Managing events in flutter's TextFormField - dart

Question

In Flutter project, I need to listen to the input text in TextFormField and do certain actions, especially when user put some character (eg. space) in this filed or when he request focus. When this kind of event happen, I need to modify filed's value.
I know there is a property called controller but I don't know how to use it in this case.
Thank you in advance.

Answer 1

You can specify a controller and focus node, then add listeners to them to monitor for changes.
Ex:
Define controllers and focus nodes
TextEditingController _controller = new TextEditingController();
FocusNode _textFocus = new FocusNode();
Define listener function
void onChange(){
String text = _controller.text;
bool hasFocus = _textFocus.hasFocus;
//do your text transforming
_controller.text = newText;
_controller.selection = new TextSelection(
baseOffset: newText.length,
extentOffset: newText.length
);
}
Add listner to controller and focusnode at initState
// you can have different listner functions if you wish
_controller.addListener(onChange);
_textFocus.addListener(onChange);
Then you can use it as
new TextFormField(
controller: _controller,
focusNode: _textFocus,
)
Hope that helps!

Answer 2

If you are just trying to transform the input to other form in TextFormField, you better use "TextInputFormatter". Using a listener with TextController cause a lot of troubles. Take a look at my sample code see if that helps you. btw, The last line of code is just trying to move the cursor to the end of text.
TextFormField(inputFormatters: [QuantityInputFormatter()])
class QuantityInputFormatter extends TextInputFormatter {
#override
TextEditingValue formatEditUpdate(TextEditingValue oldValue, TextEditingValue newValue) {
final intStr = (int.tryParse(newValue.text) ?? 0).toString();
return TextEditingValue(
text: intStr, selection: TextSelection.collapsed(offset: intStr.length),);
}
}