Why Root Widget gets build twice?

Question

I have this very basic test app:

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    print("ROOT WIDGET ");
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Container(decoration: BoxDecoration(color: Colors.blue),),
    );
  }
}

Log output debug:

Launching lib/main.dart on iPhone X in debug mode...

Found saved certificate choice "XXX". To clear, use "flutter config". Signing iOS app for device deployment using developer identity: "XXX"

Running Xcode build...

Xcode build done. 39,4s

Installing and launching...

flutter: ROOT WIDGET

Syncing files to device iPhone X...

flutter: ROOT WIDGET

Log output with flutter run --release

Launching lib/main.dart on iPhone X in release mode...

Found saved certificate choice "XXX". To clear, use "flutter config".

Signing iOS app for device deployment using developer identity: "XXX"

Running pod install...
1,2s

Running Xcode build...
├─Building Dart code... 15,8s

├─Generating dSYM file... 0,1s

├─Stripping debug symbols... 0,0s

├─Assembling Flutter resources... 0,7s

└─Compiling, linking and signing... 48,3s

Xcode build done. 67,4s

Installing and launching...
6,8s

To quit, press "q".

flutter: ROOT WIDGET

As you can see it only happens in debug mode.

The reason why I ask is, in my real app, I use a WebView. The function onWebViewCreated(WebViewController controller) of the WebView is only called the first time, so that the WebViewController is always null in debug mode.

Answer 1

I have been getting this issue on

Flutter 1.22.0 • channel stable

Deleting the app from my emulator/physical phone and reinstalling fixes the issue. Delete the app from the emulator/physical from System Settings/Apps and reinstall in debug mode.

I have no idea why but time to time my main widget keeps on getting rebuild exactly twice and I dont do anything in my main widget no SetStates or no futurebuilder. I think this is a flutter internal bug.

Answer 2

The clue might be here

Installing and launching... //We are launching the app , the code bundled with it it's executed briefly before it "sync" to allow hot reload

flutter: ROOT WIDGET //The bundled code executes

Syncing files to device iPhone X... //We start the sync, hot reload triggers an app rebuild 

flutter: ROOT WIDGET //The live code executes

If you comment the print, then hot-reload, then uncomment the print, you should only see one "ROOT WIDGET".