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Interface is a named group of messages and is an analogue of an interface in object-oriented design.
Messages have names and can have associated payload for both request and reply interactions. A simple message can be defined without any payload:
interface Device is
message Start;
message Stop;
-- ...
end Device;
Above, the Start and Stop message does not have any payload, but both messages indicate two-way interactions, which means that their processing can be confirmed by receiver.
One-way messages are used for interactions without confirmation:
interface Device is
-- ...
oneway message Keep_Alive;
end Device;
Messages can have payload for either request or reply direction, or both. The payload, if present, is always a single value of the user-defined type with an indication of direction:
type Time is
Timestamp : Long_Long;
end Time;
interface Clock is
message Set_Time (T : in Time);
message Get_Time (T : out Time);
end Clock;
Above, Set_Time is a message that will contain the Time record as a parameter and no payload in reply, whereas Get_Time will have no parameters in the request, but will deliver the Time value as a reply.
Of course, data can be transmitted in both directions as well, as can be seen in the complete package definition of the calculator example:
package Calculator is
type Operands is
A : Integer;
B : Integer;
end Operands;
type Results is
Sum : Integer;
Difference : Integer;
Product : Integer;
Ratio : optional Integer; -- does not exist if B = 0
end Results;
interface Operations is
-- message with data tramsmitted in both directions:
message Calculate (Op : in Operands; Res : out Results);
end Operations;
end Calculator;