should setState have any body in flutter widgets

Question

I was reviewing documentation for setState and it states

Notify the framework that the internal state of this object has changed.

I am guessing the doc really means to emphasis on this

Notify the framework that the internal state of this object has changed.

I tried an empty setState with the default generated code and it was able to trigger the notification.

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    _counter++; // moved it out of setState
    setState(() {
//  _counter++; // instead setting it before setState
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.display1,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}

above code does work fine,but all the examples I am finding on internet are explicitly changing the values or doing some activities withing setState, is it really necessary ?

You can refer this https://stackoverflow.com/questions/52383575/empty-set-state-what-is-the-point — Ayush Bherwani, Dec 17 '19 at 03:15
@ayush-bherwani thanks, I guess it is close, but I guess synch / asynch ? — , Dec 17 '19 at 03:19
void _incrementCounter() { _counter++; setState( xyz); // will call method xyz and then build } xyz() { print(' X '+ _counter.toString()); _counter++; } — mina, Dec 17 '19 at 16:27

score 1 · Answer 1 · answered Dec 17 '19 at 03:22

Yes it works.

The callback in setState is carries no magic as conveyed in source code. It is not being passed down, and called within some internal lifecycle.

The provided callback is immediately called synchronously. It must not return a future (the callback cannot be async), since then it would be unclear when the state was actually being set.

@protected
void setState(VoidCallback fn) {
  final dynamic result = fn() as dynamic;
  _element.markNeedsBuild();
}

Source code without assert.

It just help you to execute the code inside the closure immediately, and then mark it to rebuild in next frame. That's all. Although it works fine with blank callback, it is not the suggested practice.

It may be related to maintainability because all things inside setState should be mutating the widget's state, explicitly stated with no surprises.

score 0 · Answer 2 · answered Dec 17 '19 at 03:23

Short:

You can call setState with an empty body.

(But it'll become easier to read, understand when it has something inside body, right?)

Long version:

Flutter will check some conditions like callback not null, assert the current widget's state, the callback cannot be async... Flutter will execute anything inside setState's body before mark widget needs to re-build.

@protected
  void setState(VoidCallback fn) {
    assert(fn != null);
    assert(() {
      if (_debugLifecycleState == _StateLifecycle.defunct) {
        throw FlutterError.fromParts(<DiagnosticsNode>[
          ErrorSummary('setState() called after dispose(): $this'),
          ErrorDescription(
            'This error happens if you call setState() on a State object for a widget that '
            'no longer appears in the widget tree (e.g., whose parent widget no longer '
            'includes the widget in its build). This error can occur when code calls '
            'setState() from a timer or an animation callback.'
          ),
          ErrorHint(
            'The preferred solution is '
            'to cancel the timer or stop listening to the animation in the dispose() '
            'callback. Another solution is to check the "mounted" property of this '
            'object before calling setState() to ensure the object is still in the '
            'tree.'
          ),
          ErrorHint(
            'This error might indicate a memory leak if setState() is being called '
            'because another object is retaining a reference to this State object '
            'after it has been removed from the tree. To avoid memory leaks, '
            'consider breaking the reference to this object during dispose().'
          ),
        ]);
      }
      if (_debugLifecycleState == _StateLifecycle.created && !mounted) {
        throw FlutterError.fromParts(<DiagnosticsNode>[
          ErrorSummary('setState() called in constructor: $this'),
          ErrorHint(
            'This happens when you call setState() on a State object for a widget that '
            'hasn\'t been inserted into the widget tree yet. It is not necessary to call '
            'setState() in the constructor, since the state is already assumed to be dirty '
            'when it is initially created.'
          ),
        ]);
      }
      return true;
    }());
    final dynamic result = fn() as dynamic;
    assert(() {
      if (result is Future) {
        throw FlutterError.fromParts(<DiagnosticsNode>[
          ErrorSummary('setState() callback argument returned a Future.'),
          ErrorDescription(
            'The setState() method on $this was called with a closure or method that '
            'returned a Future. Maybe it is marked as "async".'
          ),
          ErrorHint(
            'Instead of performing asynchronous work inside a call to setState(), first '
            'execute the work (without updating the widget state), and then synchronously '
           'update the state inside a call to setState().'
          ),
        ]);
      }
      // We ignore other types of return values so that you can do things like:
      //   setState(() => x = 3);
      return true;
    }());
    _element.markNeedsBuild();
  }

should setState have any body in flutter widgets

2 Answers2