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How to: Complete Asynchronous Operations Using WRL

This document shows how to use the Windows Runtime C++ Template Library (WRL) to start asynchronous operations and perform work when the operations complete.

This document shows two examples. The first example starts an asynchronous timer and waits for the timer to expire. In this example, you specify the asynchronous action when you create the timer object. The second example runs a background worker thread. This example shows how to work with a Windows Runtime method that returns an IAsyncInfo interface. The Callback function is an important part of both examples because it enables them to specify an event handler to process the results of the asynchronous operations.

For a more basic example that creates an instance of that component and retrieves a property value, see How to: Activate and Use a Windows Runtime Component Using WRL.

Tip Tip

These examples use lambda expressions to define the callbacks. You can also use function objects (functors), function pointers, or std::function objects. For more information about C++ lambda expressions, see Lambda Expressions in C++.

The following steps start an asynchronous timer and wait for the timer to expire. The complete example follows.

Caution note Caution

Although you typically use the WRL in a Windows Store app, this example uses a console app for illustration. Functions such as wprintf_s are not available from a Windows Store app. For more information about the types and functions that you can use in a Windows Store app, see CRT functions not supported by /ZW and Win32 and COM for Windows Store apps.

  1. Include (#include) any required Windows Runtime, WRL, or standard C++ library headers.

    #include <Windows.Foundation.h>
    #include <Windows.System.Threading.h>
    #include <wrl/event.h>
    #include <stdio.h>
    #include <Objbase.h>
    
    using namespace ABI::Windows::Foundation;
    using namespace ABI::Windows::System::Threading;
    using namespace Microsoft::WRL;
    using namespace Microsoft::WRL::Wrappers;
    

    Windows.System.Threading.h declares the types that are required to use an asynchronous timer.

    We recommend that you utilize the using namespace directive in your .cpp file to make the code more readable.

  2. Initialize the Windows Runtime.

    // Initialize the Windows Runtime.
    RoInitializeWrapper initialize(RO_INIT_MULTITHREADED);
    if (FAILED(initialize))
    {
        return PrintError(__LINE__, initialize);
    }
    
  3. Create an activation factory for the ABI::Windows::System::Threading::IThreadPoolTimer interface.

    // Get the activation factory for the IThreadPoolTimer interface.
    ComPtr<IThreadPoolTimerStatics> timerFactory;
    HRESULT hr = GetActivationFactory(HStringReference(RuntimeClass_Windows_System_Threading_ThreadPoolTimer).Get(), &timerFactory);
    if (FAILED(hr))
    {
        return PrintError(__LINE__, hr);
    }
    

    The Windows Runtime uses fully-qualified names to identify types. The RuntimeClass_Windows_System_Threading_ThreadPoolTimer parameter is a string that's provided by the Windows Runtime and contains the required runtime class name.

  4. Create an Event object that synchronizes the timer callback to the main app.

    // Create an event that is set after the timer callback completes. We later use this event to wait for the timer to complete.  
    // This event is for demonstration only in a console app. In most apps, you typically don't wait for async operations to complete.
    Event timerCompleted(CreateEventEx(nullptr, nullptr, CREATE_EVENT_MANUAL_RESET, WRITE_OWNER | EVENT_ALL_ACCESS));
    hr = timerCompleted.IsValid() ? S_OK : HRESULT_FROM_WIN32(GetLastError());
    if (FAILED(hr))
    {
        return PrintError(__LINE__, hr);
    }
    
    Note Note

    This event is for demonstration only as part of a console app. This example uses the event to ensure that an async operation completes before the app exits. In most apps, you typically don’t wait for async operations to complete.

  5. Create an IThreadPoolTimer object that expires after two seconds. Use the Callback function to create the event handler (an ABI::Windows::System::Threading::ITimerElapsedHandler object).

    // Create a timer that prints a message after 2 seconds.
    
