tracer.rb

require 'ffi-tk'

root = Tk::Root.new

var = Tk::Variable.new('RubyFFI::foo')
entry = Tk::Tile::Entry.new(root, textvariable: var)
entry.pack
entry.focus

show = proc{ p var }

Tk.execute(:trace, :add, :variable, var, :write, show)

Tk.mainloop