Smalltalk and Object Orientation: an Introduction - Free

Caller Phone Line Callee
time
caller lifts receiver
dial tone begins
dial(9)
dial tone ends
dial (9)
dial (9)
ringing tone
tone stops
phone rings
answer phone
ringing stops
Figure 18.1: A sequence diagram
A variation of the basic sequence diagram (called a focus -of-control diagram) illustrates which
object has the thread of control at any one time. This is shown by a fatter line during the period when
the object has control (as illustrated in Figure 18.2). Note how the bar representing the object C only
starts when it is created and terminates when it is destroyed.
A
B
C
f()
create
g()
time
h()
destroy
Figure 18.2: Sequence diagram with focus-of-control regions
18.2.2 Collaboration diagrams
As stated above, collaboration diagrams illustrate the sequence of messages between objects based
around the object structure (rather than the temporal aspects of sequence diagrams). A collaboration
diagram is formed from the objects involved in the collaboration, the links (permanent or temporary)
between the objects and the messages (numbered in sequence) that are exchanged between the objects.
An example collaboration diagram is presented in Figure 18.3.
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