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CHAPTER 6 DIAGRAMS FOR EVENT-BASED SYSTEMS
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are diagrammed as boxes, and signals enter and leave components through an entity called a port, represented using an arrow that connects to the border of a component box. A port is a logical grouping of signals, and input ports are nothing more than exposed interfaces. An output port denotes outgoing calls. A port can contain both input and output signals, in which case the port is bidirectional. In Espresso, signals are delivered using method calls, and an arrow depicts the direction of control flow through the port. Figure 6-23 shows an example of a simple Espresso diagram.
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Figure 6-23. A simple Espresso diagram with a bidirectional port connection When it is useful to show the details of the signals involved with a port connection, you can use a special diagramming variation, as shown in Figure 6-24.
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Lines Carrying Input or Output Signals
Figure 6-24. An Espresso diagram, showing ports and lines The detailed connection is shown using a notation called port-breakout. The individual signals travel on entities called lines, which also show the direction of control flow for each method call. Espresso components can have any number of ports, and a port can have any number of lines. While input lines have a one-to-one correspondence with methods, output lines have a one-to-one correspondence with method pointers exposed by the component. Although input ports are equivalent conceptually to interfaces, the individual calls made into an input line are made at the method level, using a method reference that bypasses interfaces.
CHAPTER 6 DIAGRAMS FOR EVENT-BASED SYSTEMS
Catalysis Diagrams
Catalysis12 is a methodology for building component-based software developed by Wills and D Souza. It defines a number of different types of diagrams, some of which had a great influence on UML 2 diagrams. One type of Catalysis diagram that is of particular interest for EBSs is the Connector diagram, in which components are denoted by boxes that have outgoing and incoming signals. There are two kinds of outgoing signals: events and properties. Output events are notifications sent when a component changes state or senses that something has happened. A Button component might have a clicked output event that is fired when the button is pressed. Output properties are notifications sent when a given attribute changes value. Output events are depicted with open arrowheads; output properties are depicted with solid arrowheads, as shown in Figure 6-25.
Output Event
clicked
Button
clickCounter
Output Property
Figure 6-25. A component with two kinds of outputs The arrows on outputs show the direction of control flow. Outputs are meant to be wired to inputs, which can also be of two types: input events and input properties. Both are depicted with a lollipop symbol. Input events use a hollow lollipop and input properties use a solid lollipop, as shown in Figure 6-26.
Input Event
On Intensity
Input Property
Lamp
Figure 6-26. A component with two kinds of inputs The point at which an input or output attaches to a component is called a port. A port can handle only a single input or output. Components are wired together on a Connector diagram by connecting output arrows to lollipops. Figure 6-27 shows a simple elevator-control system depicted using a Connector diagram.
12. Desmond Francis D Souza and Alan Cameron Wills, Objects, Components, and Frameworks with UML: The Catalysis Approach (Boston: Addison-Wesley Professional, 1998).
CHAPTER 6 DIAGRAMS FOR EVENT-BASED SYSTEMS
Up Button
Pressed
closeDoors
Door Controller
Floor Number Display Input Property
doorsClosed
floorNumber
moveUp
Motor Controller
floorNumber floorReached
Floor Sensor
Output Property
Stop
Figure 6-27. A simple Connector diagram Figure 6-27 represents an early version of the Connector diagram. A later version evolved13 in which ports are shown as small squares on the border of a component. Figure 6-28 shows the four variations of the port symbol.
Output Event
Output Property
Input Event
Input Property
Figure 6-28. Port symbols used in later versions of Connector diagrams Given that outputs have a different symbol than inputs, Connection diagrams no longer need arrows to show the direction of control flow. The lollipop symbols were also dropped, so a connection is shown simply as a line connecting an output port to an input port. Figure 6-29 shows the elevator-control system with the later version of the Connector diagram.
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