Allow using wxWindowUpdateLocker conditionally

This can be useful if the window needs to be frozen only if some
run-time condition holds true.

include/wx/wupdlock.h

@@ -19,12 +19,29 @@
 class wxWindowUpdateLocker
 {
 public:
+    // Prefer using the ctor below if possible, this ctor is only useful if
+    // Lock() must be called only conditionally.
+    wxWindowUpdateLocker() : m_win(NULL) { }
+
     // create an object preventing updates of the given window (which must have
     // a lifetime at least as great as ours)
     explicit wxWindowUpdateLocker(wxWindow *win) : m_win(win) { win->Freeze(); }
 
+    // May be called only for the object constructed using the default ctor.
+    void Lock(wxWindow *win)
+    {
+        wxASSERT( !m_win );
+
+        m_win = win;
+        win->Freeze();
+    }
+
     // dtor thaws the window to permit updates again
-    ~wxWindowUpdateLocker() { m_win->Thaw(); }
+    ~wxWindowUpdateLocker()
+    {
+        if ( m_win )
+            m_win->Thaw();
+    }
 
 private:
     wxWindow *m_win;

interface/wx/wupdlock.h

@@ -35,6 +35,17 @@
 class wxWindowUpdateLocker
 {
 public:
+    /**
+        Default constructor doesn't do anything.
+
+        Prefer using the non-default constructor if possible, this constructor
+        is only useful if Lock() must be called conditionally, i.e. if it may
+        or not be called depending on some run-time condition.
+
+        @since 3.1.4
+     */
+    wxWindowUpdateLocker();
+
     /**
         Creates an object preventing the updates of the specified @e win.
         The parameter must be non-@NULL and the window must exist for longer than
@@ -42,6 +53,18 @@ public:
     */
     explicit wxWindowUpdateLocker(wxWindow* win);
 
+    /**
+        Really lock window updates.
+
+        This method can only be called on an object initialized using the
+        default constructor.
+
+        @param win Non-@NULL window which must exist for longer than this object.
+
+        @since 3.1.4
+     */
+    void Lock(wxWindow *win);
+
     /**
         Destructor reenables updates for the window this object is associated with.
     */