React-native-pager-view: Add example test to readme

[brunocrpontes]

Summary

Testing screens with ViewPager that uses setPage function fails with the following error:

TypeError: Cannot read property 'Commands' of undefined

Tests like below will fail:

import React from 'react';
import {render, act} from '@testing-library/react-native';
import ViewPager from '@react-native-community/viewpager';
import {View} from 'react-native';

describe('ViewPager', () => {
  test('view pager should move to next view', () => {
    const viewPagerRef = React.createRef();

    const {UNSAFE_getByType} = render(
      <ViewPager ref={viewPagerRef}>
        <View testID="page1" />
        <View testID="page2" />
      </ViewPager>,
    );

    expect(UNSAFE_getByType(ViewPager)._fiber.index).toBe(0);

    act(() => {
      viewPagerRef.current.setPage && viewPagerRef.current.setPage(1);
    });

    expect(UNSAFE_getByType(ViewPager)._fiber.index).toBe(1);
  });
});

Environment info

react-native info output:

 System:
    OS: macOS 10.15.6
    CPU: (4) x64 Intel(R) Core(TM) i5-5257U CPU @ 2.70GHz
    Memory: 21.24 MB / 8.00 GB
    Shell: 5.7.1 - /bin/zsh
  Binaries:
    Node: 12.18.3 - /var/folders/vl/7sf6krl17mz491vg6rqrlkr00000gn/T/yarn--1598293332713-0.5982209185240344/node
    Yarn: 1.22.4 - /var/folders/vl/7sf6krl17mz491vg6rqrlkr00000gn/T/yarn--1598293332713-0.5982209185240344/yarn
    npm: 6.14.6 - ~/.asdf/installs/nodejs/12.18.3/bin/npm
    Watchman: 4.9.0 - /usr/local/bin/watchman
  Managers:
    CocoaPods: 1.9.3 - /usr/local/bin/pod
  SDKs:
    iOS SDK:
      Platforms: iOS 13.6, DriverKit 19.0, macOS 10.15, tvOS 13.4, watchOS 6.2
    Android SDK:
      API Levels: 28
      Build Tools: 28.0.3, 30.0.1
      System Images: android-28 | Intel x86 Atom
      Android NDK: Not Found
  IDEs:
    Android Studio: 4.0 AI-193.6911.18.40.6626763
    Xcode: 11.6/11E708 - /usr/bin/xcodebuild
  Languages:
    Java: 1.8.0_261 - /usr/bin/javac
    Python: 2.7.16 - /Users/brunocrpontes/.asdf/shims/python
  npmPackages:
    @react-native-community/cli: Not Found
    react: 16.11.0 => 16.11.0 
    react-native: 0.62.2 => 0.62.2 
  npmGlobalPackages:
    *react-native*: Not Found

Library version: 4.1.6

Steps to reproduce

1. Create a test with <ViewPager />
2. Take its ref like <ViewPager ref={ref => viewPager = ref}
3. Call viewPager.setPage(/* page value */)

Describe what you expected to happen:

1. Call setPage doesn't throw any errors.
2. Update value of index in component reference.

[troZee]

Hey,
Thank you for reporting an issue. What platform is affected ?

From my POV, you should call it using current like below:
https://github.com/react-native-community/react-native-viewpager/blob/master/example/ViewPagerExample.js#L103

[Covicake]

I'm having the same issue, but only when running in test environment on Android platform. The test fails with the same error message the OP is having. I'm not using the library directly, but through a third party (react-native-scrollable-tab-view), and they are using an Animated version with: Animated.createAnimatedComponent(ViewPager)

[brunocrpontes]

Hey,
Thank you for reporting an issue. What platform is affected ?

From my POV, you should call it using current like below:
https://github.com/react-native-community/react-native-viewpager/blob/master/example/ViewPagerExample.js#L103

It's happening in testing platform, in my Jest tests to be more exactly. Run it on device works fine.

I'm mocking package to workaround the issue, like below:

jest.mock('@react-native-community/viewpager', () => {
  const RealComponent = jest.requireActual('@react-native-community/viewpager');
  const React = require('react');

  return class ViewPager extends RealComponent {
    index = 0;

    setPage = (page) => {
      this.index = Math.max(
        0,
        Math.min(page, React.Children.count(this.props.children)),
      );
    };
  };
});

Update:
I added a test example that fails in issue description.

[troZee]

@brunocrpontes
Thank you for providing an example. I don't understand, what would you like to achieve in your test ?

[brunocrpontes]

My test example isn't the best, is just to achieve error mentioned in issue.

In my case I built an paginated form using view pager as base component, part of my test is move to next page and try to capture some dependent instances of it, but in tests I can't move to next page. So current workaround is mocking it like my previous comment.

[troZee]

You cannot use below code

  const RealComponent = jest.requireActual('@react-native-community/viewpager');

because viewpager has native implementation. There is no javascript logic inside.

If you would like to test VP, you need to mock this class and then mock implantation for onPageSelected method.

[brunocrpontes]

@troZee understood, but why onPageSelected? I wish to move to next view pager page, the error came from "Commands" in setPage implementation.

[troZee]

VP changes pages using onPageSelected, so why you want you use setPage method ?
You can check in tests, if setPage has been called, once eg button clicked

[brunocrpontes]

Hmm, I will try that and back with a feedback. Thanks @troZee for help!

[EduVencovsky]

Any updates on this? Still not sure how to test it

[finalight]

likewise, i have the same error as well

[troZee]

Any updates on this? Still not sure how to test it

likewise, i have the same error as well

What is an issue with mocking this component ?

[finalight]

Any updates on this? Still not sure how to test it

likewise, i have the same error as well

What is an issue with mocking this component ?

for some reason, the link that you gave is not valid anymore

do you have the updated link for that?