    TimeSpan delay;
    delay.Duration = 20000000; // 2 seconds.
    
    auto callback = Callback<ITimerElapsedHandler>([&timerCompleted](IThreadPoolTimer* timer) -> HRESULT
    {
        wprintf_s(L"Timer fired.\n");
    
        TimeSpan delay;
        HRESULT hr = timer->get_Delay(&delay);
        if (SUCCEEDED(hr))
        {
            wprintf_s(L"Timer duration: %2.2f seconds.\n", delay.Duration / 10000000.0);
        }
    
        // Set the completion event and return.
        SetEvent(timerCompleted.Get());
        return hr;
    });
    hr = callback ? S_OK : E_OUTOFMEMORY;
    if (FAILED(hr))
    {
        return PrintError(__LINE__, hr);
    }
    
    ComPtr<IThreadPoolTimer> timer;
    hr = timerFactory->CreateTimer(callback.Get(), delay, &timer);
    if (FAILED(hr))
    {
        return PrintError(__LINE__, hr);
    }
    
  6. Print a message to the console and wait for the timer callback to complete. All ComPtr and RAII objects leave scope and are released automatically.

    // Print a message and wait for the timer callback to complete.
    wprintf_s(L"Timer started.\nWaiting for timer...\n");
    
    // Wait for the timer to complete.
    WaitForSingleObjectEx(timerCompleted.Get(), INFINITE, FALSE);
    // All smart pointers and RAII objects go out of scope here.
    

Here is the complete example:

// wrl-consume-async.cpp 
// compile with: runtimeobject.lib
#include <Windows.Foundation.h>
#include <Windows.System.Threading.h>
#include <wrl/event.h>
#include <stdio.h>
#include <Objbase.h>

using namespace ABI::Windows::Foundation;
using namespace ABI::Windows::System::Threading;
using namespace Microsoft::WRL;
using namespace Microsoft::WRL::Wrappers;

// Prints an error string for the provided source code line and HRESULT 
// value and returns the HRESULT value as an int. 
int PrintError(unsigned int line, HRESULT hr)
{
    wprintf_s(L"ERROR: Line:%d HRESULT: 0x%X\n", line, hr);
    return hr;
}

int wmain()
{
    // Initialize the Windows Runtime.
    RoInitializeWrapper initialize(RO_INIT_MULTITHREADED);
    if (FAILED(initialize))
    {
        return PrintError(__LINE__, initialize);
    }

    // Get the activation factory for the IThreadPoolTimer interface.
    ComPtr<IThreadPoolTimerStatics> timerFactory;
    HRESULT hr = GetActivationFactory(HStringReference(RuntimeClass_Windows_System_Threading_ThreadPoolTimer).Get(), &timerFactory);
    if (FAILED(hr))
    {
        return PrintError(__LINE__, hr);
    }

    // Create an event that is set after the timer callback completes. We later use this event to wait for the timer to complete.  
    // This event is for demonstration only in a console app. In most apps, you typically don't wait for async operations to complete.
    Event timerCompleted(CreateEventEx(nullptr, nullptr, CREATE_EVENT_MANUAL_RESET, WRITE_OWNER | EVENT_ALL_ACCESS));
    hr = timerCompleted.IsValid() ? S_OK : HRESULT_FROM_WIN32(GetLastError());
    if (FAILED(hr))
    {
        return PrintError(__LINE__, hr);
    }

    // Create a timer that prints a message after 2 seconds.

    TimeSpan delay;
    delay.Duration = 20000000; // 2 seconds.

    auto callback = Callback<ITimerElapsedHandler>([&timerCompleted](IThreadPoolTimer* timer) -> HRESULT
    {
        wprintf_s(L"Timer fired.\n");

        TimeSpan delay;
        HRESULT hr = timer->get_Delay(&delay);
        if (SUCCEEDED(hr))
        {
            wprintf_s(L"Timer duration: %2.2f seconds.\n", delay.Duration / 10000000.0);
        }

        // Set the completion event and return.
        SetEvent(timerCompleted.Get());
        return hr;
    });
    hr = callback ? S_OK : E_OUTOFMEMORY;
    if (FAILED(hr))
    {
        return PrintError(__LINE__, hr);
    }

    ComPtr<IThreadPoolTimer> timer;
    hr = timerFactory->CreateTimer(callback.Get(), delay, &timer);
    if (FAILED(hr))
    {
        return PrintError(__LINE__, hr);
    }

    // Print a message and wait for the timer callback to complete.
    wprintf_s(L"Timer started.\nWaiting for timer...\n");

