设置WPF窗口相对于非WPF窗口的位置

在前一个Post当中，指出了在WPF的WindowInteropHelper类中的一个BUG：通过WindowInteropHelper的Owner属性不能实现把WPF窗口的Owner属性设置为一个非WPF窗口的句柄。

在我的Post帖出后不到一天，在WPF SDK的Blog上，就针对这个BUG给出了一个非常完美的解决方案。既然不同通过设置WindowStartupLocation.CenterOwner来改变窗口的位置。那么我们就用WindowStartupLocation.Manual来手动计算设置窗口的位置。大致的代码如下：

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();

这段代码理论上没有什么问题呢？但是WPF是支持设备独立的。因此，在非WPF Owner窗口的某些情况下可能会因为DPI的而不能正常工作。解决这个问题，可以利用HwndSource类进行窗口位置的设备独立计算：

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();

在上面的代码中需要注意的是HwndSource的使用。这个类需要一个窗口句柄，因此它的代码被放在一个SourceInitialized的事件委派函数中执行。

最后，除了上面这种方法，其实我们还可以用Win32 API函数来实现，在ATL的CWindow类中，就有这样的一个函数，我直接把放在下面，有兴趣的朋友参考其中的实现原理： 

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);
}