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description Learn more about: How to: Handle Events Using WRL
title How to: Handle Events Using WRL
ms.date 11/04/2016
ms.topic reference
ms.assetid 1c77543f-7b0c-4a94-93bf-e3225885ed76

How to: Handle Events Using WRL

This document shows how to use the Windows Runtime C++ Template Library (WRL)to subscribe to and handle the events of a Windows Runtime object.

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.

Subscribing to and Handling Events

The following steps start an ABI::Windows::System::Threading::IDeviceWatcher object and use event handlers to monitor progress. The IDeviceWatcher interface enables you to enumerate devices asynchronously, or in the background, and receive notification when devices are added, removed, or changed. The Callback function is an important part of this example because it enables it to specify event handlers that process the results of the background operation. The complete example follows.

Warning

Although you typically use the Windows Runtime C++ Template Library in a Universal Windows Platform app, this example uses a console app for illustration. Functions such as wprintf_s are not available from a Universal Windows Platform app. For more information about the types and functions that you can use in a Universal Windows Platform app, see CRT functions not supported in Universal Windows Platform apps and Win32 and COM for UWP apps.

  1. Include (#include) any required Windows Runtime, Windows Runtime C++ Template Library, or C++ Standard Library headers.

    [!code-cppwrl-consume-event#2]

    Windows.Devices.Enumeration.h declares the types that are required to enumerate devices.

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

  2. Declare the local variables for the app. This example holds count of the number of enumerated devices and registration tokens that enable it to later unsubscribe from events.

    [!code-cppwrl-consume-event#7]

  3. Initialize the Windows Runtime.

    [!code-cppwrl-consume-event#3]

  4. Create an Event object that synchronizes the completion of the enumeration process to the main app.

    [!code-cppwrl-consume-event#4]

    [!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 activation factory for the IDeviceWatcher interface.

    [!code-cppwrl-consume-event#5]

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

  6. Create the IDeviceWatcher object.

    [!code-cppwrl-consume-event#6]

  7. Use the Callback function to subscribe to the Added, EnumerationCompleted, and Stopped events.

    [!code-cppwrl-consume-event#8]

    The Added event handler increments the count of enumerated devices. It stops the enumeration process after ten devices are found.

    The Stopped event handler removes the event handlers and sets the completion event.

    The EnumerationCompleted event handler stops the enumeration process. We handle this event in case there are fewer than ten devices.

    [!TIP] This example uses a lambda expression to define the callbacks. You can also use function objects (functors), function pointers, or std::function objects. For more information about lambda expressions, see Lambda Expressions.

  8. Start the enumeration process.

    [!code-cppwrl-consume-event#9]

  9. Wait for the enumeration process to complete and then print a message. All ComPtr and RAII objects leave scope and are released automatically.

    [!code-cppwrl-consume-event#10]

Here is the complete example:

[!code-cppwrl-consume-event#1]

Compiling the Code

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-events.cpp and then run the following command in a Visual Studio Command Prompt window.

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

See also

Windows Runtime C++ Template Library (WRL)