UI Automation Events for Clients

  • Article

Microsoft UI Automation allows clients to subscribe to events of interest. This capability improves performance by eliminating the need to continually poll the UI elements in the system to see if any information, structure, or state has changed.

Efficiency is also improved by the ability to listen for events only within a defined scope. For example, a client can listen for selection changes on an item in a list, on the list itself, or on an entire dialog box.

Note  Do not assume that all possible events are raised by a Microsoft UI Automation provider. For example, not all property changes cause events to be raised by the standard proxy providers for Windows Forms and Win32 controls.

For a broader view of UI Automation events, see UI Automation Events Overview.

This topic contains the following sections.

  • Subscribing to Events
  • Examples
  • Related Topics

Subscribing to Events

Client applications subscribe to events of a particular kind by registering an event handler, using one of the following methods.

Subscription Method Event Type Callback Interface
IUIAutomation::AddFocusChangedEventHandler Focus change IUIAutomationFocusChangedEventHandler
IUIAutomation::AddPropertyChangedEventHandler, IUIAutomation::AddPropertyChangedEventHandlerNativeArray Property change IUIAutomationPropertyChangedEventHandler
IUIAutomation::AddStructureChangedEventHandler Structure change IUIAutomationStructureChangedEventHandler
IUIAutomation::AddAutomationEventHandler Other events IUIAutomationEventHandler

When a client adds an event handler for all descendants (TreeScope_Descendants), UI Automation adds only one handler for the root of sub-tree, and the handler listens across all descendents. UI Automation does not add event handlers recursively.

When a client calls the IUIAutomation::RemoveAllEventHandlers method, UI Automation removes all event handlers from the client process.

To receive and handle events, you implement an event-handling object that exposes a callback interface, and you must register the object by calling an event registration method such as IUIAutomation::AddPropertyChangedEventHandler. The callback interface has a single method; UI Automation calls this method when the event is processed.

On shutdown, or when UI Automation events are no longer of interest to the application, UI Automation clients should call one or more of the following methods.

Note  A UI Automation client should not use multiple threads to add or remove event handlers. Unexpected behavior can result if one event handler is being added or removed while another is being added or removed in the same client process.

Method Description
IUIAutomation::RemoveAutomationEventHandler Unregisters an event handler that was registered by using IUIAutomation::AddAutomationEventHandler.
IUIAutomation::RemoveFocusChangedEventHandler Unregisters an event handler that was registered by using IUIAutomation::AddFocusChangedEventHandler.
IUIAutomation::RemovePropertyChangedEventHandler Unregisters an event handler that was registered by using IUIAutomation::AddPropertyChangedEventHandler or IUIAutomation::AddPropertyChangedEventHandlerNativeArray.
IUIAutomation::RemoveStructureChangedEventHandler Unregisters an event handler that was registered by using IUIAutomation::AddStructureChangedEventHandler.
IUIAutomation::RemoveAllEventHandlers Unregisters all registered event handlers.

Examples

The following code example is a Microsoft Windows console application that implements a UI Automation event handler for general UI Automation events. This example handles creation and destruction events for tooltips and other windows.

// Defines an event handler for general UI Automation events. It listens for
// tooltip and window creation and destruction events. 
#include <windows.h>
#include <stdio.h>
#include <UIAutomation.h>

class EventHandler:
    public IUIAutomationEventHandler
{
private:
    LONG _refCount;

public:
    int _eventCount;

    // Constructor.
    EventHandler(): _refCount(1), _eventCount(0) 
    {
    }

    // IUnknown methods.
    ULONG STDMETHODCALLTYPE AddRef() 
    {
        ULONG ret = InterlockedIncrement(&_refCount);
        return ret;
    }

    ULONG STDMETHODCALLTYPE Release() 
    {
        ULONG ret = InterlockedDecrement(&_refCount);
        if (ret == 0) 
        {
            delete this;
            return 0;
        }
        return ret;
    }

    HRESULT STDMETHODCALLTYPE QueryInterface(REFIID riid, void** ppInterface) 
    {
        if (riid == __uuidof(IUnknown)) 
            *ppInterface=static_cast<IUIAutomationEventHandler*>(this);
        else if (riid == __uuidof(IUIAutomationEventHandler)) 
            *ppInterface=static_cast<IUIAutomationEventHandler*>(this);
        else 
        {
            *ppInterface = NULL;
            return E_NOINTERFACE;
        }
        this->AddRef();
        return S_OK;
    }

