Summary:
**Motivation**
Whenever a user changes the system font size to its maximum allowable setting, React Native apps that allow font scaling can become unusable because the text gets too big. Experimenting with a native app like iMessage on iOS, the font size used for non-body text (e.g. header, navigational elements) is capped while the body text (e.g. text in the message bubbles) is allowed to grow.
This PR introduces a new prop on `<Text>` and `<TextInput>` called `maxFontSizeMultiplier`. This enables devs to set the maximum allowed text scale factor on a Text/TextInput. The default is 0 which means no limit.
Another PR will add this feature to Android.
**Test Plan**
I created a test app which utilizes all categories of values of `maxFontSizeMultiplier`:
- `undefined`: inherit from parent
- `0`: no limit
- `1`, `1.2`: fixed limits
I tried this with `Text`, `TextInput` with `value`, and `TextInput` with children. For `Text`, I also verified that nesting works properly (if a child `Text` doesn't specify `maxFontSizeMultiplier`, it inherits it from its parent).
Lastly, we've been using a version of this in Skype for several months.
**Release Notes**
[GENERAL] [ENHANCEMENT] [Text/TextInput] - Added maxFontSizeMultiplier prop to prevent some text from getting unusably large as user increases OS's font scale setting (iOS)
Adam Comella
Microsoft Corp.
Pull Request resolved: https://github.com/facebook/react-native/pull/20915
Differential Revision: D9646739
Pulled By: shergin
fbshipit-source-id: c823f59c1e342c22d6297b88b2cb11c5a1f10310
01d5eff425