Use Case: OpenDoor
[Describes the scenario when a user wants to open a door]

When the user pulls his card through the card reader slot, the 
signal Card is sent from the Environment to the process 
PanelController. The signal has one parameter of that should be 
able to store the carddata. 

This causes the process PanelController to send an identical 
signal Card to the process Controller and the signal Display to 
the display hardware with a parameter that can handle the display 
texts. In this case the text should be "Enter Code".

Now the user is supposed to press the 4-digit code for the card on 
the keypad.
For every digit entered on the keypad the signal KeyStroke with a 
parameter for handling the keydata will be sent to the process 
PanelController from Environment.

When a complete code (four digits) is received by PanelController, 
the code should be sent to Controller. This is done by sending the 
signal Code, with one parameter that can handle the codedata, to 
the process Controller. 

After that, process PanelController is also sending the signal 
Display with the text "Please wait" as a parameter to the 
Environment.

The card and code should now be validated against the database. 
The process  Controller does this by sending the signal 
CardAndCode to the process Central with two parameters, one for
handling the carddata and one for handling the codedata.

If the code is correct the process Central sends back the signal OK 
to process Controller. The process Controller then sends the signal 
DisplayOK to process PanelController and the signal OpenDoor to the 
process DoorController. 

To actually open the door lock the signal Open is sent from the 
process DoorController to the Environment (to the hardware lock). 
After that, the process DoorController sends the signal DoorOpened 
to process Controller. 

Process Controller reacts by sending the signal DisplayDoorOpened 
to the process PanelController which in turn sends the signal 
Display to the Environment with the text "Door opened" as a 
parameter. 
This signal completes the sequence to open a door in our system.