Add wxEventLoop::ScheduleExit().

This method allows to request exiting from the given event loop even if it's
not the currently active one, unlike Exit() which would assert in this case.
With it, it becomes possible to ask the loop to terminate as soon as possible
even if a nested loop is currently running.

See #10258.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74335 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/evtloop.h

@@ -23,6 +23,19 @@
     You can create your own event loop if you need, provided that you restore
     the main event loop once yours is destroyed (see wxEventLoopActivator).
 
+    Notice that there can be more than one event loop at any given moment, e.g.
+    an event handler called from the main loop can show a modal dialog, which
+    starts its own loop resulting in two nested loops, with the modal dialog
+    being the active one (its IsRunning() returns @true). And a handler for a
+    button inside the modal dialog can, of course, create another modal dialog
+    with its own event loop and so on. So in general event loops form a stack
+    and only the event loop at the top of the stack is considered to be active.
+    It is also the only loop that can be directly asked to terminate by calling
+    Exit() (which is done by wxDialog::EndModal()), an outer event loop can't
+    be stopped while an inner one is still running. It is however possible to
+    ask an outer event loop to terminate as soon as all its nested loops exit
+    and the control returns back to it by using ScheduleExit().
+
     @library{wxbase}
     @category{appmanagement}
 
@@ -90,9 +103,32 @@ public:
     virtual bool IsOk() const;
 
     /**
-        Exit from the loop with the given exit code.
+        Exit the currently running loop with the given exit code.
+
+        The loop will exit, i.e. its Run() method will return, during the next
+        event loop iteration.
+
+        Notice that this method can only be used if this event loop is the
+        currently running one, i.e. its IsRunning() returns @true. If this is
+        not the case, an assert failure is triggered and nothing is done as
+        outer event loops can't be exited from immediately. Use ScheduleExit()
+        if you'd like to exit this loop even if it doesn't run currently.
      */
-    virtual void Exit(int rc = 0) = 0;
+    virtual void Exit(int rc = 0);
+
+    /**
+        Schedule an exit from the loop with the given exit code.
+
+        This method is similar to Exit() but can be called even if this event
+        loop is not the currently running one -- and if it is the active loop,
+        then it works in exactly the same way as Exit().
+
+        The loop will exit as soon as the control flow returns to it, i.e.
+        after any nested loops terminate.
+
+        @since 2.9.5
+     */
+    virtual void ScheduleExit(int rc = 0) = 0;
 
     /**
         Return true if any events are available.