You just need to add a field to child widget then in parent widget change value of that field and call setState() .
Read moreDoes setState rebuild?
setState() It is used to notify the framework that “data has changed”, and the widget at this build context should be rebuilt .
Read moreIs setState synchronous flutter?
Whenever you change the internal state of a State object, make the change in a function that you pass to setState: setState(() { _myState = newValue; }); The provided callback is immediately called synchronously .
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