From StatefulWidget navigate to StatelessWidget runs the last builder but also the first builder - flutter

Question

Is there a way to prevent the build() from being called multiple times?
The next code creates two pages, each with one button, the button on FirstPage navigates to SecondPage, and the one on SecondsPage goes back to FirstPage.
The problem is that if you click the button on FirstPage it runs the SecondPage build() but it also runs the FirstPage build().
import 'package:flutter/material.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'First page',
home: FirstPage(),
);
}
}
class FirstPage extends StatefulWidget {
#override
State<StatefulWidget> createState() {
return _FirstPageState();
}
}
class _FirstPageState extends State<FirstPage> {
#override
Widget build(BuildContext context) {
print('---------- In FirstPage build ----------');
return Scaffold(
appBar: AppBar(
title: Text('First page'),
),
body: Column(
children: [
Text('First page body'),
FlatButton(
onPressed: () {
Navigator.push(
context,
MaterialPageRoute(builder: (context) => SecondPage()),
);
},
child: Text('Go to second page')
)
]
)
);
}
}
class SecondPage extends StatelessWidget {
#override
Widget build(BuildContext context) {
print('---------- In SecondPage build ----------');
return Scaffold(
appBar: AppBar(
title: Text('Second page'),
),
body: Column(
children: [
Text('Second page body'),
FlatButton(
onPressed: () {
Navigator.pop(context);
},
child: Text('Go back')
)
]
)
);
}
}
Console output:
Performing hot restart...
D/dalvikvm(23908): threadid=12: interp stack at 0x76040000
Restarted app in 3,849ms.
I/flutter (23908): ---------- In FirstPage build ----------
I/SurfaceTextureClient(23908): [STC::queueBuffer] (this:0x76b39810) fps:0.07, dur:14731.13, max:14731.13, min:14731.13
I/SurfaceTextureClient(23908): [STC::queueBuffer] this:0x76b39810, api:1, last queue time elapsed:14731.13
After click button:
V/Provider/Settings(23908): from db cache, name = sound_effects_enabled , value = 0
I/flutter (23908): ---------- In SecondPage build ----------
I/SurfaceTextureClient(23908): [STC::queueBuffer] (this:0x76b39810) fps:0.24, dur:4140.60, max:4140.60, min:4140.60
I/flutter (23908): ---------- In FirstPage build ----------
Thanks for your help.
EDIT:
I tryed using popAndPushNamed() on FirstPage it just runs the build on the SecondPage but if I try to go back with:
Navigator.popAndPushNamed(
context,
'/'
);
I get the error:
I/flutter ( 5005): ══╡ EXCEPTION CAUGHT BY GESTURE ╞═══════════════════════════════════════════════════════════════════
I/flutter ( 5005): The following assertion was thrown while handling a gesture:
I/flutter ( 5005): Cannot reuse a MaterialPageRoute<dynamic> after disposing it.
I/flutter ( 5005): 'package:flutter/src/widgets/routes.dart': Failed assertion: line 215 pos 12:
I/flutter ( 5005): '!_transitionCompleter.isCompleted'
I/flutter ( 5005): Either the assertion indicates an error in the framework itself, or we should provide substantially
I/flutter ( 5005): more information in this error message to help you determine and fix the underlying cause.
I/flutter ( 5005): In either case, please report this assertion by filing a bug on GitHub:
I/flutter ( 5005): https://github.com/flutter/flutter/issues/new
I/flutter ( 5005): When the exception was thrown, this was the stack:
I/flutter ( 5005): #2 TransitionRoute.didPopNext (package:flutter/src/widgets/routes.dart)
I/flutter ( 5005): #3 NavigatorState.pop (package:flutter/src/widgets/navigator.dart:1691:23)
I/flutter ( 5005): #4 NavigatorState.popAndPushNamed (package:flutter/src/widgets/navigator.dart:1399:5)
I/flutter ( 5005): #5 Navigator.popAndPushNamed (package:flutter/src/widgets/navigator.dart:731:34)
I/flutter ( 5005): #6 SecondPage.build.<anonymous closure> (file:///home/ncs/Documents/flutter/src/test1/lib/main.dart:83:25)
I/flutter ( 5005): #7 _InkResponseState._handleTap (package:flutter/src/material/ink_well.dart:494:14)
I/flutter ( 5005): #8 _InkResponseState.build.<anonymous closure> (package:flutter/src/material/ink_well.dart:549:30)
I/flutter ( 5005): #9 GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:102:24)
I/flutter ( 5005): #10 TapGestureRecognizer._checkUp (package:flutter/src/gestures/tap.dart:161:9)
I/flutter ( 5005): #11 TapGestureRecognizer.handlePrimaryPointer (package:flutter/src/gestures/tap.dart:94:7)
I/flutter ( 5005): #12 PrimaryPointerGestureRecognizer.handleEvent (package:flutter/src/gestures/recognizer.dart:315:9)
I/flutter ( 5005): #13 PointerRouter._dispatch (package:flutter/src/gestures/pointer_router.dart:73:12)
I/flutter ( 5005): #14 PointerRouter.route (package:flutter/src/gestures/pointer_router.dart:101:11)
I/flutter ( 5005): #15 _WidgetsFlutterBinding&BindingBase&GestureBinding.handleEvent (package:flutter/src/gestures/binding.dart:143:19)
I/flutter ( 5005): #16 _WidgetsFlutterBinding&BindingBase&GestureBinding.dispatchEvent (package:flutter/src/gestures/binding.dart:121:22)
I/flutter ( 5005): #17 _WidgetsFlutterBinding&BindingBase&GestureBinding._handlePointerEvent (package:flutter/src/gestures/binding.dart:101:7)
I/flutter ( 5005): #18 _WidgetsFlutterBinding&BindingBase&GestureBinding._flushPointerEventQueue (package:flutter/src/gestures/binding.dart:64:7)
I/flutter ( 5005): #19 _WidgetsFlutterBinding&BindingBase&GestureBinding._handlePointerDataPacket (package:flutter/src/gestures/binding.dart:48:7)
I/flutter ( 5005): #20 _invoke1 (dart:ui/hooks.dart:134:13)
I/flutter ( 5005): #21 _dispatchPointerDataPacket (dart:ui/hooks.dart:91:5)
I/flutter ( 5005): (elided 2 frames from class _AssertionError)
I/flutter ( 5005): Handler: onTap
I/flutter ( 5005): Recognizer:
I/flutter ( 5005): TapGestureRecognizer#94626(debugOwner: GestureDetector, state: possible, won arena, finalPosition:
I/flutter ( 5005): Offset(65.3, 119.6), sent tap down)
I/flutter ( 5005): ════════════════════════════════════════════════════════════════════════════════════════════════════
The only way I'm able to remove the unwanted build is by having a condition like::
class _FirstPageState extends State<FirstPage> {
bool _canBuild = true;
#override
Widget build(BuildContext context) {
if (!_canBuild) {
return Container();
}
print('---------- In FirstPage build ----------');
...
onPressed: () {
_canBuild = false;
Navigator.push(
context,
MaterialPageRoute(builder: (context) => SecondPage()),
).then((_) {
_canBuild = true;
});
},
...
Is this a bad practice or can create any problems?

