使 Qt 顶级窗口由单个进程中的其他非 Qt 窗口“拥有”

Question

Is it doable on Windows through Qt API? I need such feature/behavior because I would like to integrate my Qt code as a plugin into a 3rd party application , and I want to do it as seamless as possible. For instance, I don't want to have another item on the taskbar representing my Qt window. I'm talking about a single-process application .

I have found one way using WinAPI and HWNDs:

// The Qt frame is "alien". We need its HWND so transform it (and all its ancestors,
// which is probably fine for me) to a "native" widget first. 
Q_ASSERT(!frame->testAttribute(Qt::WA_NativeWindow));
frame->setAttribute(Qt::WA_NativeWindow);
Q_ASSERT(frame->testAttribute(Qt::WA_NativeWindow));

// Get the HWND.
QWindow* frameHandle = frame->windowHandle();
Q_ASSERT(frameHandle);
HWND frameHwnd = HWND(frameHandle->winId());
Q_ASSERT(frameHwnd);

// And use it to set the frame's "owner" (not "parent" because WS_CHILD bit is not set).
Q_ASSERT((GetWindowLongPtr(frameHwnd, GWL_STYLE) & WS_CHILD) == 0);
SetWindowLongPtr(frameHwnd, GWLP_HWNDPARENT, LONG_PTR(getOtherNonQtWindowHwnd()));

This seems to work as expected if it is done after showing the frame. But such solution is kind of strange and I'm afraid it is even dangerous. Furthermore, may be the setup could be overriden by Qt itself any time later.

As can be seen in the snippet above, I'm able to retrieve HWND of the 3rd party application's main window. So generally from WinAPI perspective it should be doable. Please, is there some Qt way of doing this? Or at least a better solution?

Answer 1

Another way to do it is by exposing your code as an ActiveX control, using the ActiveQt module. That way, Qt will handle all the nitty-gritty for you.

Your solution is fine as well. It is about the most "Qt" way of doing it you can get. With two nitpicks:

1. Since Qt is responsible for the lifetime of the child window, you must not set the owner. Set the parent instead. It's on you to set the WS_CHILD style in that case - after all, you're about to make the window a child.

2. The frame should not have any ancestor windows to begin with. You must ensure that the frame has its own window.

Thus:

bool reparentToNativeWindow(QWidget * widget, HWND newParent) {
  widget->setWindowFlags(Qt::Window);
  if (!widget->testAttribute(Qt::WA_NativeWindow));
    widget->setAttribute(Qt::WA_NativeWindow);  
  auto childHandle = widget->windowHandle();
  if (!childHandle)
    return false;
  auto child = HWND(widgetHandle->winId());
  if (!child)
    return false;

  auto style = GetWindowLongPtr(child, GWL_STYLE);
  if (!style)
    return false;
  if ((style & WS_CHILD) == 0) {
    Q_ASSERT(style & WS_POPUP);
    style &= ~WS_POPUP;
    style |= WS_CHILD;
    if (SetWindowLongPtr(child, GWL_STYLE, style) == 0)
      return false;  
  }
  SetParent(child, newParent);
}