3. Placing Values and Functions
Table of contents
Placing Values
When defining values in our @multitier environment, we have to define where those values are going to be stored. We do so using the on keyword.
To define a “message” event which can be fired by the clients in order to send a new message we write the following inside our Chat object:
val message = on[Client] { Evt[String] }
on[Client] specifies that the value is going to be stored on the clients. The actual value of the val will be defined inside the curly braces.
The Evt here is a REscala keyword used for specifying events. Evt[String] says that new messages will be represented by events of type String.
Accessing Remote Values
To access a value that lives on another peer, we have to call .asLocal on these values. If multiple peers hold a copy of the values we have to use .asLocalFromAll which returns a map from connections to values or .asLocalFromAllSeq which directly returns a sequence of the values.
In our chat example we use this to define an event on the server which combines all message events from the clients:
val publicMessage = on[Server] sbj { client: Remote[Client] =>
message.asLocalFromAllSeq collect {
case (remote, message) if remote != client => message
}
}
Notice the sbj keyword which let’s us adapt the value to each peer that is accessing it. In this example we use this to not display their own messages to clients that are accessing the public messages.
Placing Functions
Functions can be placed on peers in the same way as values. This is demonstrated in the application logic for our client peers:
def main() = on[Client] {
// observe the publicMessage event and print messages as they arrive
publicMessage.asLocal observe println
// let the user send new messages via stdin
for (line <- scala.io.Source.stdin.getLines)
message.fire(line)
}
}
Notice how we access the publicMessage event which is stored on another peer (the server) using asLocal.
We can also call functions of other peers using asLocal, however, this will not be covered in this tutorial. More information can be found in the Concepts section.