Invoking Service Methods Asynchronously

The WpdServiceApiSample application includes code that demonstrates how an application can invoke the service methods asynchronously. This sample uses the following interfaces.

When the user chooses option "10" at the command line, the application invokes the InvokeMethodsAsync method that is found in the ServiceMethods.cpp module. Note that prior to invoking the methods, the sample application opens a Contacts service on a connected device.

Service methods encapsulate functionality that each service defines and implements. They are unique to each type of service and are represented by a GUID. For example, the Contacts service defines a BeginSync method that applications call to prepare the device for synchronizing Contact objects, and an EndSync method to notify the device that synchronization has completed. Applications execute a portable device service method by calling IPortableDeviceServiceMethods::Invoke.

Service methods should not be confused with WPD commands. WPD commands are part of the standard WPD Device Driver Interface (DDI), and are the mechanism for communication between a WPD application and the driver. Commands are predefined, grouped by categories (for example, WPD_CATEGORY_COMMON), and are represented by a PROPERTYKEY structure. An application sends commands to the device driver by calling IPortableDeviceService::SendCommand. For more information, see the Commands topic.

The InvokeMethodsAsync method invokes IPortableDeviceService::Methods to retrieve an IPortableDeviceServiceMethods interface. Using this interface, it invokes the InvokeMethodAsync helper function twice; once for the BeginSync method and once for the EndSync method. In this example, , InvokeMethodAsync waits indefinitely for a global event to be signaled when IPortableDeviceServiceMethodCallback::OnComplete is called.

The following code uses the InvokeMethodsAsync method.

// Invoke methods on the Contacts Service asynchornously.
// BeginSync and EndSync are methods defined by the FullEnumerationSync Device Service.
void InvokeMethodsAsync(IPortableDeviceService* pService)
{
    HRESULT hr = S_OK;
    CComPtr<IPortableDeviceServiceMethods> pMethods;

    if (pService == NULL)
    {
        printf("! A NULL IPortableDeviceService interface pointer was received\n");
        return;
    }

    // Get an IPortableDeviceServiceMethods interface from the IPortableDeviceService interface to
    // invoke methods.
    hr = pService->Methods(&pMethods);
    if (FAILED(hr))
    {
        printf("! Failed to get IPortableDeviceServiceMethods from IPortableDeviceService, hr = 0x%lx\n",hr);
    }

    // Invoke the BeginSync method asynchronously
    if (SUCCEEDED(hr))
    {
        printf("Invoking %ws asynchronously...\n",NAME_FullEnumSyncSvc_BeginSync);

        // This method does not take any parameters, so we pass in NULL
        hr = InvokeMethodAsync(pMethods, METHOD_FullEnumSyncSvc_BeginSync, NULL);
        if (FAILED(hr))
        {
            printf("! Failed to invoke %ws asynchronously, hr = 0x%lx\n",NAME_FullEnumSyncSvc_BeginSync, hr);
        }
    }

    // Invoke the EndSync method asynchronously
    if (SUCCEEDED(hr))
    {
        printf("Invoking %ws asynchronously...\n",NAME_FullEnumSyncSvc_EndSync);

        hr = InvokeMethodAsync(pMethods, METHOD_FullEnumSyncSvc_EndSync, NULL);
        if (FAILED(hr))
        {
            printf("! Failed to invoke %ws asynchronously, hr = 0x%lx\n",NAME_FullEnumSyncSvc_EndSync, hr);
        }
    }
}

The InvokeMethodAsync helper function does the following for each method that it invokes:

• Creates a global event handle that it monitors to determine method completion.
• Creates a CMethodCallback object that is supplied as an argument to IPortableDeviceServiceMethods:InvokeAsync.
• Calls the IPortableDeviceServiceMethods::InvokeAsync method to invoke the given method.
• Monitors the global event handle for completion.
• Performs cleanup.

The CMethodCallback class demonstrates how an application can implement IPortableDeviceServiceMethodCallback. The implementation of OnComplete in this class signals an event to notify the application that the service method has completed. In addition to the OnComplete method, this class implements AddRef, QueryInterface, and Release, which are used to maintain the object's reference count and the interfaces that it implements.

class CMethodCallback : public IPortableDeviceServiceMethodCallback
{
public:
   CMethodCallback () : m_cRef(1)
   {
   }

   ~CMethodCallback ()
   {
   }

public:
    // IPortableDeviceServiceMethodCallback::QueryInterface
    virtual HRESULT STDMETHODCALLTYPE OnComplete(
        HRESULT                 hrStatus,
        IPortableDeviceValues*  /*pResults*/) // We are ignoring results as our methods will not return any results
    {
        printf("** Method completed, status HRESULT = 0x%lx **\n", hrStatus);

        if (g_hMethodCompleteEvent != NULL)
        {
            SetEvent(g_hMethodCompleteEvent);
        }          
        return S_OK;
    }

    // IUnknown::AddRef
    virtual ULONG STDMETHODCALLTYPE AddRef(void)
    {
        InterlockedIncrement((long*) &m_cRef);
        return m_cRef;
    }

    // IUnknown::QueryInterface
    virtual HRESULT STDMETHODCALLTYPE QueryInterface(
        REFIID  riid,
        LPVOID* ppvObj)
    {
        HRESULT hr = S_OK;
        if (ppvObj == NULL)
        {
            hr = E_INVALIDARG;
            return hr;
        }

        if ((riid == IID_IUnknown) ||
            (riid == IID_IPortableDeviceServiceMethodCallback))
        {
            AddRef();
            *ppvObj = this;
        }
        else
        {
            *ppvObj = NULL;
            hr = E_NOINTERFACE;
        }
        return hr;
    }

    // IUnknown::Release
    virtual ULONG STDMETHODCALLTYPE Release(void)
    {
        ULONG ulRefCount = m_cRef - 1;

        if (InterlockedDecrement((long*) &m_cRef) == 0)
        {
            delete this;
            return 0;
        }
        return ulRefCount;
    }

private:
    DWORD   m_cRef;
};

Related topics

IPortableDeviceService

IPortableDeviceServiceMethodCallback

IPortableDeviceServiceMethods

WpdServicesApiSample