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如何：使用 WRL 处理事件

本文档演示如何使用 Windows 运行时 C++ 模板库 (WRL) 订阅和处理 Windows 运行时 对象的事件。

有关创建该组件实例并检索属性值的更基本示例，请参见 如何：使用 WRL 激活和使用 Windows 运行时组件。

订阅事件和操作事件

以下步骤开始 ABI::Windows::System::Threading::IDeviceWatcher 对象，并使用监视器的事件处理程序继续执行。在设备添加，移除或已更改时，IDeviceWatcher 接口可以枚举计算机异步，或者在后台，并接收通知。回调 功能是此示例的重要，因为这使它指定处理后台操作的结果的事件处理程序。下面是完整的示例。

警告说明警告

虽然可以在 Windows 应用商店 应用程序通常使用 WRL，此示例使用图的控件个 app。功能 (如 wprintf_s 从 Windows 应用商店 app 不可用。有关可在 Windows 应用商店 app 可以使用和函数的类型的更多信息，请参见 CRT 函数不支持与 /ZW 和 Win32 和 COM windows 中的 apps。

  1. 包括 (#include) 所需的 Windows 运行时、WRL或标准 C++ 库标头。

    #include <Windows.Devices.Enumeration.h>
#include <wrl/event.h>
#include <stdio.h>

using namespace ABI::Windows::Devices::Enumeration;
using namespace ABI::Windows::Foundation;
using namespace Microsoft::WRL;
using namespace Microsoft::WRL::Wrappers;

    Windows.Devices.Enumeration.h 声明需要枚举计算机的类型。

    建议您使用中的 .cpp 文件中 using namespace 命名空间指令使代码更具可读性。

  2. 声明一个 app 的局部变量。此示例包含枚举计算机和注册标记的计数使其从事件之后取消订阅。

    // Counts the number of enumerated devices.
unsigned int deviceCount = 0;

// Event registration tokens that enable us to later unsubscribe from events.
EventRegistrationToken addedToken;
EventRegistrationToken stoppedToken;
EventRegistrationToken enumCompletedToken;

  3. 初始化 Windows 运行时。

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

  4. 创建同步枚举完成处理到主应用程序的一 事件 对象。

    // Create an event that is set after device enumeration 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 enumerationCompleted(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);
}
    说明说明

    作为控件个 app 的一部分，此操作仅用于演示。此示例使用事件确保异步操作完成，该应用程序退出之前。在大多数 apps，通常不等待异步操作完成。

  5. 创建 IDeviceWatcher 接口的启动工厂。

    // Get the activation factory for the IDeviceWatcher interface.
ComPtr<IDeviceInformationStatics> watcherFactory;
HRESULT hr = ABI::Windows::Foundation::GetActivationFactory(HStringReference(RuntimeClass_Windows_Devices_Enumeration_DeviceInformation).Get(), &watcherFactory);
if (FAILED(hr))
{
    return PrintError(__LINE__, hr);
}

    Windows 运行时 使用完全限定名来标识类型。RuntimeClass_Windows_Devices_Enumeration_DeviceInformation 参数是 Windows 运行时 提供并包含需的运行时类名的字符串。

  6. 创建 IDeviceWatcher 对象。

    // Create a IDeviceWatcher object from the factory.
ComPtr<IDeviceWatcher> watcher;
hr = watcherFactory->CreateWatcher(&watcher);
if (FAILED(hr))
{
    return PrintError(__LINE__, hr);
}

  7. 使用 Callback 函数订阅 Added、EnumerationCompleted和 Stopped 事件。

    // Subscribe to the Added event.
hr = watcher->add_Added(Callback<AddedHandler>([&deviceCount](IDeviceWatcher* watcher, IDeviceInformation*) -> HRESULT
{
    // Print a message and increment the device count.
    // When we reach 10 devices, stop enumerating devices.
    wprintf_s(L"Added device...\n");
    deviceCount++;
    if (deviceCount == 10)
    {
        return watcher->Stop();
    }
    return S_OK;

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

hr = watcher->add_Stopped(Callback<StoppedHandler>([=, &enumerationCompleted](IDeviceWatcher* watcher, IInspectable*) -> HRESULT
{
    wprintf_s(L"Device enumeration stopped.\nRemoving event handlers...");

    // Unsubscribe from the events. This is shown for demonstration.
    // The need to remove event handlers depends on the requirements of 
    // your app. For instance, if you only need to handle an event for 
    // a short period of time, you might remove the event handler when you
    // no longer need it. If you handle an event for the duration of the app,
    // you might not need to explicitly remove it.
    HRESULT hr1 = watcher->remove_Added(addedToken);
    HRESULT hr2 = watcher->remove_Stopped(stoppedToken);
    HRESULT hr3 = watcher->remove_EnumerationCompleted(enumCompletedToken);

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

    return FAILED(hr1) ? hr1 : FAILED(hr2) ? hr2 : hr3;

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

// Subscribe to the EnumerationCompleted event.
hr = watcher->add_EnumerationCompleted(Callback<EnumerationCompletedHandler>([](IDeviceWatcher* watcher, IInspectable*) -> HRESULT
{
    wprintf_s(L"Enumeration completed.\n");

    return watcher->Stop();

