Introduce Calls and Casts

GenServer calls

More often than not, we want a meaningful response when we send a GenServer process a message. We might query the process’s state, or we might want to see the result of a command we’ve sent it. This is where calls come in.

GenServer calls are synchronous. They can return any arbitrary value to the caller, and if the caller is waiting for a return, it blocks until it gets one. handle_call/3 is the GenServer callback that handles calls. It’s similar to handle_info/2 in that its pattern matches a message as its first argument.

It’s different from handle_info/2 since it doesn’t accept messages sent directly from other processes. Instead, it’s triggered whenever we call GenServer.call/2.

Let’s try this out. In lib/islands_engine/game.ex, we add a clause of handle_call/3 that looks like this:

def handle_call(:demo_call, _from, state) do 
  {:reply, state, state}
end

Our aim is to have it return to the initial server state.

The key here is the first argument, :demo_call. This is the pattern that determines which clause of handle_call/3 to execute. We’ll see where it comes from shortly. Don’t worry about the other arguments. GenServer provides them internally.

The return value is ...