A focus of control rectangle, or activation bar, is drawn on an object lifeline to show the period during which the object is performing an action. The activation bar represents both the duration of the action in time and the control relationship between the activation and its callers. The terms 'focus of control' and 'activation bar' are interchangeable.
The top of the activation bar is aligned with the initiation time of the object activation, and the bottom of the activation bar is aligned with the completion time. The action being performed may be labeled in text next to the activation bar.
In procedural flow of control, the top of the activation bar is at the tip of an incoming arrow (the one that initiates the action) and the base of the activation bar is at the tail of a return arrow.
How to draw activation bars (focus of control)
You may add one or more activation bars to object lifelines as follows:
1 Select the Focus of Control tool from the toolbar or Draw menu, and place it on an object lifeline on the diagram. If you try to draw the Focus of Control rectangle into empty diagram workspace, it will afix itself to the closest object lifeline. Once dropped onto an object lifeline, the Focus of Control rectangle remains fixed to the object lifeline.
2 You normally leave the Focus of Control rectangle unnamed. You can name it by double-clicking on it or right-mouse clicking on it and choosing Edit. A message appears that says “The Symbol must be named to edit its definition”. You could name the Focus of Control with the name of the action being performed. Once a name is added, you may not hide the name.
Stacking
You may stack activation bars to model a recursive call to an Instance that has an existing activation. The second activation bar is drawn slightly to the right of the first one, so that they appear to 'stack up' visually. Recursive calls may be nested to an arbitrary depth.
To stack activation bars
1 Drag on the first activation bar to make it longer than the second bar that you will stack on top of it.
2 Draw the second activation bar so that it stacks on top of the first.