    // IUIAutomationEventHandler methods
    HRESULT STDMETHODCALLTYPE HandleAutomationEvent(IUIAutomationElement * pSender, EVENTID eventID)
    {
        _eventCount++;
        switch (eventID) 
        {
            case UIA_ToolTipOpenedEventId:
                wprintf(L">> Event ToolTipOpened Received! (count: %d)\n", _eventCount);
                break;
            case UIA_ToolTipClosedEventId:
                wprintf(L">> Event ToolTipClosed Received! (count: %d)\n", _eventCount);
                break;
            case UIA_Window_WindowOpenedEventId:
                wprintf(L">> Event WindowOpened Received! (count: %d)\n", _eventCount);
                break;
            case UIA_Window_WindowClosedEventId:
                wprintf(L">> Event WindowClosed Received! (count: %d)\n", _eventCount);
                break;
            default:
                wprintf(L">> Event (%d) Received! (count: %d)\n", eventID, _eventCount);
                break;
        }
        return S_OK;
    }
};

int main(int argc, char* argv[])
{
    HRESULT hr;
    int ret = 0;
    IUIAutomationElement* pTargetElement = NULL;
    EventHandler* pEHTemp = NULL;

    CoInitializeEx(NULL,COINIT_MULTITHREADED);
    IUIAutomation* pAutomation = NULL;
    hr = CoCreateInstance(__uuidof(CUIAutomation), NULL,CLSCTX_INPROC_SERVER, __uuidof(IUIAutomation), (void**)&pAutomation);
    if(FAILED(hr) || pAutomation==NULL) 
    {
        ret = 1;
        goto cleanup;
    }
    // Use root element for listening to window and tooltip creation and destruction.
    hr = pAutomation->GetRootElement(&pTargetElement);
    if (FAILED(hr) || pTargetElement==NULL) 
    {
        ret = 1;
        goto cleanup;
    }

    pEHTemp = new EventHandler();
    if (pEHTemp == NULL) 
    {
        ret = 1;
        goto cleanup;
    }

    wprintf(L"-Adding Event Handlers.\n");
    hr = pAutomation->AddAutomationEventHandler(UIA_ToolTipOpenedEventId, pTargetElement, TreeScope_Subtree, NULL, (IUIAutomationEventHandler*) pEHTemp);
    if (FAILED(hr)) 
    {
        ret = 1;
        goto cleanup;
    }
    hr = pAutomation->AddAutomationEventHandler(UIA_ToolTipClosedEventId, pTargetElement, TreeScope_Subtree, NULL, (IUIAutomationEventHandler*) pEHTemp);
    if (FAILED(hr)) 
    {
        ret = 1;
        goto cleanup;
    }
    hr = pAutomation->AddAutomationEventHandler(UIA_Window_WindowOpenedEventId, pTargetElement, TreeScope_Subtree, NULL, (IUIAutomationEventHandler*) pEHTemp);
    if (FAILED(hr)) 
    {
        ret = 1;
        goto cleanup;
    }
    hr = pAutomation->AddAutomationEventHandler(UIA_Window_WindowClosedEventId, pTargetElement, TreeScope_Subtree, NULL, (IUIAutomationEventHandler*) pEHTemp);
    if (FAILED(hr)) 
    {
        ret = 1;
        goto cleanup;
    }

    wprintf(L"-Press any key to remove event handlers and exit\n");
    getchar();

    wprintf(L"-Removing Event Handlers.\n");

cleanup:
    // Remove event handlers, release resources, and terminate
    if (pAutomation != NULL) 
    {
        hr = pAutomation->RemoveAllEventHandlers();
        if (FAILED(hr))
            ret = 1;
        pAutomation->Release();
    }

    if (pEHTemp != NULL) 
        pEHTemp->Release();

    if (pTargetElement != NULL) 
        pTargetElement->Release();

    CoUninitialize();
    return ret;
}

The following code example is a Windows console application that implements a handler for focus-changed events.

