iOS/Android: Text Scale

[amarlette]

Requires API Proposal

This issue may require a new API, or a change to an existing API. An API proposal should be added and discussed before proceeding with implementation. The API proposal can be added in the comments of this issue or linked as a separate issue.

Description

Both iOS and Android allow users to change the current size of their font at a system level. Apps should respect this setting, and when rendering text via some custom manner (graphs, charts, image composition, etc.), may need to take it into account.

React Native version:

v0.63

Expected Behavior

Currently the AccessibilityInfo API has no method to detect the current font scale that a user has chosen. It should have a method that works on both iOS and Android. iOS and Android treat scaling differently, with iOS having named sizes, and Android using a multiplier, so this information may need to be normalized.

Android Details

The setting on Android may show named sizes or a slider of different multiplier values (depending on the device manufacturer). On the hood these are always treated as a multiplier. The current value can be queried using the Settings.System.FONT_SCALE property.

https://developer.android.com/reference/android/provider/Settings.System#FONT_SCALE

iOS Details

On iOS you can use the preferredContentSizeCategory property to query this value.
https://developer.apple.com/documentation/uikit/uitraitcollection/1771746-preferredcontentsizecategory

[blavalla]

This is definitely a "good first issue" on Android due to it's relatively simple multiplier that is returned, but on iOS the named sizes to not always impact all fonts in the same way, so it may be a bit trickier to know exactly how to map them to numbers.

[grgr-dkrk]

Currently, getting font scales seems to be supported by Dimensions.get('screen').FontScale and PixelRatio.getFontScale().
Does this need to be implemented?

#11008
#12268

[blavalla]

@grgr-dkrk , good catch! These both return exactly what I was looking for.

@amarlette, we can close this one out.