How may I start/stop an endless loop with Tk?

Using a Tk widget to start such a loop, Tk will hang while the loop loops...endlessly. What I want is for another Tk widget to stop said loop. As below, where one Tk widget sets $foo to one and another Tk widget shall later unset it to zero.

while ($foo) {
  ...do something...
}

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In reply to Tk for start/stop of endless loop? by Anonymous Monk

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