Function write_completion 

pub fn write_completion<F, D: DispatchData>(
    fd: dispatch_fd_t,
    data: &D,
    queue: &Unmanaged,
    handler: F,
)
where
    F: FnOnce(Option<&Unmanaged>, c_int) + Send + 'static,

Calls dispatch_write with the specified completion handler. You can use a blocksr::continuation to wrap this in an async method if desired.