首先我們要明白截圖軟件的基本原理，截圖時實際上是新建了一個全屏窗口，然后將當前桌面的截圖畫在上面，大部分截圖軟件，包括QQ都是這么做的。根據鼠標位置獲取下層窗口，有好幾個類似的API可以用(WindowFromPoint, ChildWindowFromPoint, ChildWindowFromPointEx,RealChildWindowFromPoint)。
這里我們重點關注ChildWindowFromPointEx，因為我們知道截圖時有個全屏窗口覆蓋在上面，通過鼠標位置去取得窗口，肯定首先取到的是這個全屏窗口，所以我們要把這個窗口過濾掉，而只有ChildWindowFromPointEx這個API有窗口過濾功能。
所以我們有理由相信QQ的全屏窗口用了WS_EX_LAYERED屬性，然后QQ通過調用ChildWindowFromPointEx(hWndDesktop,ptCursor, CWP_SKIPINVISIBLE|CWP_SKIPTRANSPARENT), 這樣就可以過濾掉不可見的和Layered窗口，然后通過遞歸調用該API，就可以獲取里面的子窗口了。
另外我們可以在啟動QQ截圖后，通過Windows鍵激活任務欄，然后改變通過任務欄最小化或是關閉某個包含在截圖內的窗口，再繼續截圖就會發現QQ沒法識別了。這也說明了QQ截圖是實時通過ChildWindowFromPointEx來獲取下層窗口的。這也算是QQ截圖的一個Bug。
很多截圖軟件卻沒有上述問題，我想他們應該是在開始截圖時保存了桌面上所有窗口的層次關系和所在區域，后面用的都是當時保存的信息來識別的，這樣即使后面下面的窗口變化了，識別也不會受到影響。
另外，有些截圖軟件能夠識別到比窗口粒度更小的元素，比如Toolbar控件上的每個Item，他們用的應該是MSAA(Microsoft Active Accessibility),標準控件一般都支持該接口。
class CSCWinFilter
{
public:
static BOOL IsFilterWindow(HWND hWnd)
{
_ASSERTE(hWnd != NULL);
DWORD dwProcessID = GetCurrentProcessId();
if(hWnd != NULL && IsWindow(hWnd))
{
DWORD dwWinProcessId(0);
GetWindowThreadProcessId(hWnd, &dwWinProcessId);
if(dwProcessID == dwWinProcessId)
{
return TRUE;
}
}
return FALSE;
}
static DWORD GetIncludeStyle()
{
return WS_VISIBLE;
}
static DWORD GetExcludeStyleEx()
{
return WS_EX_TRANSPARENT;
}
static BOOL IsTargetPopupWindow()
{
return FALSE;
}
};
class CSCWinInfo
{
public:
HWND m_hWnd;
CRect m_rtWin;
//window rect
INT m_nLevel;
// 1 - pop up window ; 2
N - child window
};
//pop up win 1 (level 1).. first Z order
// child11 (level 2)
// child12 (level 2)
// chilld121 (level 3)
// chilld122 (level 3)
//
// child3 (level 2)
//pop up win2
// child21 (level 2)
// child21 (level 2)
// .
// .
//pop up winN . last Z order
template<typename CWinFilterTraits = CSCWinFilter>
class CSCWinSpy:
public CHYSingleton<CSCWinSpy>
{
public:
BOOL SnapshotAllWinRect()
{
ClearData();
// cache current window Z order when call this function
EnumWindows(EnumWindowsSnapshotProc, 1);
return TRUE;
}
//get from current Z order of desktop
HWND GetHWNDByPoint(CPoint pt)
{
m_hWndTarget = NULL;
EnumWindows(EnumWindowsRealTimeProc, MAKELPARAM(pt.x, pt.y));
return m_hWndTarget;
}
CRect GetWinRectByPoint(CPoint ptHit, BOOL bGetInRealTime = FALSE)
{
CRect rtRect(0, 0, 0, 0);
if(bGetInRealTime)
//get from current Z order
{
HWND hWndTarget = GetHWNDByPoint(ptHit);
if(hWndTarget != NULL )
{
GetWindowRect(hWndTarget, &rtRect);
}
}
else //get from snapshot cache
{
GetRectByPointFromSnapshot(ptHit, rtRect);
}
return rtRect;
}
protected:
static BOOL CALLBACK EnumWindowsRealTimeProc(HWND hwnd, LPARAM lParam)
{
if(!PtInWinRect(hwnd, CPoint(GET_X_LPARAM(lParam), GET_Y_LPARAM(lParam))))
return TRUE;
if(ShouldWinBeFiltered(hwnd))
return TRUE;
m_hWndTarget = hwnd;
if(CWinFilterTraits::IsTargetPopupWindow())
return FALSE;
//this is the target window, exit search
EnumChildWindows(hwnd, EnumChildRealTimeProc, lParam);
return FALSE;
}
static BOOL CALLBACK EnumChildRealTimeProc(HWND hwnd, LPARAM lParam)
{
if(!PtInWinRect(hwnd, CPoint(GET_X_LPARAM(lParam), GET_Y_LPARAM(lParam))))
return TRUE;
if(ShouldWinBeFiltered(hwnd))
return TRUE;
m_hWndTarget = hwnd;
