Function idle_add_once 

pub fn idle_add_once<F>(func: F) -> SourceId
where
    F: FnOnce() + Send + 'static,

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

func will be called repeatedly 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.

In comparison to idle_add(), this only requires func to be FnOnce, and will automatically return ControlFlow::Break.