A completion source sample

This topic demonstrates how you can author and consume your own completion source class, similar to .NET's TaskCompletionSource.

Source code for the completion_source sample

The code in the listing below is offered as a sample. Its purpose is to illustrate how you could write your own version of it. For example, support for cancellation and error propagation are outside the scope of this sample.

#include <winrt/base.h>
#include <windows.h>

template <typename T>
struct completion_source
{
 completion_source()
 {
 m_signal.attach(::CreateEvent(nullptr, true, false, nullptr));
 }

 void set(T const& value)
 {
 m_value = value;
 ::SetEvent(m_signal.get());
 }

 bool await_ready() const noexcept
 {
 return ::WaitForSingleObject(m_signal.get(), 0) == 0;
 }

 void await_suspend(std::experimental::coroutine_handle<> resume)
 {
 m_wait.attach(winrt::check_pointer(::CreateThreadpoolWait(callback, resume.address(), nullptr)));
 ::SetThreadpoolWait(m_wait.get(), m_signal.get(), nullptr);
 }

 T await_resume() const noexcept
 {
 return m_value;
 }

private:

 static void __stdcall callback(PTP_CALLBACK_INSTANCE, void* context, PTP_WAIT, TP_WAIT_RESULT) noexcept
 {
 std::experimental::coroutine_handle<>::from_address(context)();
 }

 struct wait_traits
 {
 using type = PTP_WAIT;

 static void close(type value) noexcept
 {
 ::CloseThreadpoolWait(value);
 }

 static constexpr type invalid() noexcept
 {
 return nullptr;
 }
 };

 winrt::handle m_signal;
 winrt::handle_type<wait_traits> m_wait;
 T m_value{};
};

Offload completion to a separate coroutine

This section demonstrates one use case for completion_source. Create a new project in Visual Studio based on the Windows Console Application (C++/WinRT) project template, and paste the following code listing into main.cpp (expanding the definition of completion_source based on the listing in the previous section).

// main.cpp
#include "pch.h"

#include <winrt/base.h>
#include <windows.h>

template <typename T>
struct completion_source
{
 ... // Paste the listing of completion_source here.
}

using namespace std::literals;
using namespace winrt;
using namespace Windows::Foundation;

fire_and_forget CompleteAfterFiveSecondsAsync(completion_source<bool>& completionSource)
{
 co_await 5s;
 completionSource.set(true);
}

IAsyncAction CompletionSourceExample1Async()
{
 completion_source<bool> completionSource;
 CompleteAfterFiveSecondsAsync(completionSource);
 co_await completionSource;
}

int main()
{
 auto asyncAction { CompletionSourceExample1Async() };
 puts("waiting");
 asyncAction.get();
 puts("done");
}

Encapsulate a completion_source in a class, and return a value

In this next example, a simple App class is used to encapsulate a completion_source, and return a value when it completes. Create a new project in Visual Studio based on the Windows Console Application (C++/WinRT) project template, and paste the following code listing into main.cpp (expanding the definition of completion_source based on the listing in the previous section).

// main.cpp
#include "pch.h"

#include <winrt/base.h>
#include <windows.h>

template <typename T>
struct completion_source
{
 ... // Paste the listing of completion_source here.
}

using namespace std::literals;
using namespace winrt;
using namespace Windows::Foundation;

struct App
{
 completion_source<winrt::hstring> m_completionSource;

 IAsyncOperation<winrt::hstring> CompletionSourceExample2Async()
 {
 co_return co_await m_completionSource;
 }

 winrt::fire_and_forget CompleteAfterFiveSecondsAsync()
 {
 co_await 5s;
 m_completionSource.set(L"Hello, World!");
 }
};

int main()
{
 App app;
 auto asyncAction{ app.CompletionSourceExample2Async() };
 app.CompleteAfterFiveSecondsAsync();
 puts("waiting");
 auto message = asyncAction.get();
 printf("%ls\n", message.c_str());
}

Related topics

• Concurrency and asynchronous operations
• Advanced concurrency and asynchrony with C++/WinRT