The keyboard is flashing when focusing on the TextFormField

Question

I have this login screen in which when i tap of the TextFormField the keyboard shows up and hide quickly.

@override
  Widget build(BuildContext context) {
    return Scaffold(
      body: LayoutBuilder(
        builder: (BuildContext context, BoxConstraints viewportConstraints) {
          return SingleChildScrollView(
            child: ConstrainedBox(
              constraints: BoxConstraints(
                minHeight: viewportConstraints.maxHeight,
              ),
              child: LoginScreenContent(),
            ),
          );
        },
      ),
    );
  }

it worked for me before in another project, but in this one i couldn't identify the cause of the problem even after trying several solutions.

there is no error per say, as i said the keyboard is displayed and immediately get hide just after tapping on one of the TextFormInputs. — joe_inz, Feb 13 '21 at 20:58

score 0 · Accepted Answer · answered Feb 14 '21 at 04:43

When the drawer or soft keyboard opened the state of the screen change and sometimes the build method reload automatically, please check this link for more information.

The build method is designed in such a way that it should be pure/without side effects. This is because many external factors can trigger a new widget build, such as:

Route pop/push Screen resize, usually due to keyboard appearance or orientation change Parent widget recreated its child An InheritedWidget the widget depends on (Class.of(context) pattern) change This means that the build method should not trigger an http call or modify any state.

How is this related to the question?

The problem you are facing is that your build method has side-effects/is not pure, making extraneous build call troublesome.

Instead of preventing build calls, you should make your build method pure, so that it can be called anytime without impact.

In the case of your example, you'd transform your widget into a StatefulWidget then extract that HTTP call to the initState of your State:

class Example extends StatefulWidget {
  @override
  _ExampleState createState() => _ExampleState();
}

class _ExampleState extends State<Example> {
  Future<int> future;

  @override
  void initState() {
    future = Future.value(42);
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return FutureBuilder(
      future: future,
      builder: (context, snapshot) {
        // create some layout here
      },
    );
  }
}

I know this already. I came here because I really want to optimize rebuilds

It is also possible to make a widget capable of rebuilding without forcing its children to build too.

When the instance of a widget stays the same; Flutter purposefully won't rebuild children. It implies that you can cache parts of your widget tree to prevent unnecessary rebuilds.

The easiest way is to use dart const constructors:

@override
Widget build(BuildContext context) {
  return const DecoratedBox(
    decoration: BoxDecoration(),
    child: Text("Hello World"),
  );
}

Thanks to that const keyword, the instance of DecoratedBox will stay the same even if the build were called hundreds of times.

But you can achieve the same result manually:

@override
Widget build(BuildContext context) {
  final subtree = MyWidget(
    child: Text("Hello World")
  );

  return StreamBuilder<String>(
    stream: stream,
    initialData: "Foo",
    builder: (context, snapshot) {
      return Column(
        children: <Widget>[
          Text(snapshot.data),
          subtree,
        ],
      );
    },
  );
}

In this example when StreamBuilder is notified of new values, the subtree won't rebuild even if the StreamBuilder/Column does. It happens because, thanks to the closure, the instance of MyWidget didn't change.

This pattern is used a lot in animations. Typical uses are AnimatedBuilder and all transitions such as AlignTransition.

You could also store subtree into a field of your class, although less recommended as it breaks the hot-reload feature.

a part of your answer had the solution implied in it, in my LoginScreenContent i had several widgets in which one of them is a Form in it's own widget class, it was stateless and i converted it to statefull and that's it. — joe_inz, Feb 14 '21 at 10:31
Ok, my friend @joeloudjinz if you get your answer please upvote and check my answer, thanks. — Abbas Jafari, Feb 14 '21 at 16:37

The keyboard is flashing when focusing on the TextFormField

1 Answers1