def _schedule_callback(prev, next):
    '''
    Called when a context is stopped or a new context is made runnable.
    '''
    try:
        if not prev and not next:
            return

        current_frame = sys._getframe()

        if next:
            register_tasklet_info(next)

            # Ok, making next runnable: set the tracing facility in it.
            debugger = get_global_debugger()
            if debugger is not None:
                next.trace_function = debugger.trace_dispatch
                frame = next.frame
                if frame is current_frame:
                    frame = frame.f_back
                if hasattr(frame, 'f_trace'):  # Note: can be None (but hasattr should cover for that too).
                    frame.f_trace = debugger.trace_dispatch

            debugger = None

        if prev:
            register_tasklet_info(prev)

        try:
            for tasklet_ref, tasklet_info in dict_items(_weak_tasklet_registered_to_info):  # Make sure it's a copy!
                tasklet = tasklet_ref()
                if tasklet is None or not tasklet.alive:
                    # Garbage-collected already!
                    try:
                        del _weak_tasklet_registered_to_info[tasklet_ref]
                    except KeyError:
                        pass
                    if tasklet_info.frame_id is not None:
                        remove_custom_frame(tasklet_info.frame_id)
                else:
                    is_running = stackless.get_thread_info(tasklet.thread_id)[1] is tasklet
                    if tasklet is prev or (tasklet is not next and not is_running):
                        # the tasklet won't run after this scheduler action:
                        # - the tasklet is the previous tasklet
                        # - it is not the next tasklet and it is not an already running tasklet
                        frame = tasklet.frame
                        if frame is current_frame:
                            frame = frame.f_back
                        if frame is not None:
                            base = get_abs_path_real_path_and_base_from_frame(frame)[-1]
                            # print >>sys.stderr, "SchedCB: %r, %d, '%s', '%s'" % (tasklet, frame.f_lineno, _filename, base)
                            is_file_to_ignore = base in DONT_TRACE
                            if not is_file_to_ignore:
                                tasklet_info.update_name()
                                if tasklet_info.frame_id is None:
                                    tasklet_info.frame_id = add_custom_frame(frame, tasklet_info.tasklet_name, tasklet.thread_id)
                                else:
                                    update_custom_frame(tasklet_info.frame_id, frame, tasklet.thread_id, name=tasklet_info.tasklet_name)

                    elif tasklet is next or is_running:
                        if tasklet_info.frame_id is not None:
                            # Remove info about stackless suspended when it starts to run.
                            remove_custom_frame(tasklet_info.frame_id)
                            tasklet_info.frame_id = None


        finally:
            tasklet = None
            tasklet_info = None
            frame = None

    except:
        import traceback;traceback.print_exc()

    if _application_set_schedule_callback is not None:
        return _application_set_schedule_callback(prev, next)

    Called when a context is stopped or a new context is made runnable.
    
debugger/src/_pydevd_bundle/pydevd_stackless.py