How do I hook in a post validate function in a form element
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I am writing a more advanced file form element, more of a widget that displays the currently loaded uploaded image as well as validating image sizes etc.

If the file validates, I want the form element itself to copy the file to a publicly accessible folder. This will mean that a) the image can be displayed and b) if the whole form doesn't validate, the user does not need to re-upload the image.

Anyway that's the back story, but essentially I need to figure out how to get a function from within a form element to run when the form validates that element. Something like creating an onValidate() function in the form element class.

Most importantly I need a solution that doesn't spill out into the controller (or as little as possible). I am struggling to find a clean way to implement this.