Can't access PageController.page even when hasClients is true

[caseycrogers]

Steps to reproduce

1. Create a page view with a page controller
2. Put a builder or subwidget inside of pages
3. Display PageController.page from inside the builder/subwidget, checking controller.hasClients first

Expected results

The current page is displayed or a fallback if no page is available (has client is false or the page is null)

Actual results

Page value is only available after content dimensions are established. is thrown and the screen flashes red before properly displaying the page number.

Code sample

Code sample

class PageDemo extends StatefulWidget {
  const PageDemo({super.key});

  @override
  State<PageDemo> createState() => _PageDemoState();
}

class _PageDemoState extends State<PageDemo> {
  final PageController controller = PageController();

  @override
  Widget build(BuildContext context) {
    if (controller.hasClients) {
      print(controller.page);
    }
    return MaterialApp(
      home: Scaffold(
        body: PageView(
          controller: controller,
          children: [
            // Only throws if a builder or sub widget is used here.
            Builder(
              builder: (context) {
                final double currentPage =
                    controller.hasClients ? controller.page ?? -1 : -2;
                return Center(child: Text(currentPage.toString()));
              },
            ),
          ],
        ),
      ),
    );
  }
}

Screenshots or Video

Screenshots / Video demonstration

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Logs

Logs

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Flutter Doctor output

Doctor output

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[caseycrogers]

Relatedly, why is PageController.page nullable when it throws if the page isn't available? It feels like an API should either return null if a valid value isn't available or throw-not both?

[danagbemava-nc]

Reproducible using the code sample provided above.

complete sample

import 'package:flutter/material.dart';

void main() async {
  runApp(const PageDemo());
}


class PageDemo extends StatefulWidget {
  const PageDemo({super.key});

  @override
  State<PageDemo> createState() => _PageDemoState();
}

class _PageDemoState extends State<PageDemo> {
  final PageController controller = PageController();

  @override
  Widget build(BuildContext context) {
    if (controller.hasClients) {
      print(controller.page);
    }
    return MaterialApp(
      home: Scaffold(
        body: PageView(
          controller: controller,
          children: [
            // Only throws if a builder or sub widget is used here.
            Builder(
              builder: (context) {
                final double currentPage =
                    controller.hasClients ? controller.page ?? -1 : -2;
                return Center(child: Text(currentPage.toString()));
              },
            ),
          ],
        ),
      ),
    );
  }
}

flutter doctor -v

[!] Flutter (Channel stable, 3.19.6, on macOS 14.4.1 23E224 darwin-arm64, locale en-GB)
    • Flutter version 3.19.6 on channel stable at /Users/nexus/dev/sdks/flutter
    ! Warning: `flutter` on your path resolves to /Users/nexus/dev/sdks/flutters/bin/flutter, which is not inside your current Flutter SDK checkout at /Users/nexus/dev/sdks/flutter. Consider adding /Users/nexus/dev/sdks/flutter/bin to the front of your path.
    ! Warning: `dart` on your path resolves to /Users/nexus/dev/sdks/flutters/bin/dart, which is not inside your current Flutter SDK checkout at /Users/nexus/dev/sdks/flutter. Consider adding /Users/nexus/dev/sdks/flutter/bin to the front of your path.
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision 54e66469a9 (17 hours ago), 2024-04-17 13:08:03 -0700
    • Engine revision c4cd48e186
    • Dart version 3.3.4
    • DevTools version 2.31.1
    • If those were intentional, you can disregard the above warnings; however it is recommended to use "git" directly to perform update checks and upgrades.

[✓] Android toolchain - develop for Android devices (Android SDK version 34.0.0)
    • Android SDK at /Users/nexus/Library/Android/sdk
    • Platform android-34, build-tools 34.0.0
    • Java binary at: /Users/nexus/Applications/Android Studio.app/Contents/jbr/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 17.0.7+0-17.0.7b1000.6-10550314)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 15.3)
    • Xcode at /Applications/Xcode-15.3.0.app/Contents/Developer
    • Build 15E204a
    • CocoaPods version 1.14.3

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 2023.1)
    • Android Studio at /Users/nexus/Applications/Android Studio.app/Contents
    • Flutter plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/6351-dart
    • Java version OpenJDK Runtime Environment (build 17.0.7+0-17.0.7b1000.6-10550314)

[✓] IntelliJ IDEA Ultimate Edition (version 2023.2.5)
    • IntelliJ at /Users/nexus/Applications/IntelliJ IDEA Ultimate.app
    • Flutter plugin version 77.2.2
    • Dart plugin version 232.10286

[✓] VS Code (version 1.88.1)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.86.0

[✓] Connected device (4 available)
    • Nexus (mobile)       • 00008020-001875E83A38002E • ios            • iOS 17.4.1 21E236
    • Dean’s iPad (mobile) • 00008103-000825C811E3401E • ios            • iOS 17.4.1 21E236
    • macOS (desktop)      • macos                     • darwin-arm64   • macOS 14.4.1 23E224 darwin-arm64
    • Chrome (web)         • chrome                    • web-javascript • Google Chrome 124.0.6367.60

[✓] Network resources
    • All expected network resources are available.

! Doctor found issues in 1 category.

[✓] Flutter (Channel master, 3.22.0-13.0.pre.38, on macOS 14.4.1 23E224 darwin-arm64, locale en-GB)
    • Flutter version 3.22.0-13.0.pre.38 on channel master at /Users/nexus/dev/sdks/flutters
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision 86135b7774 (10 hours ago), 2024-04-17 20:08:36 -0700
    • Engine revision 20638b75c8
    • Dart version 3.5.0 (build 3.5.0-71.0.dev)
    • DevTools version 2.35.0-dev.8

[✓] Android toolchain - develop for Android devices (Android SDK version 34.0.0)
    • Android SDK at /Users/nexus/Library/Android/sdk
    • Platform android-34, build-tools 34.0.0
    • Java binary at: /Users/nexus/Applications/Android Studio.app/Contents/jbr/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 17.0.7+0-17.0.7b1000.6-10550314)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 15.3)
    • Xcode at /Applications/Xcode-15.3.0.app/Contents/Developer
    • Build 15E204a
    • CocoaPods version 1.14.3

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 2023.1)
    • Android Studio at /Users/nexus/Applications/Android Studio.app/Contents
    • Flutter plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/6351-dart
    • Java version OpenJDK Runtime Environment (build 17.0.7+0-17.0.7b1000.6-10550314)

[✓] IntelliJ IDEA Ultimate Edition (version 2023.2.5)
    • IntelliJ at /Users/nexus/Applications/IntelliJ IDEA Ultimate.app
    • Flutter plugin version 77.2.2
    • Dart plugin version 232.10286

[✓] VS Code (version 1.88.1)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.86.0

[✓] Connected device (5 available)
    • Nexus (mobile)                  • 00008020-001875E83A38002E • ios            • iOS 17.4.1 21E236
    • Dean’s iPad (mobile)            • 00008103-000825C811E3401E • ios            • iOS 17.4.1 21E236
    • macOS (desktop)                 • macos                     • darwin-arm64   • macOS 14.4.1 23E224 darwin-arm64
    • Mac Designed for iPad (desktop) • mac-designed-for-ipad     • darwin         • macOS 14.4.1 23E224 darwin-arm64
    • Chrome (web)                    • chrome                    • web-javascript • Google Chrome 124.0.6367.60

[✓] Network resources
    • All expected network resources are available.

• No issues found!