5.1 The State Transition Diagram Editor (TSTD)

    

5.1.1 Nodes and Edges

TSTD follows the notational convention of Mealy machines. In short, a Mealy machine  is a finite state machine in which each transition can have one or more actions. See figure 5.1 for the shapes and subjects.

  

Figure 5.1: State transition diagram nodes and edges.

5.1.2 States

There are tree kinds of nodes: initial states, (normal) states and decision points. All nodes should have mutually unique names. Nameless states, also called transitory states, are not permitted.  Non-deterministic state transition diagrams are permitted.

An initial state can have one or more initialization actions. You can add an action by selecting the initial state, and click on it for editing near the arrow on top of the box. Then an edit cursor appears at the right side of the arrow. You can type in the actions, each line of text will become a separate action. If you stop editing, the actions will become left aligned, the arrow will be resized if necessary and on top of the actions a horizontal separator line will be drawn. Only when the initial state has actions then the separator line is drawn. Note that these actions are in this notational convention part of the initial state node, whereas the other actions (and events) of a diagram are as a notational convention part of an edge.

5.1.3 Transitions, Events and Actions

    Transitions are drawn as arrows. They do not have an editable name label. Instead they have an event label and a number of action labels. The action labels each occupy exactly one line of text; the event label can contain multiple lines. The event and the actions are separated by a horizontal separator line . When a transition is created, a separator line will also be created and it will be positioned near where a name label would be positioned. When the transition line segment near the separator is drawn vertically, the separator is connected to the right side of the line segment. When the segment is drawn horizontally, the separator is positioned a little above the line, see figure 5.2 for the two default positions.

  

Figure 5.2: Default STD separator positions.

If you click for editing a bit above the separator line, an edit cursor appears and you can enter an event string. When you stop editing, the event string will be positioned above the separator line and the separator line will be resized, if necessary.

If you click for editing a bit below the separator, an edit cursor appears there and you can enter a list of actions. Each line of text will become a separate action. If you stop editing, the actions will become left aligned and, if necessary, the separator line will be resized. When the transition line segment is horizontal and the separator is positioned above the segment, all labels are moved up so that they are all positioned above the line. See figure 5.3 for an example diagram with actions and events.

  

Figure 5.3: STD with events and actions.

It is possible to drag the separator line together with the event and action labels. There are two different sorts of movements possible :

1.

Move it to another line segment. You do this by selecting the transition and click for editing on the desired transition segment. The separator line and the existing event and action labels are moved to that line segment. When you stop editing, the separator will remain in that line segment. Summarizing: to move it to a different segment, empty the selection and then double click the desired segment.

2.

Move within a line segment. Drag with button-1 the separator or one of the labels to the desired position. The final position is determined by the direction of the transition line segment.

• If the line segment is more or less horizontal, you can freely move the separator and labels from left to right. But you can only move the separator up and down to two predefined positions: either all labels are positioned entirely above the line segment or all labels are positioned entirely below the line segment.
• If the line segment is more or less vertical, you can freely move the separator and labels up and down the segment, but you can only move the separator left and right to two predefined positions: either all labels are entirely positioned at the left side of the line segment or all labels are entirely positioned at the right of the line segment.

The automatic positioning of labels looks best when the segments are horizontal or vertical. When the transition arrow segments are diagonal, then label positioning still works but sometimes the result does not look as great as figure 5.3.

5.1.4 Constraint Checking

TSTD checks the following constraints that are summarized in figure 5.4.

  

Figure 5.4: Immediately checked and soft constraints on STDs.