How to use onKeyUp event in TextField in flutter - dart

Question

I am using flutter to make a simple application.
In the below code, when TextBox changed event is fired then I call the method named updateTitle().
But I have to call the same method updateTitle(), when key is up, as we use in javascript and other languages too.
TextField(
controller: titleController,
style: textStyle,
onChanged: (value) => updateTitle(),
decoration: InputDecoration(
labelText: "Title",
labelStyle: textStyle,
border: OutlineInputBorder(
borderRadius: BorderRadius.circular(5.0),
)),
),

Answer 1

For your use case, the onChangedworks just like onkyeup would work. Everytime that the user tap a new character in the textfield, it is fired.
In order to enable/disable a button you should listen to this event, do the test to see if the field isn't empty, modify a variable that will handle the button state, and call setState().
This is just a sample code. Not tested, but should work as is.
class _SoAnswerState extends State<SoAnswer> {
bool _buttonActive = false;
#override
Widget build(BuildContext context) {
...
TextField(
controller: titleController,
style: textStyle,
onChanged: (value) => updateButtonState(value), // onChanged return the value of the field
decoration: InputDecoration(
labelText: "Title",
labelStyle: textStyle,
border: OutlineInputBorder(
borderRadius: BorderRadius.circular(5.0),
)
),
),
...
}
void updateButtonState(String text){
// if text field has a value and button is inactive
if(text != null && text.length > 0 && !_buttonActive){
setState(() {
_buttonActive = true;
}
}else if((text == null || text.length == 0) && _buttonActive){
setState(() {
_buttonActive = false;
}
}
}
Edit: add more information about the events
In JavaScript, the onkeyup event handler fires when the user releases a key that was previously pressed. When the user press and release a key inside a text field, the text field value changes. The onChanged listener in Flutter fires when the text field value changes. When working with a typing interface, where the user uses a tradicional keyboard, it is not a good idea to listen to all the changes of a text field, because the user can press and hold a key, leading the application to repeat the onchanged event too many times, once for every character repetition. That's not the case with a mobile interface, where the user (usually) can't press and hold a key.