TextFormField: setState() or markNeedsBuild() called during build - dart

Question

I'm trying to set first time button as disabled and when user enters amount it gets enabled, button disable works fine but when i enter amount in TextFormField it gives below error.
I/flutter (29519): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY
╞═══════════════════════════════════════════════════════════
I/flutter (29519): The following assertion was thrown building
TextFormField(dirty, state: _TextFormFieldState#b89da):
I/flutter (29519): setState() or markNeedsBuild() called during build.
I/flutter (29519): This HomePage widget cannot be marked as needing to
build because the framework is already in the
I/flutter (29519): process of building widgets. A widget can be marked as
needing to be built during the build phase
I/flutter (29519): only if one of its ancestors is currently building.
This exception is allowed because the framework
I/flutter (29519): builds parent widgets before children, which means a
dirty descendant will always be built.
I/flutter (29519): Otherwise, the framework might not visit this widget
during this build phase.
I/flutter (29519): The widget on which setState() or markNeedsBuild() was
called was:
my code:
var _onPressed;
if (isButtonDisabled) {
_onPressed = () {
print("Hello");
};
}
TextFormField code:
child: TextFormField(
decoration: InputDecoration(
contentPadding: EdgeInsets.only(
left: 12.0,
right: 12.0,
top: 12.0,
bottom: 12.0
),
labelText: 'Enter amount',
hintText: 'Enter amount',
hintStyle: TextStyle(
color: Colors.red,
fontSize: 14.0
),
border: OutlineInputBorder(
borderRadius: BorderRadius.circular(0.0)
),
),
keyboardType: TextInputType.number,
autovalidate: true,
validator: (String txt) {
if (txt.length == 2) {
setState(() {
isButtonDisabled = true;
});
} else {
setState(() {
isButtonDisabled = false;
});
}
}
),
Button Code:
FlatButton(
child: Text("Confirm"),
onPressed: _onPressed,
)

Answer 1

Remove setState from
if (txt.length == 2){
setState((){
isButtonDisabled = true;
});
} else {
setState((){
isButtonDisabled = false;
});
}}),
to
if (txt.length == 2){
isButtonDisabled = true;
} else {
isButtonDisabled = false;
}}),
You don't need call setState() in code executed directly in build(), only if you pass functions like
onPressed: () {
// code here is not executed in `build()`,
it's just passed to `onPressed` to be executed when the button is tapped
// therefore here `setState()` is required for state changes
}
validator: () {...} looks similar, but doesn't update the state of your widget.