CS 522 Assignment 3 - Statecharts

Spencer Borland
(sborla@cs.mcgill.ca)

[PDF version of this document]

1  Introduction

You may wish to have a look at the COMP522 Statecharts projects where slightly modified requirements were actually implemented, linking a GUI to a CD device driver.

One possible solution can be seen in Figure 1 (dia source).

The CD state has been zoomed-out and its inner dynamics can be seen in Figure 2 (dia source).

One advantage of modelling a system with Statecharts is that it is easy to verify that each requirement for the system has been met. Let us now go through each requirement in the assignment specification and ensure that each has been fulfilled.

 

Requirement 1

The CLOSE_WIN event is generated by the GUI when closing the application window. The system ends in the DEAD state.

Requirement 2

There are 7 events generated by the 7 corresponding buttons: PLAY-PAUSE, STOP, PREVIOUS, NEXT, REVERSE, FORWARD, EJECT.

Requirement 3

There is a Display object which has a method setTime(time) to set the time text field.

Requirement 4

There is a Display object which has a method setTrack(track) to set the track text field.

Requirement 5

The state No CD is entered via the condition not cd_loaded() which is within the Tray In contour.

Requirement 6

If cd_loaded() is true, the system moves to within the CD state, which consists of either PLAYING, PAUSED or STOPPED.

Requirement 7

The transition from the Tray In state to the Tray Out state is activated by the EJECT event.

Requirement 8

The transition from the Tray Out state to the Tray In state is activated by the EJECT event. Once in the Tray In state, the system will determine if a CD is present via the condition C.

Requirement 9

The only transition leading out of the No CD state is the transition from the Tray In state to the Tray Out state. Thus, no other event will have an effect on the system while in this state.

Requirement 10

There is a transition from the Stop state (which contains Stopped) to the Playing state. This transition is activated by the PLAY-PAUSE event.

Requirement 11

There is a transition from the Playing state to the Stop state. The default transition in the Stop contour leads to the Stopped state. This transition is activated by the STOP event.

Requirement 12

There is a transition from the Playing state to the Paused state which is activated by the PLAY-PAUSE event.

Requirement 13

There is a transition from the Paused state to the Playing state which is activated by the PLAY-PAUSE event.

Requirement 14

The transitions leading into the Stop state cause the default transition to Stopped to fire. One of the actions of this default transition is to set the time to 0 and the track number to 1. Notice that this requirement conflicts with requirements 20 and 21. If the system is in the Stopped state, then the display should have the track number set to 1. However, while in the Stopped state, the Next button may be pressed. Once the system returns to the Stopped state, the track number should now read 2. Thus, it is necessary to move the state of the system to the Paused state, otherwise we will violate this requirement.

Requirement 15

The default transition in the Stop state has the action StopButton.disable().

Requirement 16

Requirement 17

There is a transition from the CD contour to within CD activated by the NEXT event. This transition has the action next_track() which increments the track number. If the current track is the last track, then next_track() automatically set the track number to 1, which is the desired effect.

Requirement 18

Requirement 19

There is a transition from the CD contour to within CD activated by the PREVIOUS event. This transition has the action previous_track() which decrements the track number. If the current track is the first track, then next_track() is called before the call to previous_track(). This ensures that we get back to the beginning of the first track.

Requirement 20

Requirement 21

Without loss of generality, we will only discuss the re-wind action for these two requirements. There is a transition from the CD contour to the RW state (re-wind), activated by the RW_DOWN event. RW has a timer set on it which expires after 0.1 seconds. After this time, the transition fires and several actions are executed. One if these actions is to move the hardware's clock back_one_sec(). We also update the display accordingly. Notice that at each iteration of the re-wind cycle, the system checks if the end of the CD has been reached. This check is implemented in the transition from the CD Loaded contour to within the CD contour. Once the re-wind button is released, the RW_UP event is generated. This causes the system to move to last known state within the Play contour. If no history is available, then we revert to the default state, Paused. This ensures that after the re-wind button is pushed and released, and if the system is in the Stopped state, we end up in the Paused state so that requirement 14 is not violated.

Requirement 22

The Playing state has a transition from itself to itself which is activated on a 1 second timer. When the timer expires the time and track number displays are updated accordingly.

Requirement 23

The internal dynamics of the Paused state alternate between the Blank and Not Blank state. This alternation first calls the blank() method of the display object, then later sets the time and track number to the current_time() and current_track(), respectively.

1.1  Added Behaviour

Since most CD players allow you to press stop while the player is paused, we have added the contour Play around the Playing and Paused states to allow the user to stop the playback if the system is either playing or paused.

1.2  Extra Notes

Note that we must reset the history for the Play contour when moving to the Stopped state since we may leave the CD state via the re-winding/fast-forward or track increment/decrement events. We do this because, if we reach the Stopped state, then a fast forward action occurs, we want the system to return to the Paused state upon the release of the fast forward button so that requirement 14 is not violated.