iOS: Add a maxFontSizeMultiplier prop to <Text> and <TextInput> (facebook#20915)

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: facebook#20915

Differential Revision: D9646739

Pulled By: shergin

fbshipit-source-id: c823f59c1e342c22d6297b88b2cb11c5a1f10310

Author: Adam Comella

Libraries/Components/TextInput/TextInput.js

@@ -177,6 +177,7 @@ type Props = $ReadOnly<{|
   autoCorrect?: ?boolean,
   autoFocus?: ?boolean,
   allowFontScaling?: ?boolean,
+  maxFontSizeMultiplier?: ?boolean,
   editable?: ?boolean,
   keyboardType?: ?KeyboardType,
   returnKeyType?: ?ReturnKeyType,
@@ -367,6 +368,14 @@ const TextInput = createReactClass({
      * default is `true`.
      */
     allowFontScaling: PropTypes.bool,
+    /**
+     * Specifies largest possible scale a font can reach when `allowFontScaling` is enabled.
+     * Possible values:
+     * `null/undefined` (default): inherit from the parent node or the global default (0)
+     * `0`: no max, ignore parent/global default
+     * `>= 1`: sets the maxFontSizeMultiplier of this node to this value
+     */
+    maxFontSizeMultiplier: PropTypes.number,
     /**
      * If `false`, text is not editable. The default value is `true`.
      */
@@ -933,7 +942,11 @@ const TextInput = createReactClass({
       );
       if (childCount >= 1) {
         children = (
-          <Text style={props.style} allowFontScaling={props.allowFontScaling}>
+          <Text
+            style={props.style}
+            allowFontScaling={props.allowFontScaling}
+            maxFontSizeMultiplier={props.maxFontSizeMultiplier}
+          >
             {children}
           </Text>
         );

Libraries/Text/BaseText/RCTBaseTextViewManager.m

@@ -36,6 +36,7 @@ - (RCTShadowView *)shadowView
 RCT_REMAP_SHADOW_PROPERTY(fontStyle, textAttributes.fontStyle, NSString)
 RCT_REMAP_SHADOW_PROPERTY(fontVariant, textAttributes.fontVariant, NSArray)
 RCT_REMAP_SHADOW_PROPERTY(allowFontScaling, textAttributes.allowFontScaling, BOOL)
+RCT_REMAP_SHADOW_PROPERTY(maxFontSizeMultiplier, textAttributes.maxFontSizeMultiplier, CGFloat)
 RCT_REMAP_SHADOW_PROPERTY(letterSpacing, textAttributes.letterSpacing, CGFloat)
 // Paragraph Styles
 RCT_REMAP_SHADOW_PROPERTY(lineHeight, textAttributes.lineHeight, CGFloat)

Libraries/Text/RCTTextAttributes.h

@@ -31,6 +31,7 @@ extern NSString *const RCTTextAttributesTagAttributeName;
 @property (nonatomic, copy, nullable) NSString *fontFamily;
 @property (nonatomic, assign) CGFloat fontSize;
 @property (nonatomic, assign) CGFloat fontSizeMultiplier;
+@property (nonatomic, assign) CGFloat maxFontSizeMultiplier;
 @property (nonatomic, copy, nullable) NSString *fontWeight;
 @property (nonatomic, copy, nullable) NSString *fontStyle;
 @property (nonatomic, copy, nullable) NSArray<NSString *> *fontVariant;
@@ -71,7 +72,7 @@ extern NSString *const RCTTextAttributesTagAttributeName;
 - (UIFont *)effectiveFont;
 
 /**
- * Font size multiplier reflects `allowFontScaling` and `fontSizeMultiplier`.
+ * Font size multiplier reflects `allowFontScaling`, `fontSizeMultiplier`, and `maxFontSizeMultiplier`.
  */
 - (CGFloat)effectiveFontSizeMultiplier;
 

