Module ch02_waiting_for_processes 

Chapter 2: Waiting for another process.

Processes can start other processes and wait for them — the building block for “do this sub-task, then continue”. In SimPy you yield env.process(...); in simu, spawn returns a ProcessHandle which is itself a future: awaiting it suspends you until the child process finishes and hands you its return value.

Our car is now electric. After every trip it must charge before it can drive again — and charging is its own process:

use simu::SimEnv;

let mut env = SimEnv::with_seed(42);
let h = env.handle();

env.spawn(async move {
    loop {
        println!("Start driving at {}", h.now());
        h.timeout(2.0).await;

        println!("Start charging at {}", h.now());
        let hc = h.clone();
        let charging = h.spawn(async move {
            hc.timeout(5.0).await;
            42.0 // a process can return a value, e.g. the kWh charged
        });
        let kwh = charging.await; // suspend until charging finishes
        assert_eq!(kwh, 42.0);
    }
});

env.run_until(15.0);

Output:

Start driving at 0
Start charging at 2
Start driving at 7
Start charging at 9
Start driving at 14

Two details:

• Handles are cheap clones sharing one env; clone freely (let hc = h.clone()) whenever a child process needs its own.
• If you don’t need the result, just drop the ProcessHandle — the child keeps running, fire-and-forget.