Janic Duplessis
96d7c92a06
Summary: `maintainVisibleContentPosition` is broken when using virtualization and the new content pushes visible content outside its "window". This can be reproduced in the example from this diff. When using a large page size it will always push visible content outside of the list "window" which will cause currently visible views to be unmounted so the implementation of `maintainVisibleContentPosition` can't adjust the content inset since the visible views no longer exist. The first illustration shows the working case, when the new content doesn't push visible content outside the window. The red box represents the window, all views outside the box are not mounted, which means the native implementation of `maintainVisibleContentPosition` has no way to know it exists. In that case the first visible view is https://github.com/facebook/react-native/issues/2, after new content is added https://github.com/facebook/react-native/issues/2 is still inside the window so there's not problem adjusting content offset to maintain position. As you can see Step 1 and 3 result in the same position for all initial views. The second illustation shows the broken case, when new content is added and pushes the first visible view outside the window. As you can see in step 2 the view https://github.com/facebook/react-native/issues/2 is no longer rendered so there's no way to maintain its position. #### Illustration 1  #### Illustration 2  To fix `maintainVisibleContentPosition` when using `VirtualizedList` we need to make sure the visible items stay rendered when new items are added at the start of the list. In order to do that we need to do the following: - Detect new items that will cause content to be adjusted - Add cells to render mask so that previously visible cells stay rendered - Ignore certain updates while scroll metrics are invalid ### Detect new items that will cause content to be adjusted The goal here is to know that scroll position will be updated natively by the `maintainVisibleContentPosition` implementation. The problem is that the native code uses layout heuristics which are not easily available to JS to do so. In order to approximate the native heuristic we can assume that if new items are added at the start of the list, it will cause `maintainVisibleContentPosition` to be triggered. This simplifies JS logic a lot as we don't have to track visible items. In the worst case if for some reason our JS heuristic is wrong, it will cause extra cells to be rendered until the next scroll event, or content position will not be maintained (what happens all the time currently). I think this is a good compromise between complexity and accuracy. We need to find how many items have been added before the first one. To do that we save the key of the first item in state `firstItemKey`. When data changes we can find the index of `firstItemKey` in the new data and that will be the amount we need to adjust the window state by. Note that this means that keys need to be stable, and using index won't work. ### Add cells to render mask so that previously visible cells stay rendered Once we have the adjusted number we can save this in a new state value `maintainVisibleContentPositionAdjustment` and add the adjusted cells to the render mask. This state is then cleared when we receive updated scroll metrics, once the native implementation is done adding the new items and adjusting the content offset. This value is also only set when `maintainVisibleContentPosition` is set so this makes sure this maintains the currently behavior when that prop is not set. ### Ignore certain updates while scroll metrics are invalid While the `maintainVisibleContentPositionAdjustment` state is set we know that the current scroll metrics are invalid since they will be updated in the native `ScrollView` implementation. In that case we want to prevent certain code from running. One example is `onStartReached` that will be called incorrectly while we are waiting for updated scroll metrics. ## Changelog [General] [Fixed] - Fix VirtualizedList with maintainVisibleContentPosition Pull Request resolved: https://github.com/facebook/react-native/pull/35993 Test Plan: Added bidirectional paging to RN tester FlatList example. Note that for this to work RN tester need to be run using old architecture on iOS, to use new architecture it requires https://github.com/facebook/react-native/pull/35319 Using debug mode we can see that virtualization is still working properly, and content position is being maintained. https://user-images.githubusercontent.com/2677334/163294404-e2eeae5b-e079-4dba-8664-ad280c171ae6.mov Reviewed By: yungsters Differential Revision: D45294060 Pulled By: NickGerleman fbshipit-source-id: 8e5228318886aa75da6ae397f74d1801d40295e8
React Native
Learn once, write anywhere:
Build mobile apps with React.
Getting Started · Learn the Basics · Showcase · Contribute · Community · Support
React Native brings React's declarative UI framework to iOS and Android. With React Native, you use native UI controls and have full access to the native platform.
- Declarative. React makes it painless to create interactive UIs. Declarative views make your code more predictable and easier to debug.
- Component-Based. Build encapsulated components that manage their state, then compose them to make complex UIs.
- Developer Velocity. See local changes in seconds. Changes to JavaScript code can be live reloaded without rebuilding the native app.
- Portability. Reuse code across iOS, Android, and other platforms.
React Native is developed and supported by many companies and individual core contributors. Find out more in our ecosystem overview.
Contents
- Requirements
- Building your first React Native app
- Documentation
- Upgrading
- How to Contribute
- Code of Conduct
- License
📋 Requirements
React Native apps may target iOS 12.4 and Android 5.0 (API 21) or newer. You may use Windows, macOS, or Linux as your development operating system, though building and running iOS apps is limited to macOS. Tools like Expo can be used to work around this.
🎉 Building your first React Native app
Follow the Getting Started guide. The recommended way to install React Native depends on your project. Here you can find short guides for the most common scenarios:
📖 Documentation
The full documentation for React Native can be found on our website.
The React Native documentation discusses components, APIs, and topics that are specific to React Native. For further documentation on the React API that is shared between React Native and React DOM, refer to the React documentation.
The source for the React Native documentation and website is hosted on a separate repo, @facebook/react-native-website.
🚀 Upgrading
Upgrading to new versions of React Native may give you access to more APIs, views, developer tools, and other goodies. See the Upgrading Guide for instructions.
React Native releases are discussed in this discussion repo.
👏 How to Contribute
The main purpose of this repository is to continue evolving React Native core. We want to make contributing to this project as easy and transparent as possible, and we are grateful to the community for contributing bug fixes and improvements. Read below to learn how you can take part in improving React Native.
Code of Conduct
Facebook has adopted a Code of Conduct that we expect project participants to adhere to. Please read the full text so that you can understand what actions will and will not be tolerated.
Contributing Guide
Read our Contributing Guide to learn about our development process, how to propose bugfixes and improvements, and how to build and test your changes to React Native.
Open Source Roadmap
You can learn more about our vision for React Native in the Roadmap.
Good First Issues
We have a list of good first issues that contain bugs which have a relatively limited scope. This is a great place to get started, gain experience, and get familiar with our contribution process.
Discussions
Larger discussions and proposals are discussed in @react-native-community/discussions-and-proposals.
📄 License
React Native is MIT licensed, as found in the LICENSE file.
React Native documentation is Creative Commons licensed, as found in the LICENSE-docs file.