Invokes callback on each non-internal child of this.
See forall for details on what constitutes an “internal” child. For all
practical purposes, this function should iterate over precisely those child widgets that were added to the container by the application
with explicit add calls.
It is permissible to remove the child from the callback handler.
Most applications should use @foreach, rather than forall .
| this | |
| callback |
a callback |
| callback_data |
callback user data |