Supporting the WPD Infrastructure

The WPD infrastructure is command driven. When a WPD application calls one of the methods in a given interface, the WPD serializer (an in-process COM server) converts the method call into one or more commands that it sends to the driver. The driver then processes these commands and returns a response. For more information about the infrastructure, see the Architecture Overview topic.

The following image of the WpdMon.exe tool shows the result of an application calling the IPortableDeviceProperties::GetPropertyAttributes method to retrieve the property attributes for the temperature and humidity sensor.

The WPD Monitor

In the previous image, the WPD serializer converted the GetPropertyAttributes call into the WPD_COMMAND_OBJECT_PROPERTIES_GET_ATTRIBUTES command and the corresponding parameters. The driver processed this command and issued a response to the WPD API.

The WPD_COMMAND_OBJECT_PROPERTIES_GET_ATTRIBUTES command is first handled in the WpdObjectProperties::DispatchWpdMessage method. This method, in turn, invokes the WpdObjectProperties::OnGetPropertyAttributes method. Both of these methods are found in the WpdObjectProperties.cpp source file.

The following excerpt from the WpdObjectProperties::DispatchWpdMessage method in WpdObjectProperties.cpp shows the call to the WpdObjectProperties::OnGetPropertyAttributes handler for the retrieval of property attributes. The DispatchWpdMessage method passes the command parameters to the handler. These parameters include an object identifier for the object whose properties the driver should retrieve. Additionally, the parameters include a collection of property keys that identify the attributes to be retrieved.


HRESULT WpdObjectProperties::DispatchWpdMessage(
    const PROPERTYKEY&     Command,
    IPortableDeviceValues* pParams,
    IPortableDeviceValues* pResults)
{
    HRESULT hr = S_OK;

    if (hr == S_OK)
    {
        if (Command.fmtid != WPD_CATEGORY_OBJECT_PROPERTIES)
        {
            hr = E_INVALIDARG;
            CHECK_HR(hr, 
                     "This object does not support this command category %ws",
                     CComBSTR(Command.fmtid));
        }
    }

    if (hr == S_OK)
    {
     …
        else if(IsEqualPropertyKey(Command, 
                                   WPD_COMMAND_OBJECT_PROPERTIES_GET_ATTRIBUTES))
        {
            hr = OnGetPropertyAttributes(pParams, pResults);
            if(FAILED(hr))
            {
                CHECK_HR(hr, "Failed to get property attributes");
            }
        }
        …
    return hr;
}


The WpdObjectProperties::OnGetPropertyAttributes method returns the collection of requested attributes in a collection of device values (IPortableDeviceValues).

Related topics

The WpdBasicHardwareDriverSample
The WPD Driver Samples

 

 

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