Experiment 1.2: Understanding how it works

What was changed

I added a println! statement right after the spawner.spawn(...) line in main.rs:

spawner.spawn(async {
    println!("Rifqi's Komputer: howdy!");
    TimerFuture::new(Duration::new(2, 0)).await;
    println!("Rifqi's Komputer: done!");
});

println!("Rifqi's Komputer: hey hey...");

Output

When running the program, the output is:

Explanation

The reason Rifqi's Komputer: hey hey... appears before the async task output is because spawner.spawn(...) only schedules the async task to be run later. The code inside the async block does not execute immediately. Instead, the main thread continues and prints hey hey... right away.

The async task (printing "howdy!", waiting, then printing "done!") only starts running when the executor (executor.run()) begins processing the scheduled tasks. That happens after "hey hey..." is printed.

In summary:

• spawn schedules the async task, but does not run it immediately.
• The main thread prints "hey hey..." right away.
• The executor then runs the async task, printing "howdy!", waiting, then "done!".

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Experiment 1.3: Multiple Spawn and removing drop

What was changed

I replicated the spawn three times to create multiple async tasks, and then experimented with commenting out the drop(spawner); line in main.rs:

spawner.spawn(async {
    println!("Rifqi's Komputer: howdy!");
    TimerFuture::new(Duration::new(2, 0)).await;
    println!("Rifqi's Komputer: done!");
});
spawner.spawn(async {
    println!("Rifqi's Komputer: howdy2!");
    TimerFuture::new(Duration::new(2, 0)).await;
    println!("Rifqi's Komputer: done2!");
});
spawner.spawn(async {
    println!("Rifqi's Komputer: howdy3!");
    TimerFuture::new(Duration::new(2, 0)).await;
    println!("Rifqi's Komputer: done3!");
});

println!("Rifqi's Komputer: hey hey...

// drop(spawner); // This line was commented out

executor.run();

Output

When running the program with drop(spawner); commented out, the program hangs and does not terminate after printing the async task outputs.

When drop(spawner); is present, the program runs all tasks and then exits as expected.

Explanation

The drop(spawner); statement is important because it signals to the executor that no more tasks will be spawned. If you do not drop the spawner, the executor's task queue never closes, so executor.run() waits forever for new tasks, causing the program to hang.

In summary:

• With drop(spawner);, the executor knows when all tasks are done and exits cleanly.
• Without drop(spawner);, the executor waits forever, and the program never finishes.

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