Not all interactions of a block with its environment are based on services in the
sense of interface descriptions. For example, a block can have an interaction point
over which it is supplied with electric power. An UML standard port would not
be suitable. You ??™ d have to describe the fl ow of electric power in an operation of
an interface ( givePower( ):Power ).
However, a fl ow port doesn ??™ t let pass just any objects one can think of. The
fl ow specifi cation is a special interface that describes incoming and outgoing data.
It has no operations.
You can see a fl ow specifi cation in Figure 4.37 . The data descriptions have
the heading fl ow properties . Incoming data is prefi xed with the keyword in , and
outgoing data is prefi xed with the keyword out . Data that is transported in both
directions have the keyword inout . The fl ow port of the block engine is denoted
with small arrowheads, like an UML port. The fl ow specifi cation is the type of the
port ( Figure 4.38 ).
The opposite fl ow port requires the same fl ow specifi cation, except that the
incoming data and the outgoing data are swapped.
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