allow processing of window close event - e.g. to disable closing if there is unsaved data - issue #60 implemented for win32 backend

src/dlangui/platforms/common/platform.d

@@ -276,6 +276,22 @@ class Window {
     /// set handler for files dropped to app window
     @property Window onFilesDropped(void delegate(string[]) handler) { _onFilesDropped = handler; return this; }
 
+    protected bool delegate() _onCanClose;
+    /// get handler for closing of app (it must return true to allow immediate close, false to cancel close or close window later)
+    @property bool delegate() onCanClose() { return _onCanClose; }
+    /// set handler for closing of app (it must return true to allow immediate close, false to cancel close or close window later)
+    @property Window onCanClose(bool delegate() handler) { _onCanClose = handler; return this; }
+
+    /// calls onCanClose handler if set to check if system may close window
+    bool handleCanClose() {
+        if (!_onCanClose)
+            return true;
+        bool res = _onCanClose();
+        if (!res)
+            update(true); // redraw window if it was decided to not close immediately
+        return res;
+    }
+
 
     /// hide tooltip if shown and cancel tooltip timer if set
     void hideTooltip() {

src/dlangui/platforms/windows/winapp.d

@@ -1164,6 +1164,13 @@ LRESULT WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
                 DragFinish(hdrop);
             }
             return 0;
+        case WM_CLOSE:
+			if (window !is null) {
+                if (!window.handleCanClose())
+                    return 0; // prevent closing
+            }
+            // default handler inside DefWindowProc will close window
+            break;
         case WM_GETMINMAXINFO:
         case WM_NCCREATE:
         case WM_NCCALCSIZE: