require "callback-locker"
locker = CallbackLocker::new

foo = nil
locker.synchronize do
    foo = "bar"
end

# ^^^ locker is unlocked, so #synchronize will execute callback
#     immediately

foo = nil
locker.lock!
locker.synchronize do
    foo = "1"
end
locker.synchronize do
    foo << "2"
end
locker.unlock!

# ^^^ locker is locked, so callbacks are stacked and executed
#     immediately after the #unlock! method is call, so foo
#     will contain "12"