ok

Think of it as of a socket connection. You cannot perform a function invocation directly through a socket

if i where to just substitute the callback in my service as hardcoded would it be inefiecent to use Messenger to say accomplish this block of code

            @Override
            public void onTextChanged(TerminalSession changedSession) {
                if (!terminalController.mIsVisible) return;
                if (terminalController.mTerminalView.getCurrentSession() == changedSession) terminalController.mTerminalView.onScreenUpdated();
            }

We dont know what your terminal session is and what the guts of all this is

eg would it... be slow enough to lag the UI assuming the service dictates what the UI will look like

since it cannot be multi-threaded

Not really. But it is not recommended to send f.e. each frame for 60fps animation through an aidl connection

Everything can be multithreaded with a proper engineering

and through a Messenger Handler connection?

If it is not ipc it is fine

the service runs in a seperate process

And so what

It is was your choice to put in exactly this api and now struggle with it : )

so idk if that qualifies it as requiring ipc or not since the service is contained within the same application as its client

Important

IF the process is in the same package AND in the same process

Then you can throw messenger and aidl off completely

i have it as

        <service
            android:name=".TerminalService"
            android:process=":TerminalService"
            android:exported="true" />

IPC is when you talk from Photoshop to Paint

so that the terminal can survive app crashes