Function idle_add_full 

pub fn idle_add_full<F>(priority: Priority, func: F) -> SourceId
where
    F: FnMut() -> ControlFlow + Send + 'static,

Adds a closure to be called by the default main loop when it’s idle.

func will be called repeatedly with priority until it returns ControlFlow::Break.

The default main loop almost always is the main loop of the main thread. Thus, the closure is called on the main thread.