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Implementing IAccessibleEx for Providers

  • Článek

This section explains how to add Microsoft UI Automation provider capabilities to a Microsoft Active Accessibility server by implementing the IAccessibleEx interface.

Before implementing IAccessibleEx, consider the following requirements:

  • The baseline Microsoft Active Accessibility accessible object hierarchy must be clean. IAccessibleEx cannot correct problems with existing accessible object hierarchies. Any problems with the object model structure must be corrected in the Microsoft Active Accessibility implementation before implementing IAccessibleEx.
  • The IAccessibleEx implementation must comply with both the Microsoft Active Accessibility specification and the UI Automation specification. Tools are available to validate compliance under both specifications. For more information, see Testing for accessibility and UI Automation Verify (UIA Verify) Test Automation Framework.

Implementing IAccessibleEx requires these main steps:

  • Implement IServiceProvider on the accessible object so that the IAccessibleEx interface can be found on this or a separate object.
  • Implement IAccessibleEx on the accessible object.
  • Create accessible objects for any Microsoft Active Accessibility child items, which in Microsoft Active Accessibility are represented by the IAccessible interface on the parent object (for example, list items). Implement IAccessibleEx on these objects.
  • Implement IRawElementProviderSimple on all the accessible objects.
  • Implement the appropriate control pattern interfaces on the accessible objects.

Implementing the IServiceProvider Interface

Because the implementation of IAccessibleEx for a control may reside in a separate object, client applications cannot rely on QueryInterface to obtain this interface. Instead, clients are expected to call IServiceProvider::QueryService. In the following example implementation of this method, it is assumed that IAccessibleEx is not implemented on a separate object; therefore the method simply calls through to QueryInterface.

           
HRESULT CListboxAccessibleObject::QueryService(REFGUID guidService, REFIID riid, LPVOID *ppvObject)
{
    if (ppvObject == NULL)
    {
        return E_INVALIDARG;
    }
    *ppvObject = NULL;
    if (guidService == __uuidof(IAccessibleEx))
    {
        return QueryInterface(riid, ppvObject);
    }
    else 
    {
        return E_NOINTERFACE;
    }
};

Implementing the IAccessibleEx Interface

In Microsoft Active Accessibility, a UI element is always identified by an IAccessible interface and a child ID. A single instance of IAccessible can represent multiple UI elements.

Because each IAccessibleEx instance represents only a single UI element, each IAccessible and child ID pair must be mapped to a single IAccessibleEx instance. IAccessibleEx includes two methods to handle this mapping:

  • GetObjectForChild—Retrieves the IAccessibleEx interface for the specified child. This method returns NULL if the IAccessibleEx implementation does not recognize the specified child ID, does not have an IAccessibleEx for the specified child, or represents a child element.
  • GetIAccessiblePair—Retrieves the IAccessible interface and child ID for the IAccessibleEx element. For IAccessible implementations that do not use a child ID, this method retrieves the corresponding IAccessible object and CHILDID_SELF.

The following example shows the implementation of the GetObjectForChild and GetIAccessiblePair methods for an item in a custom list view. The methods enable UI Automation to map the IAccessible and child ID pair to a corresponding IAccessibleEx instance.

           
HRESULT CListboxAccessibleObject::GetObjectForChild(
    long idChild, IAccessibleEx **ppRetVal)
{ 
    VARIANT vChild;
    vChild.vt = VT_I4;
    vChild.lVal = idChild;
    HRESULT hr = ValidateChildId(vChild);
    if (FAILED(hr))
    {
        return E_INVALIDARG;
    }
    // List item accessible objects are stored as an array of
    // pointers; for the purpose of this example it is assumed that 
    // the list contents will not change. Accessible objects are
    // created only when needed.
    if (itemProviders[idChild - 1] == NULL)
    {
        // Create an object that supports UI Automation and
        // IAccessibleEx for the item.
        itemProviders[idChild - 1] = 
          new CListItemAccessibleObject(idChild, 
          g_pListboxControl);
        if (itemProviders[idChild - 1] == NULL)
        {
            return E_OUTOFMEMORY;
        }
    }
    IAccessibleEx* pAccEx = static_cast<IAccessibleEx*>
      (itemProviders[idChild - 1]);
    if (pAccEx != NULL)
    {
      pAccEx->AddRef();
    }
    *ppRetVal = pAccEx;
    return S_OK; 
}

HRESULT CListItemAccessibleObject::GetIAccessiblePair(
    IAccessible **ppAcc, long *pidChild)
{ 
    if (ppAcc == NULL || pidChild == NULL)
    {
        return E_INVALIDARG;   
    }

                CListboxAccessibleObject* pParent = 
        m_control->GetAccessibleObject();

    HRESULT hr = pParent->QueryInterface( 
        __uuidof(IAccessible), (void**)ppAcc);
    if (FAILED(hr))
    {
        *pidChild = 0;
        return E_NOINTERFACE;
    }
    *pidChild = m_childID; 
    return S_OK; 
}
}

If an accessible object implementation does not use a child ID, the methods can still be implemented as shown in the following code snippet.

           

    // This sample implements IAccessibleEx on the same object; it could use a tear-off
    // or inner object instead.
    class MyAccessibleImpl: public IAccessible,
                        public IAccessibleEx,
                        public IRawElementProviderSimple
    {
    public:
    ...
   HRESULT STDMETHODCALLTYPE GetObjectForChild( long idChild, IAccessibleEx **ppRetVal )
    {
        // This implementation does not support child IDs.
        *ppRetVal = NULL;
        return S_OK;
    }

    HRESULT STDMETHODCALLTYPE GetIAccessiblePair( IAccessible ** ppAcc, long * pidChild )
    {
        // This implementation assumes that IAccessibleEx is implemented on same object as
        // IAccessible.
        *ppAcc = static_cast<IAccessible *>(this);
        (*ppAcc)->AddRef();
        *pidChild = CHILDID_SELF;
        return S_OK;
    }

Implement the IRawElementProviderSimple Interface

Servers use IRawElementProviderSimple to expose information about UI Automation properties and control patterns. IRawElementProviderSimple includes the following methods:

An IAccessibleEx server exposes control patterns by implementing IRawElementProviderSimple::GetPatternProvider. This method takes an integer parameter that specifies the control pattern. The server returns NULL if the pattern is not supported. If the control pattern interface is supported, servers return an IUnknown and the client then calls QueryInterface to get the appropriate control pattern.

An IAccessibleEx server can support UI Automation properties (such as LabeledBy, and IsRequiredForForm) by implementing IRawElementProviderSimple::GetPropertyValue and supplying an integer PROPERTYID identifying the property as a parameter. This technique applies only to UI Automation properties that are not included in a control pattern interface. Properties associated with a control pattern interface are exposed through the control pattern interface method. For example, the IsSelected property from the SelectionItem control pattern would be exposed with ISelectionItemProvider::get_IsSelected.