    // Wait for the timer to complete.
    WaitForSingleObjectEx(timerCompleted.Get(), INFINITE, FALSE);
    // All smart pointers and RAII objects go out of scope here.
}
/*
Output:
Timer started.
Waiting for timer...
Timer fired.
Timer duration: 2.00 seconds.
*/

To compile the code, copy it and then paste it in a Visual Studio project, or paste it in a file that is named wrl-consume-async.cpp and then run the following command in a Visual Studio Command Prompt window.

cl.exe wrl-consume-async.cpp runtimeobject.lib

The following steps start a worker thread and define the action that's performed by that thread. The complete example follows.

Tip Tip

This example demonstrates how to work with the ABI::Windows::Foundation::IAsyncAction interface. You can apply this pattern to any interface that implements IAsyncInfo: IAsyncAction, IAsyncActionWithProgress, IAsyncOperation, and IAsyncOperationWithProgress.

  1. Include (#include) any required Windows Runtime, WRL, or standard C++ library headers.

    #include <Windows.Foundation.h>
    #include <Windows.System.Threading.h>
    #include <wrl/event.h>
    #include <stdio.h>
    #include <Objbase.h>
    
    using namespace ABI::Windows::Foundation;
    using namespace ABI::Windows::System::Threading;
    using namespace Microsoft::WRL;
    using namespace Microsoft::WRL::Wrappers;
    

    Windows.System.Threading.h declares the types that are required to use a worker thread.

    We recommend that you use the using namespace directive in your .cpp file to make the code more readable.

  2. Initialize the Windows Runtime.

    // Initialize the Windows Runtime.
    RoInitializeWrapper initialize(RO_INIT_MULTITHREADED);
    if (FAILED(initialize))
    {
        return PrintError(__LINE__, initialize);
    }
    
  3. Create an activation factory for the ABI::Windows::System::Threading::IThreadPoolStatics interface.

    // Get the activation factory for the IThreadPoolStatics interface.
    ComPtr<IThreadPoolStatics> threadPool;
    HRESULT hr = GetActivationFactory(HStringReference(RuntimeClass_Windows_System_Threading_ThreadPool).Get(), &threadPool);
    if (FAILED(hr))
    {
        return PrintError(__LINE__, hr);
    }
    
  4. Create an Event object that synchronizes completion of the worker thread to the main app.

        // Create an event that is set after the timer callback completes. We later use this event to wait for the timer to complete.  
        // This event is for demonstration only in a console app. In most apps, you typically don't wait for async operations to complete.
        Event threadCompleted(CreateEventEx(nullptr, nullptr, CREATE_EVENT_MANUAL_RESET, WRITE_OWNER | EVENT_ALL_ACCESS));
        hr = threadCompleted.IsValid() ? S_OK : HRESULT_FROM_WIN32(GetLastError());
        if (FAILED(hr))
        {
            return PrintError(__LINE__, hr);
        }
    
    Note Note

    This event is for demonstration only as part of a console app. This example uses the event to ensure that an async operation completes before the app exits. In most apps, you typically don’t wait for async operations to complete.

  5. Call the IThreadPoolStatics::RunAsync method to create a worker thread. Use the Callback function to define the action.

    wprintf_s(L"Starting thread...\n");
    
    // Create a thread that computes prime numbers.
    ComPtr<IAsyncAction> asyncAction;
    hr = threadPool->RunAsync(Callback<IWorkItemHandler>([&threadCompleted](IAsyncAction* asyncAction) -> HRESULT
    {
        // Print a message. 
        const unsigned int start = 0;
        const unsigned int end = 100000;
        unsigned int primeCount = 0;
        for (int n = start; n < end; n++)
        {
            if (IsPrime(n))
            {
                primeCount++;
            }
        }
    
        wprintf_s(L"There are %u prime numbers from %u to %u.\n", primeCount, start, end);
    
        // Set the completion event and return.
        SetEvent(threadCompleted.Get());
        return S_OK;
    
    }).Get(), &asyncAction);
    if (FAILED(hr))
    {
        return PrintError(__LINE__, hr);
    }
    

    The IsPrime function is defined in the complete example that follows.

