AsyncStatus::Completed)
{
    puts("done");
}

You can wait for an IAsyncOperation as follows:

IAsyncOperation<hstring> async = ...

if (async.wait_for(5s) == AsyncStatus::Completed)
{
    printf("result: %ls\n", async.get().c_str());
}

Or more comprehensively:

IAsyncOperation<hstring> async = ...

switch (async.wait_for(2s))
{
case AsyncStatus::Completed:
    printf("result: %ls\n", async.get().c_str());
    break;
case AsyncStatus::Started:
    puts("still running");
    break;
case AsyncStatus::Canceled:
    puts("canceled");
    break;
case AsyncStatus::Error:
    puts("failed");
    break;
}

[Scottj1s]

impl:: [](start = 19, length = 6)

needed for disambiguation with current wait_for? (not used on wait_get)

[kennykerr]

Right. wait_for calls impl::wait_for but get calls impl::wait_get so no confusion. I didn't want to call it impl::get and rather had the handful of impl functions start with wait_xxx for clarity.

[Scottj1s]

[cScalpel]

Thank you for this, kennykerr!!! It is exactly what I was looking for when using SerialDevice! I do have a question regarding my implementation:
After I determine that the AsyncStatus is not completed (no data on serial port when using SerialDevice, DataReader.LoadAsync() and IAsyncOperation.wait_for()) I call the IAsyncOperation.Cancel() function. It seems to work but when I return to make a subsequent DataReader.LoadAsync() call, I get a take_ownership_from_abi exception thrown. How can I perform any additional Async functions on the DataReader object after I cancelled the previous operation?

[kennykerr]

@cScalpel Glad you like it! You'll need to check the specific information in the thrown exception to figure out what might be wrong (take_ownership_from_abi is not an exception). Whether you can reuse the object that produced the async object following cancellation depends on the implementation of that API. You might need to create a new DataReader, but I'd first test by wrapping the failing request in a catch block and inspecting the error message. Something like this:

try { ... } catch (hresult_error const& e)
{
    printf("%ls\n", e.message().c_str());
}

In general, we don't have expertise to answer questions about specific APIs. For that, I'd suggest Stack Overflow. Tag the question with c++-winrt or windows-runtime and we'll do our best to respond if we are able.

[cScalpel]

Will do, the exception was indeed "The operation identifier is not valid." I'll update my question on stackoverflow. Thanks!