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

    Added 事件处理程序添加计数枚举计算机。在找到后，即会停止枚举过程的计算机。

    Stopped 事件处理程序中移除事件处理程序并将完成事件。

    EnumerationCompleted 事件处理程序将停止枚举过程。我们处理此事件，以防小于的设备具有。

    提示提示

    此示例使用 lambda 表达式定义回调。也可以使用函数对象 (functors)，函数指针或 std::function 对象。有关 lambda 表达式的更多信息，请参见 在C++中Lambda表达式。

  8. 开始枚举过程。

    wprintf_s(L"Starting device enumeration...\n");
hr = watcher->Start();
if (FAILED(hr))
{
    return PrintError(__LINE__, hr);
}

  9. 等待枚举过程完成然后打印消息。所有 ComPtr 和 RAII 对象离开范围和自动释放。

    // Wait for the operation to complete.
WaitForSingleObjectEx(enumerationCompleted.Get(), INFINITE, FALSE);

wprintf_s(L"Enumerated %u devices.\n", deviceCount);

// All smart pointers and RAII objects go out of scope here.

这是完整示例：

// wrl-consume-events.cpp
// compile with: runtimeobject.lib
#include <Windows.Devices.Enumeration.h>
#include <wrl/event.h>
#include <stdio.h>

using namespace ABI::Windows::Devices::Enumeration;
using namespace ABI::Windows::Foundation;
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()
{
    // Type define the event handler types to make the code more readable.
    typedef __FITypedEventHandler_2_Windows__CDevices__CEnumeration__CDeviceWatcher_Windows__CDevices__CEnumeration__CDeviceInformation AddedHandler;
    typedef __FITypedEventHandler_2_Windows__CDevices__CEnumeration__CDeviceWatcher_IInspectable EnumerationCompletedHandler;
    typedef __FITypedEventHandler_2_Windows__CDevices__CEnumeration__CDeviceWatcher_IInspectable StoppedHandler;

    // Counts the number of enumerated devices.
    unsigned int deviceCount = 0;

    // Event registration tokens that enable us to later unsubscribe from events.
    EventRegistrationToken addedToken;
    EventRegistrationToken stoppedToken;
    EventRegistrationToken enumCompletedToken;

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

    // Create an event that is set after device enumeration 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 enumerationCompleted(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);
    }

    // Get the activation factory for the IDeviceWatcher interface.
    ComPtr<IDeviceInformationStatics> watcherFactory;
    HRESULT hr = ABI::Windows::Foundation::GetActivationFactory(HStringReference(RuntimeClass_Windows_Devices_Enumeration_DeviceInformation).Get(), &watcherFactory);
    if (FAILED(hr))
    {
        return PrintError(__LINE__, hr);
    }

    // Create a IDeviceWatcher object from the factory.
    ComPtr<IDeviceWatcher> watcher;
    hr = watcherFactory->CreateWatcher(&watcher);
    if (FAILED(hr))
    {
        return PrintError(__LINE__, hr);
    }

    // Subscribe to the Added event.
    hr = watcher->add_Added(Callback<AddedHandler>([&deviceCount](IDeviceWatcher* watcher, IDeviceInformation*) -> HRESULT
    {
        // Print a message and increment the device count.
        // When we reach 10 devices, stop enumerating devices.
        wprintf_s(L"Added device...\n");
        deviceCount++;
        if (deviceCount == 10)
        {
            return watcher->Stop();
        }
        return S_OK;

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

    hr = watcher->add_Stopped(Callback<StoppedHandler>([=, &enumerationCompleted](IDeviceWatcher* watcher, IInspectable*) -> HRESULT
    {
        wprintf_s(L"Device enumeration stopped.\nRemoving event handlers...");

        // Unsubscribe from the events. This is shown for demonstration.
        // The need to remove event handlers depends on the requirements of 
        // your app. For instance, if you only need to handle an event for 
        // a short period of time, you might remove the event handler when you
        // no longer need it. If you handle an event for the duration of the app,
        // you might not need to explicitly remove it.
        HRESULT hr1 = watcher->remove_Added(addedToken);
        HRESULT hr2 = watcher->remove_Stopped(stoppedToken);
        HRESULT hr3 = watcher->remove_EnumerationCompleted(enumCompletedToken);

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

        return FAILED(hr1) ? hr1 : FAILED(hr2) ? hr2 : hr3;

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

    // Subscribe to the EnumerationCompleted event.
    hr = watcher->add_EnumerationCompleted(Callback<EnumerationCompletedHandler>([](IDeviceWatcher* watcher, IInspectable*) -> HRESULT
    {
        wprintf_s(L"Enumeration completed.\n");

        return watcher->Stop();

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

    wprintf_s(L"Starting device enumeration...\n");
    hr = watcher->Start();
    if (FAILED(hr))
    {
        return PrintError(__LINE__, hr);
    }

    // Wait for the operation to complete.
    WaitForSingleObjectEx(enumerationCompleted.Get(), INFINITE, FALSE);

    wprintf_s(L"Enumerated %u devices.\n", deviceCount);

    // All smart pointers and RAII objects go out of scope here.
}
/*
Sample output:
Starting device enumeration...
Added device...
Added device...
Added device...
Added device...
Added device...
Added device...
Added device...
Added device...
Added device...
Added device...
Device enumeration stopped.
Removing event handlers...
Enumerated 10 devices.
*/

编译代码

若要编译代码，请将其复制并粘贴到 Visual Studio 项目或一个名为 wrl 使用 events.cpp 然后运行在 Visual Studio 命令提示符窗口中运行以下命令的文件。

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

请参见

概念

Windows 运行时 C++ 模板库 (WRL)

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