EnumChildWindows(hwnd, EnumChildRealTimeProc, lParam);
return FALSE;
}
protected:
static BOOL CALLBACK EnumWindowsSnapshotProc(HWND hwnd, LPARAM lParam)
{
INT nLevel = lParam;
if(ShouldWinBeFiltered(hwnd))
return TRUE;
SaveSnapshotWindow(hwnd, nLevel);
if(!CWinFilterTraits::IsTargetPopupWindow())
{
++nLevel;
EnumChildWindows(hwnd, EnumChildSnapshotProc, nLevel);
}
return TRUE;
}
static BOOL CALLBACK EnumChildSnapshotProc(HWND hwnd, LPARAM lParam)
{
INT nLevel = lParam;
if(ShouldWinBeFiltered(hwnd))
return TRUE;
SaveSnapshotWindow(hwnd, nLevel);
++nLevel;
EnumChildWindows(hwnd, EnumChildSnapshotProc, nLevel);
return TRUE;
}
protected:
static BOOL PtInWinRect(HWND hWnd, CPoint pt)
{
CRect rtWin(0, 0, 0, 0);
GetWindowRect(hWnd, &rtWin);
return PtInRect(&rtWin, pt);
}
static BOOL ShouldWinBeFiltered(HWND hWnd)
{
if(CWinFilterTraits::IsFilterWindow(hWnd))
return TRUE;
DWORD dwStyle = GetWindowLong(hWnd, GWL_STYLE);
DWORD dwStyleMust = CWinFilterTraits::GetIncludeStyle();
if((dwStyle & dwStyleMust) != dwStyleMust)
return TRUE;
DWORD dwStyleEx = GetWindowLong(hWnd, GWL_EXSTYLE);
DWORD dwStyleMustNot = CWinFilterTraits::GetExcludeStyleEx();
if((dwStyleMustNot & dwStyleEx) != 0)
return TRUE;
return FALSE;
}
//find the first window that level is biggest
static BOOL GetRectByPointFromSnapshot(CPoint ptHit, CRect& rtRet)
{
int nCount = m_arSnapshot.size();
_ASSERTE(nCount > 0);
CSCWinInfo* pInfo = NULL;
CSCWinInfo* pTarget = NULL;
for(
int i=0; i<nCount; ++i)
{
pInfo = m_arSnapshot[i];
_ASSERTE(pInfo != NULL);
//target window is found
//and level is not increasing,
//that is checking its sibling or parent window, exit search
if(pTarget != NULL
&& pInfo->m_nLevel <= pTarget->m_nLevel)
{
break;
}
if(PtInRect(&pInfo->m_rtWin, ptHit))
{
if(pTarget == NULL)
{
pTarget = pInfo;
}
else {
if( pInfo->m_nLevel > pTarget->m_nLevel)
{
pTarget = pInfo;
}
}
}
}
if(pTarget != NULL)
{
#ifdef _DEBUG
if(pTarget != NULL)
{
HWND hWnd = pTarget->m_hWnd;
TCHAR szText[128] = {0};
_sntprintf(szText, 127, _T("GetRectByPointFromSnapshot: pt(%d, %d), hWnd(%x)"),
ptHit.x, ptHit.y, (UINT)(pInfo->m_hWnd));
OutputDebugString(szText);
}
#endif rtRet.CopyRect(&pTarget->m_rtWin);
return TRUE;
}
return FALSE;
}
static VOID SaveSnapshotWindow(HWND hWnd, INT nLevel)
{
_ASSERTE(hWnd != NULL && IsWindow(hWnd));
CRect rtWin(0, 0, 0, 0);
GetWindowRect(hWnd, &rtWin);
if(rtWin.IsRectEmpty())
return;
CSCWinInfo* pInfo =
new CSCWinInfo;
if(pInfo == NULL)
return;
pInfo->m_hWnd = hWnd;
pInfo->m_nLevel = nLevel;
pInfo->m_rtWin = rtWin;
m_arSnapshot.push_back(pInfo);
}
static VOID ClearData()
{
int nCount = m_arSnapshot.size();
for(
int i=0; i<nCount; ++i)
{
delete m_arSnapshot[i];
}
m_arSnapshot.clear();
}
protected:
friend
class CHYSingleton<CSCWinSpy>;
CSCWinSpy() { NULL; }
~CSCWinSpy() { ClearData(); }
static HWND m_hWndTarget;
static std::vector<CSCWinInfo*> m_arSnapshot;
};
template<typename T> HWND CSCWinSpy<T>::m_hWndTarget = NULL;
template<typename T> std::vector<CSCWinInfo*> CSCWinSpy<T>::m_arSnapshot;
CRect rtSelect = CSCWinSpy<CSCWinFilter>::GetInstance()->GetWinRectByPoint(pt, FALSE);