PageView page value is never absolute for last page

[lukef]

[✓] Flutter (Channel master, v0.11.8-pre.12, on Linux, locale en_US.UTF-8)

Tested on an Android Emulator and iOS Simulator.

The page value of a PageController is always very close to absolute but never exact.
e.g.: A PageView with a page count of 3 will result in a page value of 2.9999999999999... but never 3.0.

[mallardduck]

The framework seems to just apply .round() to the page getter for internal use. For example in the nextPage & previousPage methods. Based on that the obvious solution is to have the same applied within the getter itself. That would require removing that from those and it seems like it would be a breaking change tho too.

Update: I just wanted to report back and confirm that the bug described in #25213 can be resolved by simply adding the .round() (or .roundToDouble()) to your implementation. In my case I used round() since I want an integer and not a double. I also realized that my implementation was a little different - I was using this:

    pageController.addListener(() {
      setState(() {
        this._pageIndex = pageController.page.toInt();
      });
    });

And for some reason, the toInt() will never return a properly rounded integer. This can be seen by setting throwaway varialbes with watchers like so:

    pageController.addListener(() {
      setState(() {
        var _dummy = pageController.page.toInt();
        var _dummy2 = pageController.page.round();

        this._pageIndex = pageController.page.round();
      });
    });

Doing this will make it clear that _dummy never reaches the last item number, while _dummy2 does. Initially, this felt like a bug of doubles toInt(), since 2.99999999 to an integer should logically be 3 - however reading the docs of it's clear this is the correct result. While it feels like a bug the toInt() documentation makes it clear that it simply truncates the value.

With that in mind, a more BC friendly change could be to just apply .roundToDouble() on the page getter internally (somewhere) - then users needing ints should get accurate integers when using toInt(). After all, this is a page getter so in most cases you expect a rational integer to be the value.

However, this would likely break implementations that require a non-rounded double value. So the solution doesn't seem so clear cut as I hoped it would be.

[miguelcmedeiros]

This issue has a dangerous implication: when PageView shows the last page, it actually builds the last 2.

Due to the rounding issues mentioned in this ticket, (2.99999999 instead of 3.0), PageView thinks that it needs to build page 2 and 3 since the PageController.page is neither 2.0 or 3.0.
Depending on the contents of each page (complex rendering), this behaviour could impact the functioning of the app.

Also, it’s really difficult issue for developers to debug. More people will have it.

[mallardduck]

@miguelcmedeiros while this certainly should be addressed by Flutter team, there's a fairly simple workaround for the issue as described in my comment.

Essentially using this will fix it:

    pageController.addListener(() {
      setState(() {
        this._pageIndex = pageController.page.roundToDouble();
      });
    });

Or if you want an Int you can simply do round().

Personally I will admit part of my issue was learning from tutorials that weren't fully accurate. So it gave me the wrong impression of this component. Only after diving into the code did I see the flaws in the tutorial.

Again I totally think Flutter could address this directly - but given how long it's been since the original report and that there's an easy work around, I'm not sure when that will be.

[chunhtai]

the original reproducible will throw exception. here is the modified version

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => MyHomePageState();
}

class MyHomePageState extends State<MyHomePage> {
  final pageController = PageController();
  double page;
  @override
  void initState() {
    super.initState();
    pageController.addListener(() => setState(() {
      page = pageController.page;
    }));
  }

  @override
  Widget build(BuildContext context) => Scaffold(
    body: PageView.builder(
      controller: pageController,
      itemBuilder: (_, index) => Container(
        color: Colors.red,
        child: Center(child: Text("${MediaQuery.of(context).size.width} --  $index -- $page")),
      ),
      itemCount: 3,
      onPageChanged: (page) => print("onPageChanged: $page"),
    ),
  );
}

[github-actions]

This thread has been automatically locked since there has not been any recent activity after it was closed. If you are still experiencing a similar issue, please open a new bug, including the output of flutter doctor -v and a minimal reproduction of the issue.