[LC++][ot] - how do GUI-libs notify the program of changes?

[Julien Claassen]

Hi all!
  I'm currently working on a c++-GUI library and I wonder, how the GUI tells
the program that something changed.
  Simple scenario: I press a button or move a slider. I would want the engine
of the program to react directly.
  Can anyone help me? A pointer to well written tutorial for this problem
would suffice. Only: I'm not to bright a programmer, really!
  Kindest regards
      Julien

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