[Discussion] setPage should not trigger onPageSelected event

Bug

We are storing and updating the active index of the ViewPager in Redux. When the user swipes to another page the onPageSelected event is correctly triggered and the active index will be updated like that:

<ViewPager
      ref={pager}
      pageMargin={0}
      onPageSelected={({ nativeEvent }) => setActiveIndex(nativeEvent.position)}
      style={[styles.pager, { width }]}
      initialPage={0}
>

But we also want to set the page programatically when the active index is changed by some other component. To achieve this we are using an useEffect hook like this:

useEffect(() => {
    pager.current && pager.current.setPageWithoutAnimation(activeIndex);
}, [activeIndex]);

Unfortunately calling setPageWithoutAnimation triggers another onPageSelected event which updates the active index which triggers the useEffect hook and so on.

We think that there should be no onPageSelected event triggered when setting the page programatically.

Environment info

React native info output:

System:
    OS: macOS Mojave 10.14.6
    CPU: (4) x64 Intel(R) Core(TM) i5-7360U CPU @ 2.30GHz
    Memory: 5.96 GB / 16.00 GB
    Shell: 3.2.57 - /bin/bash
  Binaries:
    Node: 10.16.3 - ~/.nvm/versions/node/v10.16.3/bin/node
    Yarn: 1.19.1 - /usr/local/bin/yarn
    npm: 6.13.1 - ~/.nvm/versions/node/v10.16.3/bin/npm
    Watchman: 4.9.0 - /usr/local/bin/watchman
  SDKs:
    iOS SDK:
      Platforms: iOS 13.2, DriverKit 19.0, macOS 10.15, tvOS 13.2, watchOS 6.1
    Android SDK:
      API Levels: 21, 22, 23, 24, 25, 26, 27, 28
      Build Tools: 23.0.1, 25.0.2, 25.0.3, 26.0.1, 26.0.2, 26.0.3, 27.0.3, 28.0.1, 28.0.3, 29.0.2
      System Images: android-19 | Google APIs Intel x86 Atom, android-21 | Google APIs Intel x86 Atom_64, android-22 | Intel x86 Atom_64, android-22 | Google APIs Intel x86 Atom_64, android-23 | Google APIs Intel x86 Atom_64, android-26 | Intel x86 Atom_64, android-27 | Google APIs Intel x86 Atom, android-28 | Google APIs Intel x86 Atom
      Android NDK: 20.1.5948944
  IDEs:
    Android Studio: 3.5 AI-191.8026.42.35.6010548
    Xcode: 11.3/11C29 - /usr/bin/xcodebuild
  npmPackages:
    react: 16.9.0 => 16.9.0
    react-native: 0.61.5 => 0.61.5

Library version: 3.3.0

[enagorny]

Ideally would be good to have react-native-viewpager as controlled component. Instead of initialPage might be better to have page which will control selected page.

[oliverdolgener]

@crystalneth not yet. what you describe is exactly our problem. I’ll take a closer look this week and let you know.