Libraries/Text/RCTTextAttributes.m

@@ -24,6 +24,7 @@ - (instancetype)init
     _lineHeight = NAN;
     _textDecorationStyle = NSUnderlineStyleSingle;
     _fontSizeMultiplier = NAN;
+    _maxFontSizeMultiplier = NAN;
     _alignment = NSTextAlignmentNatural;
     _baseWritingDirection = NSWritingDirectionNatural;
     _textShadowRadius = NAN;
@@ -49,6 +50,7 @@ - (void)applyTextAttributes:(RCTTextAttributes *)textAttributes
   _fontFamily = textAttributes->_fontFamily ?: _fontFamily;
   _fontSize = !isnan(textAttributes->_fontSize) ? textAttributes->_fontSize : _fontSize;
   _fontSizeMultiplier = !isnan(textAttributes->_fontSizeMultiplier) ? textAttributes->_fontSizeMultiplier : _fontSizeMultiplier;
+  _maxFontSizeMultiplier = !isnan(textAttributes->_maxFontSizeMultiplier) ? textAttributes->_maxFontSizeMultiplier : _maxFontSizeMultiplier;
   _fontWeight = textAttributes->_fontWeight ?: _fontWeight;
   _fontStyle = textAttributes->_fontStyle ?: _fontStyle;
   _fontVariant = textAttributes->_fontVariant ?: _fontVariant;
@@ -191,7 +193,15 @@ - (UIFont *)effectiveFont
 
 - (CGFloat)effectiveFontSizeMultiplier
 {
-  return !RCTHasFontHandlerSet() && _allowFontScaling && !isnan(_fontSizeMultiplier) ? _fontSizeMultiplier : 1.0;
+  bool fontScalingEnabled = !RCTHasFontHandlerSet() && _allowFontScaling;
+
+  if (fontScalingEnabled) {
+    CGFloat fontSizeMultiplier = !isnan(_fontSizeMultiplier) ? _fontSizeMultiplier : 1.0;
+    CGFloat maxFontSizeMultiplier = !isnan(_maxFontSizeMultiplier) ? _maxFontSizeMultiplier : 0.0;
+    return maxFontSizeMultiplier >= 1.0 ? fminf(maxFontSizeMultiplier, fontSizeMultiplier) : fontSizeMultiplier;
+  } else {
+    return 1.0;
+  }
 }
 
 - (UIColor *)effectiveForegroundColor
@@ -260,6 +270,7 @@ - (BOOL)isEqual:(RCTTextAttributes *)textAttributes
     RCTTextAttributesCompareObjects(_fontFamily) &&
     RCTTextAttributesCompareFloats(_fontSize) &&
     RCTTextAttributesCompareFloats(_fontSizeMultiplier) &&
+    RCTTextAttributesCompareFloats(_maxFontSizeMultiplier) &&
     RCTTextAttributesCompareStrings(_fontWeight) &&
     RCTTextAttributesCompareObjects(_fontStyle) &&
     RCTTextAttributesCompareObjects(_fontVariant) &&

Libraries/Text/Text.js

@@ -58,6 +58,7 @@ const viewConfig = {
     numberOfLines: true,
     ellipsizeMode: true,
     allowFontScaling: true,
+    maxFontSizeMultiplier: true,
     disabled: true,
     selectable: true,
     selectionColor: true,
@@ -263,6 +264,7 @@ const RCTVirtualText =
         validAttributes: {
           ...ReactNativeViewAttributes.UIView,
           isHighlighted: true,
+          maxFontSizeMultiplier: true,
         },
         uiViewClassName: 'RCTVirtualText',
       }));

Libraries/Text/Text/RCTTextViewManager.m

@@ -24,7 +24,6 @@ @interface RCTTextViewManager () <RCTUIManagerObserver>
 @implementation RCTTextViewManager
 {
   NSHashTable<RCTTextShadowView *> *_shadowViews;
-  CGFloat _fontSizeMultiplier;
 }
 
 RCT_EXPORT_MODULE(RCTText)

Libraries/Text/TextPropTypes.js

@@ -100,6 +100,14 @@ module.exports = {
    * See https://facebook.github.io/react-native/docs/text.html#allowfontscaling
    */
   allowFontScaling: PropTypes.bool,
+  /**
+   * Specifies largest possible scale a font can reach when `allowFontScaling` is enabled.
+   * Possible values:
+   * `null/undefined` (default): inherit from the parent node or the global default (0)
+   * `0`: no max, ignore parent/global default
+   * `>= 1`: sets the maxFontSizeMultiplier of this node to this value
+   */
+  maxFontSizeMultiplier: PropTypes.number,
   /**
    * Indicates whether the view is an accessibility element.
    *