Answer 1

it is just because of widget is still in to the stack of routes. i think that flutter re-render widgets which are in the stack. if you pop FirstPage before routing to SecondPage the it will not display the message of first widget.
EDIT:
how you can use.
onPressed: () {
Navigator.pop(context);
Navigator.push(
context,
MaterialPageRoute(builder: (context) => SecondPage()),
);
},

Answer 2

Flutter always builds every "page" (widget) in the navigation stack after every push/pop. This is normal and intended behavior, so that every widget can update itself if needed.
Widget's build methods are supposed to be idempotent, whilst usually it only builds when you think it will, the framework might decide at any point to build a widget for any reason - you can't rely on any widget not building.
This is not actually a problem normally, Flutter compares all new widgets with all old widgets on a build, if the widgets are identical they will not be replaced or in any other way affected.
The short answer is thus "No", of course if you want to you can hack around this (e.g. by overriding the navigator), you definitely can.
You can view the code related to building the children of a widget (including navigator) here: https://github.com/flutter/flutter/blob/master/packages/flutter/lib/src/widgets/framework.dart
But ideally you just want to assume anything can build at any time -- as it can. Flutter makes no promises as to when it will (or wont) build your widgets, any code that is not written to take this into account is liable to break!
See this issue: https://github.com/flutter/flutter/issues/11655