// Defines an event handler for focus changed events and starts
// listening to them.
#include <windows.h>
#include <stdio.h>
#include <UIAutomation.h>

class EventHandler:
    public IUIAutomationFocusChangedEventHandler
{
private:
    LONG _refCount;

public:
    int _eventCount;

    //Constructor.
    EventHandler(): _refCount(1), _eventCount(0) 
    {
    }

    //IUnknown methods.
    ULONG STDMETHODCALLTYPE AddRef() 
    {
        ULONG ret = InterlockedIncrement(&_refCount);
        return ret;
    }

    ULONG STDMETHODCALLTYPE Release() 
    {
        ULONG ret = InterlockedDecrement(&_refCount);
        if (ret == 0) 
        {
            delete this;
            return 0;
        }
        return ret;
    }

    HRESULT STDMETHODCALLTYPE QueryInterface(REFIID riid, void** ppInterface) 
    {
        if (riid == __uuidof(IUnknown))
            *ppInterface = static_cast<IUIAutomationFocusChangedEventHandler*>(this);
        else if(riid == __uuidof(IUIAutomationFocusChangedEventHandler)) 
            *ppInterface=static_cast<IUIAutomationFocusChangedEventHandler*>(this);
        else 
        {
            *ppInterface = NULL;
            return E_NOINTERFACE;
        }
        this->AddRef();
        return S_OK;
    }

    // IUIAutomationFocusChangedEventHandler methods.
    HRESULT STDMETHODCALLTYPE HandleFocusChangedEvent(IUIAutomationElement * pSender) 
    {
        _eventCount++;
        wprintf(L">> FocusChangedEvent Received! (count: %d)\n", _eventCount);
        return S_OK;
    }
};

int main(int argc, char* argv[]) 
{
    HRESULT hr;
    int ret = 0;
    EventHandler* pEHTemp = NULL;

    CoInitializeEx(NULL,COINIT_MULTITHREADED);
    IUIAutomation* pAutomation = NULL;
    hr = CoCreateInstance(__uuidof(CUIAutomation), NULL,CLSCTX_INPROC_SERVER, __uuidof(IUIAutomation), (void**)&pAutomation);
    if (FAILED(hr) || pAutomation == NULL)
    {
        ret = 1;
        goto cleanup;
    }

    pEHTemp = new EventHandler();
    if (pEHTemp == NULL)
    {
        ret = 1;
        goto cleanup;
    }

    wprintf(L"-Adding Event Handlers.\n");
    hr = pAutomation->AddFocusChangedEventHandler(NULL, (IUIAutomationFocusChangedEventHandler*) pEHTemp); 
    if (FAILED(hr)) 
    {
        ret = 1;
        goto cleanup;
    }

    wprintf(L"-Press any key to remove event handler and exit\n");
    getchar();

    wprintf(L"-Removing Event Handlers.\n");
    hr = pAutomation->RemoveFocusChangedEventHandler((IUIAutomationFocusChangedEventHandler*) pEHTemp);
    if (FAILED(hr)) 
    {
        ret = 1;
        goto cleanup;
    }

    // Release resources and terminate.
cleanup:
    if (pEHTemp != NULL) 
        pEHTemp->Release();

    if (pAutomation != NULL) 
        pAutomation->Release();

    CoUninitialize();
    return ret;
}

The following code example is a Windows console application that implements a handler for property-changed events.

// Defines an event handler for property-changed events, and listens for
// ToggleState property changes on the element specifies by the user.
#include <windows.h>
#include <stdio.h>
#include <UIAutomation.h>

class EventHandler:
    public IUIAutomationPropertyChangedEventHandler
{
private:
    LONG _refCount;

public:
    int _eventCount;

    //Constructor.
    EventHandler(): _refCount(1), _eventCount(0) 
    {
    }

    //IUnknown methods.
    ULONG STDMETHODCALLTYPE AddRef() 
    {
        ULONG ret = InterlockedIncrement(&_refCount);
        return ret;
    }

    ULONG STDMETHODCALLTYPE Release() 
    {
        ULONG ret = InterlockedDecrement(&_refCount);
        if (ret == 0) 
        {
            delete this;
            return 0;
        }
        return ret;
    }

