Thread#join

join.rb

# A reminder that join conceptually combines the stacks of the parent and child threads.
# If you consider that the stack grows downard, the child stack will sit below the parent
# stack. See output below.
def main?
  Thread.current == Thread.main
end

def name
  if main?
    "main"
  else
    "thread"
  end
end

def shout(msg)
  puts "#{name}: #{msg}"
end

def a
  b
  shout "done with a"
end

def b
  c
  shout "done with b"
end

def c
  if main?
    Thread.new do
      sleep 3
      a
      shout "done with thread"
    end.join
  end
  shout "done with c"
end

a

=begin
thread: done with c
thread: done with b
thread: done with a
thread: done with thread
main: done with c
main: done with b
main: done with a
=end