How can I be notified when a new window is created on Win32?

Solution 1:

Use SetWindowsHookEx to set up a WH_SHELL hook and look for the HSHELL_WINDOWCREATED event.

Solution 2:

Sure - you can write a CBT hook and watch for HCBT_CREATEWND. See also SetWindowsHookEx().

Note that this will allow you to be notified of all window creation, before the windows being created are even fully initialized. If all you need are unowned, top-level windows, RichieHindle's suggestion may work better...

Solution 3:

Here is some code based on UI automation events. It gives window opened, closed, and focused events.

C#

[STAThread]
public static void Main(string[] args)
{
    Automation.AddAutomationEventHandler(WindowPattern.WindowOpenedEvent, AutomationElement.RootElement, TreeScope.Children, (sender, e) =>
    {
        var element = (AutomationElement)sender;
        var name = element.Current.Name;
        Console.WriteLine("open: " + name + " hwnd:" + element.Current.NativeWindowHandle);
        Automation.AddAutomationEventHandler(WindowPattern.WindowClosedEvent, element, TreeScope.Element, (s, e2) =>
        {
            Console.WriteLine("close: " + name + " hwnd:" + element.Current.NativeWindowHandle);
        });
    });

    Automation.AddAutomationFocusChangedEventHandler((sender, e) =>
    {
        var element = (AutomationElement)sender;
        var name = element.Current.Name;
        Console.WriteLine("focused: " + name + " hwnd:" + element.Current.NativeWindowHandle);
    });
    Console.ReadLine();
    Automation.RemoveAllEventHandlers();
}

C++ equivalent:

#include <windows.h>
#include <stdio.h>
#include <uiautomation.h>

// some useful macros
#define WIDEN2(x) L ## x
#define WIDEN(x) WIDEN2(x)
#define __WFILE__ WIDEN(__FILE__)

#define MYTRACE wprintf
#define CHECKHR(expr) {hr=(expr);if(FAILED(hr)){ MYTRACE(L"HR FAILED line:%u file:%s\n", __LINE__, __WFILE__); goto cleanup; } }  
#define CHECKWIN32(expr) {if(!(expr)){hr = HRESULT_FROM_WIN32(GetLastError()); MYTRACE(L"WIN32 FAILED line:%u file:%s\n", __LINE__, __WFILE__); goto cleanup; } }  
#define CHECKARG(expr) {if(!(expr)){ MYTRACE(L"ARG FAILED line:%u file:%s\n", __LINE__, __WFILE__); hr = E_INVALIDARG; goto cleanup; } }  
#define CHECKMEM(expr) {if(!(expr)){ MYTRACE(L"MEM FAILED line:%u file:%s\n", __LINE__, __WFILE__); hr = E_OUTOFMEMORY; goto cleanup; } } 
#define CORELEASE(expr) {if(expr){ expr->Release(); expr = NULL; } } 
#define HR HRESULT hr=S_OK;

class EventHandler :
  public IUIAutomationEventHandler,
  public IUIAutomationFocusChangedEventHandler
{
private:
  LONG _ref;
  IUIAutomation* _automation;
  HWND _hwnd;
  IUIAutomationElement* _sender;

public:
  EventHandler(IUIAutomation* automation, IUIAutomationElement* sender, HWND hwnd) :
    _ref(1),
    _automation(automation),
    _sender(sender),
    _hwnd(hwnd)
  {
    if (sender)
    {
      sender->AddRef();
    }
  }

  ~EventHandler()
  {
    CORELEASE(_sender);
  }

  // IUnknown
  ULONG STDMETHODCALLTYPE AddRef() { ULONG ret = InterlockedIncrement(&_ref); return ret; }
  ULONG STDMETHODCALLTYPE Release() { ULONG ret = InterlockedDecrement(&_ref); if (!ret) { delete this; return 0; } return ret; }
  HRESULT STDMETHODCALLTYPE QueryInterface(REFIID riid, void** ppInterface)
  {
    if (riid == __uuidof(IUnknown))
    {
      *ppInterface = (IUIAutomationEventHandler*)this;
    }
    else if (riid == __uuidof(IUIAutomationEventHandler))
    {
      *ppInterface = (IUIAutomationEventHandler*)this;
    }
    else if (riid == __uuidof(IUIAutomationFocusChangedEventHandler))
    {
      *ppInterface = (IUIAutomationFocusChangedEventHandler*)this;
    }
    else
    {
      *ppInterface = NULL;
      return E_NOINTERFACE;
    }

    AddRef();
    return S_OK;
  }

  // IUIAutomationFocusChangedEventHandler
  HRESULT STDMETHODCALLTYPE HandleFocusChangedEvent(IUIAutomationElement* sender)
  {
    HWND hwnd = NULL;
    sender->get_CurrentNativeWindowHandle((UIA_HWND*)&hwnd);
    wprintf(L"Window focused hwnd:%p'\n", hwnd);
    return S_OK;
  }

  // IUIAutomationEventHandler
  HRESULT STDMETHODCALLTYPE HandleAutomationEvent(IUIAutomationElement* sender, EVENTID eventID)
  {
    HR;
    HWND hwnd = NULL;
    EventHandler* windowHandler;

    switch (eventID)
    {
    case UIA_Window_WindowOpenedEventId:
      sender->get_CurrentNativeWindowHandle((UIA_HWND*)&hwnd);
      wprintf(L"Window opened hwnd:%p\n", hwnd);

      // register for close on this window
      // we build a new handler, this is the only way to remember the hwnd (the close event doesn't have anything)
      windowHandler = new EventHandler(_automation, sender, hwnd); // implicit addref
      CHECKMEM(windowHandler);
      CHECKHR(_automation->AddAutomationEventHandler(UIA_Window_WindowClosedEventId, sender, TreeScope_Element, NULL, windowHandler));
      break;

    case UIA_Window_WindowClosedEventId:
      wprintf(L"Window closed hwnd:%p\n", _hwnd);
      CHECKHR(_automation->RemoveAutomationEventHandler(UIA_Window_WindowClosedEventId, _sender, this));
      Release(); // we release our own reference, 'this' we be deleted sometime when all COM references are gone. don't do 'delete this'!
      break;
    }

  cleanup:
    return hr;
  }
};

int main()
{
  HR;
  IUIAutomationElement* root = NULL;
  EventHandler* handler = NULL;
  IUIAutomation* automation = NULL;

  CoInitializeEx(NULL, COINIT_MULTITHREADED);
  CHECKHR(CoCreateInstance(__uuidof(CUIAutomation), NULL, CLSCTX_INPROC_SERVER, __uuidof(IUIAutomation), (void**)&automation));

  CHECKHR(automation->GetRootElement(&root));

  handler = new EventHandler(automation, NULL, NULL);
  CHECKMEM(handler);

  CHECKHR(automation->AddAutomationEventHandler(UIA_Window_WindowOpenedEventId, root, TreeScope_Subtree, NULL, handler));
  CHECKHR(automation->AddFocusChangedEventHandler(NULL, handler));

  wprintf(L"Press any key to stop listening for events.\n");
  getchar();

cleanup:
  if (automation != NULL)
  {
    automation->RemoveAllEventHandlers();
    CORELEASE(automation);
  }

  CORELEASE(handler);
  CORELEASE(root);
  CoUninitialize();
  return hr;
}