    HRESULT STDMETHODCALLTYPE QueryInterface(REFIID riid, void** ppInterface) 
    {
        if (riid == __uuidof(IUnknown))
            *ppInterface=static_cast<IUIAutomationPropertyChangedEventHandler*>(this);
        else if (riid == __uuidof(IUIAutomationPropertyChangedEventHandler)) 
            *ppInterface=static_cast<IUIAutomationPropertyChangedEventHandler*>(this);
        else 
        {
            *ppInterface = NULL;
            return E_NOINTERFACE;
        }
        this->AddRef();
        return S_OK;
    }

    // IUIAutomationPropertyChangedEventHandler methods.
    HRESULT STDMETHODCALLTYPE HandlePropertyChangedEvent(IUIAutomationElement* pSender, PROPERTYID propertyID, VARIANT newValue) 
    {
        _eventCount++;
        if (propertyID == UIA_ToggleToggleStatePropertyId) 
            wprintf(L">> Property ToggleState changed! ");
        else 
            wprintf(L">> Property (%d) changed! ", propertyID);

        if (newValue.vt == VT_I4) 
            wprintf(L"(0x%0.8x) ", newValue.lVal);

        wprintf(L"(count: %d)\n", _eventCount);
        return S_OK;
    }
};

int main(int argc, char* argv[]) 
{
    HRESULT hr;
    int ret = 0;
    IUIAutomationElement* pTargetElement = NULL;
    EventHandler* pEHTemp = NULL;

    CoInitializeEx(NULL,COINIT_MULTITHREADED);

    IUIAutomation* pAutomation = NULL;
    hr = CoCreateInstance(__uuidof(CUIAutomation), NULL, CLSCTX_INPROC_SERVER, __uuidof(IUIAutomation), (void**)&pAutomation);
    if (FAILED(hr) || pAutomation==NULL) 
    {
        ret = 1;
        goto cleanup;
    }

    wprintf(L"-Use the mouse to point to the element you want to listen from.\n");
    Sleep(3000);

    // Make this application dots-per-inch (DPI) aware.
    SetProcessDPIAware();

    // Get mouse cursor position and get element from point.
    POINT pt;
    GetPhysicalCursorPos(&pt);
    hr = pAutomation->ElementFromPoint(pt, &pTargetElement);
    if (FAILED(hr) || pTargetElement==NULL) 
    {
        ret = 1;
        goto cleanup;
    }

    pEHTemp = new EventHandler();
    if (pEHTemp == NULL) 
    {
        ret = 1;
        goto cleanup;
    }

    PROPERTYID pPIDProperties[] = { UIA_ToggleToggleStatePropertyId };

    wprintf(L"-Adding Event Handler.\n");
    hr = pAutomation->AddPropertyChangedEventHandlerNativeArray(pTargetElement, TreeScope_Subtree, NULL, (IUIAutomationPropertyChangedEventHandler*) pEHTemp, pPIDProperties, sizeof(pPIDProperties) / sizeof(pPIDProperties[0]));
    if (FAILED(hr)) 
    {
        ret = 1;
        goto cleanup;
    }

    wprintf(L"-Press any key to remove event handler and exit\n");
    getchar();

    wprintf(L"-Removing Event Handler.\n");
    hr = pAutomation->RemovePropertyChangedEventHandler(pTargetElement, (IUIAutomationPropertyChangedEventHandler*) pEHTemp);
    if (FAILED(hr)) 
    {
        ret = 1;
        goto cleanup;
    }

    // Release resources and terminate.
cleanup:
    if (pEHTemp != NULL) 
        pEHTemp->Release();

    if (pTargetElement != NULL) 
        pTargetElement->Release();

    if (pAutomation != NULL) 
        pAutomation->Release();

    CoUninitialize();
    return ret;
}

The following code example is a Windows console application that implements a handler for structure-changed events.

// Defines an event handler for structure-changed events, and 
// listens for them on the element specifies by the user.
#include <windows.h>
#include <stdio.h>
#include <UIAutomation.h>

class EventHandler:
    public IUIAutomationStructureChangedEventHandler
{
private:
    LONG _refCount;

public:
    int _eventCount;

    // Constructor.
    EventHandler(): _refCount(1), _eventCount(0) 
    {
    }

    // IUnknown methods.
    ULONG STDMETHODCALLTYPE AddRef() 
    {
        ULONG ret = InterlockedIncrement(&_refCount);
        return ret;
    }

    ULONG STDMETHODCALLTYPE Release() 
    {
        ULONG ret = InterlockedDecrement(&_refCount);
        if (ret == 0) 
        {
            delete this;
            return 0;
        }
        return ret;
    }

