Question

My Attempt at creating a simple animated shell spinner guy…

STDOUT.sync

# spinner stuff
spinner_running = false
chars = ['|', '/', '-', '\\']
@spinner = Thread.new do
  loop do
    unless spinner_running
      Thread.stop
      print "\b"
    end
    print chars[0]
    sleep(1)
    print "\b"
    chars.push chars.shift
  end
end

def start_spinner
  spinner_running = true
  @spinner.wakeup
end

def stop_spinner
  spinner_running = false
end

print ".......X"

start_spinner()
sleep(5)
stop_spinner()

Just trying to create a simple spinner, and yes, I know there’s a gem for that…

Why wouldn’t I be seeing the progress of the spinner while it’s executing in this code?

Current Output: ruby spinner.rb

.......X< new prompt (no new line) > 

Answer 1

spinner_running needs to be @spinner_running.

It’s first defined as a local variable and then used in the thread, which is fine as the block will have access to the scope it’s defined in. However the start_ and stop_ spinner methods define their own scope, so when spinner_running is used there it is a newly defined, local variable.

So the spinner_running used by the thread is never set to true – so the thread exits straight away when it is run.

To fix it, it needs to be made into an instance variable by prefixing it with @.

In answer to your comment, I don’t think there’s an easy way for the thread to spot this – because the bug isn’t in the thread.