  6. Print a message to the console and wait for the thread to complete. All ComPtr and RAII objects leave scope and are released automatically.

    // Print a message and wait for the thread to complete.
    wprintf_s(L"Waiting for thread...\n");
    
    // Wait for the thread to complete.
    WaitForSingleObjectEx(threadCompleted.Get(), INFINITE, FALSE);
    
    wprintf_s(L"Finished.\n");
    
    // All smart pointers and RAII objects go out of scope here.
    

Here is the complete example:

// wrl-consume-asyncOp.cpp 
// compile with: runtimeobject.lib 
#include <Windows.Foundation.h>
#include <Windows.System.Threading.h>
#include <wrl/event.h>
#include <stdio.h>
#include <Objbase.h>

using namespace ABI::Windows::Foundation;
using namespace ABI::Windows::System::Threading;
using namespace Microsoft::WRL;
using namespace Microsoft::WRL::Wrappers;

// Prints an error string for the provided source code line and HRESULT 
// value and returns the HRESULT value as an int. 
int PrintError(unsigned int line, HRESULT hr)
{
    wprintf_s(L"ERROR: Line:%d HRESULT: 0x%X\n", line, hr);
    return hr;
}

// Determines whether the input value is prime. 
bool IsPrime(int n)
{
    if (n < 2)
    {
        return false;
    }
    for (int i = 2; i < n; ++i)
    {
        if ((n % i) == 0)
        {
            return false;
        }
    }
    return true;
}

int wmain()
{
    // Initialize the Windows Runtime.
    RoInitializeWrapper initialize(RO_INIT_MULTITHREADED);
    if (FAILED(initialize))
    {
        return PrintError(__LINE__, initialize);
    }

    // Get the activation factory for the IThreadPoolStatics interface.
    ComPtr<IThreadPoolStatics> threadPool;
    HRESULT hr = GetActivationFactory(HStringReference(RuntimeClass_Windows_System_Threading_ThreadPool).Get(), &threadPool);
    if (FAILED(hr))
    {
        return PrintError(__LINE__, hr);
    }

    // Create an event that is set after the timer callback completes. We later use this event to wait for the timer to complete.  
    // This event is for demonstration only in a console app. In most apps, you typically don't wait for async operations to complete.
    Event threadCompleted(CreateEventEx(nullptr, nullptr, CREATE_EVENT_MANUAL_RESET, WRITE_OWNER | EVENT_ALL_ACCESS));
    hr = threadCompleted.IsValid() ? S_OK : HRESULT_FROM_WIN32(GetLastError());
    if (FAILED(hr))
    {
        return PrintError(__LINE__, hr);
    }


    wprintf_s(L"Starting thread...\n");

    // Create a thread that computes prime numbers.
    ComPtr<IAsyncAction> asyncAction;
    hr = threadPool->RunAsync(Callback<IWorkItemHandler>([&threadCompleted](IAsyncAction* asyncAction) -> HRESULT
    {
        // Print a message. 
        const unsigned int start = 0;
        const unsigned int end = 100000;
        unsigned int primeCount = 0;
        for (int n = start; n < end; n++)
        {
            if (IsPrime(n))
            {
                primeCount++;
            }
        }

        wprintf_s(L"There are %u prime numbers from %u to %u.\n", primeCount, start, end);

        // Set the completion event and return.
        SetEvent(threadCompleted.Get());
        return S_OK;

    }).Get(), &asyncAction);
    if (FAILED(hr))
    {
        return PrintError(__LINE__, hr);
    }

    // Print a message and wait for the thread to complete.
    wprintf_s(L"Waiting for thread...\n");

    // Wait for the thread to complete.
    WaitForSingleObjectEx(threadCompleted.Get(), INFINITE, FALSE);

    wprintf_s(L"Finished.\n");

    // All smart pointers and RAII objects go out of scope here.
}
/*
Output:
Starting thread...
Waiting for thread...
There are 9592 prime numbers from 0 to 100000.
Finished.
*/

To compile the code, copy it and then paste it in a Visual Studio project, or paste it in a file that is named wrl-consume-asyncOp.cpp and then run the following command in a Visual Studio Command Prompt window.

cl.exe wrl-consume-asyncOp.cpp runtimeobject.lib

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