    HRESULT STDMETHODCALLTYPE QueryInterface(REFIID riid, void** ppInterface) 
    {
        if (riid == __uuidof(IUnknown))
            *ppInterface=static_cast<IUIAutomationStructureChangedEventHandler*>(this);
        else if (riid == __uuidof(IUIAutomationStructureChangedEventHandler)) 
            *ppInterface=static_cast<IUIAutomationStructureChangedEventHandler*>(this);
        else 
        {
            *ppInterface = NULL;
            return E_NOINTERFACE;
        }
        this->AddRef();
        return S_OK;
    }

    // IUIAutomationStructureChangedEventHandler methods
    HRESULT STDMETHODCALLTYPE HandleStructureChangedEvent(IUIAutomationElement* pSender, StructureChangeType changeType, SAFEARRAY* pRuntimeID) {
        _eventCount++;
        switch (changeType) 
        {
            case StructureChangeType_ChildAdded:
                wprintf(L">> Structure Changed: ChildAdded! (count: %d)\n", _eventCount);
                break;
            case StructureChangeType_ChildRemoved:
                wprintf(L">> Structure Changed: ChildRemoved! (count: %d)\n", _eventCount);
                break;
            case StructureChangeType_ChildrenInvalidated:
                wprintf(L">> Structure Changed: ChildrenInvalidated! (count: %d)\n", _eventCount);
                break;
            case StructureChangeType_ChildrenBulkAdded:
                wprintf(L">> Structure Changed: ChildrenBulkAdded! (count: %d)\n", _eventCount);
                break;
            case StructureChangeType_ChildrenBulkRemoved:
                wprintf(L">> Structure Changed: ChildrenBulkRemoved! (count: %d)\n", _eventCount);
                break;
            case StructureChangeType_ChildrenReordered:
                wprintf(L">> Structure Changed: ChildrenReordered! (count: %d)\n", _eventCount);
                break;
        }
        return S_OK;
    }
};

int main(int argc, char* argv[]) 
{
    HRESULT hr;
    int ret = 0;
    IUIAutomationElement* pTargetElement = NULL;
    EventHandler* pEHTemp = NULL;

    CoInitializeEx(NULL,COINIT_MULTITHREADED);

    IUIAutomation* pAutomation=NULL;
    hr = CoCreateInstance(__uuidof(CUIAutomation), NULL, CLSCTX_INPROC_SERVER, __uuidof(IUIAutomation), (void**)&pAutomation);
    if (FAILED(hr) || pAutomation == NULL) 
    {
        ret = 1;
        goto cleanup;
    }

    wprintf(L"-Use the mouse to point to the element you want to listen from.\n");
    Sleep(3000);

    // Make this application dots-per-inch (DPI) aware.
    SetProcessDPIAware();

    // Get mouse cursor position and get element from point.
    POINT pt;
    GetPhysicalCursorPos(&pt);
    hr = pAutomation->ElementFromPoint(pt, &pTargetElement);
    if (FAILED(hr) || pTargetElement == NULL) 
    {
        ret = 1;
        goto cleanup;
    }

    pEHTemp = new EventHandler();
    if (pEHTemp == NULL) 
    {
        ret = 1;
        goto cleanup;
    }

    wprintf(L"-Adding Event Handler.\n");
    hr = pAutomation->AddStructureChangedEventHandler(pTargetElement, TreeScope_Subtree, NULL, (IUIAutomationStructureChangedEventHandler*) pEHTemp);
    if (FAILED(hr)) 
    {
        ret = 1;
        goto cleanup;
    }

    wprintf(L"-Press any key to remove event handler and exit\n");
    getchar();

    wprintf(L"-Removing Event Handler.\n");
    hr = pAutomation->RemoveStructureChangedEventHandler(pTargetElement, (IUIAutomationStructureChangedEventHandler*) pEHTemp);
    if (FAILED(hr)) 
    {
        ret = 1;
        goto cleanup;
    }

    // Release resources and terminate.
cleanup:
    if (pEHTemp != NULL) 
        pEHTemp->Release();

    if (pTargetElement != NULL) 
        pTargetElement->Release();

    if (pAutomation != NULL) 
        pAutomation->Release();

    CoUninitialize();
    return ret;
}