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在前一個Post當中，指出了在WPF的WindowInteropHelper類中的一個BUG：通過WindowInteropHelper的Owner屬性不能實現把WPF窗口的Owner屬性設置為一個非WPF窗口的句柄。

在我的Post帖出后不到一天，在WPF SDK的Blog上，就針對這個BUG給出了一個非常完美的解決方案。既然不同通過設置WindowStartupLocation.CenterOwner來改變窗口的位置。那么我們就用WindowStartupLocation.Manual來手動計算設置窗口的位置。大致的代碼如下：

using System.Windows;
using System.Windows.Interop; // WindowInteropHelper

...

// Instantiate the owned WPF window
Window cw = new Window();

// Set the owned WPF window’s owner with the non-WPF owner window
IntPtr ownerWindowHandle = ...;
 
// Set the owned WPF window’s owner with the non-WPF owner window

WindowInteropHelper helper = new WindowInteropHelper(cw);
helper.Owner = ownerWindowHandle;

// Manually calculate Top/Left to appear centered
int nonWPFOwnerLeft = ...;  // Get non-WPF owner’s Left
int nonWPFOwnerWidth = ...;  // Get non-WPF owner’s Width
int nonWPFOwnerTop = ...;  // Get non-WPF owner’s Top
int nonWPFOwnerHeight = ...;  // Get non-WPF owner’s Height

cw.WindowStartupLocation = WindowStartupLocation.Manual;
cw.Left = nonWPFOwnerLeft + (nonWPFOwnerWidth - cw.Width) / 2;
cw.Top = nonWPFOwnerTop + (nonWPFOwnerHeight - cw.Height) / 2;

// Show the owned WPF window
cw.Show();

這段代碼理論上沒有什么問題呢？但是WPF是支持設備獨立的。因此，在非WPF Owner窗口的某些情況下可能會因為DPI的而不能正常工作。解決這個問題，可以利用HwndSource類進行窗口位置的設備獨立計算：

using System.Windows; // Window, WindowStartupLocation, Point
using System.Windows.Interop; // WindowInteropHelper, HwndSource
using System.Windows.Media; // Matrix

...

// Instantiate the owned WPF window
CenteredWindow cw = new CenteredWindow();

// Get the handle to the non-WPF owner window
IntPtr ownerWindowHandle = ...; // Get hWnd for non-WPF window
 
// Set the owned WPF window’s owner with the non-WPF owner window
WindowInteropHelper helper = new WindowInteropHelper(cw);
helper.Owner = ownerWindowHandle;
 

// Center window
// Note - Need to use HwndSource to get handle to WPF owned window,
//        and the handle only exists when SourceInitialized has been
//        raised

cw.SourceInitialized += delegate
{
    // Get WPF size and location for non-WPF owner window
    int nonWPFOwnerLeft = ...; // Get non-WPF owner’s Left
    int nonWPFOwnerWidth = ...; // Get non-WPF owner’s Width
    int nonWPFOwnerTop = ...; // Get non-WPF owner’s Top
    int nonWPFOwnerHeight = ...; // Get non-WPF owner’s Height

    // Get transform matrix to transform non-WPF owner window
    // size and location units into device-independent WPF
    // size and location units

    HwndSource source = HwndSource.FromHwnd(helper.Handle);
    if (source == null) return;
    Matrix matrix = source.CompositionTarget.TransformFromDevice;
    Point ownerWPFSize = matrix.Transform(
      new Point(nonWPFOwnerWidth, nonWPFOwnerHeight));
    Point ownerWPFPosition = matrix.Transform(
      new Point(nonWPFOwnerLeft, nonWPFOwnerTop));

    // Center WPF window
    cw.WindowStartupLocation = WindowStartupLocation.Manual;
    cw.Left = ownerWPFPosition.X + (ownerWPFSize.X - cw.Width) / 2;
    cw.Top = ownerWPFPosition.Y + (ownerWPFSize.Y - cw.Height) / 2;

};

// Show WPF owned window
cw.Show();

在上面的代碼中需要注意的是HwndSource的使用。這個類需要一個窗口句柄，因此它的代碼被放在一個SourceInitialized的事件委派函數中執(zhí)行。

最后，除了上面這種方法，其實我們還可以用Win32 API函數來實現，在ATL的CWindow類中，就有這樣的一個函數，我直接把放在下面，有興趣的朋友參考其中的實現原理： 

BOOL CenterWindow(HWND hWndCenter = NULL) throw()

...{

    ATLASSERT(::IsWindow(m_hWnd));

    // determine owner window to center against

    DWORD dwStyle = GetStyle();

    if(hWndCenter == NULL)

    ...{

        if(dwStyle & WS_CHILD)

            hWndCenter = ::GetParent(m_hWnd);

        else

            hWndCenter = ::GetWindow(m_hWnd, GW_OWNER);

    }

    // get coordinates of the window relative to its parent

    RECT rcDlg;

    ::GetWindowRect(m_hWnd, &rcDlg);

    RECT rcArea;

    RECT rcCenter;

    HWND hWndParent;

    if(!(dwStyle & WS_CHILD))

    ...{

        // don‘t center against invisible or minimized windows

        if(hWndCenter != NULL)

        ...{

            DWORD dwStyleCenter = ::GetWindowLong(hWndCenter, GWL_STYLE);

            if(!(dwStyleCenter & WS_VISIBLE) || (dwStyleCenter & WS_MINIMIZE))

                hWndCenter = NULL;

        }

        // center within screen coordinates

        ::SystemParametersInfo(SPI_GETWORKAREA, NULL, &rcArea, NULL);

        if(hWndCenter == NULL)

            rcCenter = rcArea;

        else

            ::GetWindowRect(hWndCenter, &rcCenter);

    }

    else

    ...{

        // center within parent client coordinates

        hWndParent = ::GetParent(m_hWnd);

        ATLASSERT(::IsWindow(hWndParent));

        ::GetClientRect(hWndParent, &rcArea);

        ATLASSERT(::IsWindow(hWndCenter));

        ::GetClientRect(hWndCenter, &rcCenter);

        ::MapWindowPoints(hWndCenter, hWndParent, (POINT*)&rcCenter, 2);

    }

    int DlgWidth = rcDlg.right - rcDlg.left;

    int DlgHeight = rcDlg.bottom - rcDlg.top;

    // find dialog‘s upper left based on rcCenter

    int xLeft = (rcCenter.left + rcCenter.right) / 2 - DlgWidth / 2;

    int yTop = (rcCenter.top + rcCenter.bottom) / 2 - DlgHeight / 2;

    // if the dialog is outside the screen, move it inside

    if(xLeft < rcArea.left)

        xLeft = rcArea.left;

    else if(xLeft + DlgWidth > rcArea.right)

        xLeft = rcArea.right - DlgWidth;

    if(yTop < rcArea.top)

        yTop = rcArea.top;

    else if(yTop + DlgHeight > rcArea.bottom)

        yTop = rcArea.bottom - DlgHeight;

    // map screen coordinates to child coordinates

    return ::SetWindowPos(m_hWnd, NULL, xLeft, yTop, -1, -1,

        SWP_NOSIZE | SWP_NOZORDER | SWP_NOACTIVATE);

}