Updating a text widget using onchanged - flutter

Question

I am trying to update a text widget as the user types, its working but i don't see the input as I type
TextFormField(
autofocus: true,
controller: amountController,
keyboardType: TextInputType.number,
cursorColor: Colors.amber,
decoration: InputDecoration(
hintText: "Enter Amount",
prefixIcon: Material(
elevation: 0,
borderRadius: BorderRadius.all(Radius.circular(30)),
child: Icon(
Icons.phone_outlined,
size: 15,
color: Color(0xff020529),
),
),
border: InputBorder.none,
contentPadding:
EdgeInsets.symmetric(horizontal: 25, vertical: 13)),
onChanged: (value) {
add(value);
},
),
this is the widget
Text( x.toString(),
style: TextStyle(fontWeight: FontWeight.bold),
),
This is my method
add(String value) {
var ans = 36 * double.parse(value);
print('ddd');
setState(() {
x = ans;
});

Answer 1

What's happening is double.parse is failing because the given value can't be converted to a number.
One option to handle this is by using a try catch (see following code sample), and then setting the displayed value to 0 (or could also do something like set an error message saying something like "invalid input" if the user enters something like "123d"
add(String value) {
double ans;
try {
ans = 36 * double.parse(value);
} on FormatException catch (e) {
// string couldn't be formatted to double
ans = 0;
}
setState(() {
x = ans;
});
}
I used a try catch, but you could also check if value is empty, and/or add an onError callback to double.parse.