react-native/docs/Text.md

Nested Text

In iOS, the way to display formatted text is by using NSAttributedString: you give the text that you want to display and annotate ranges with some specific formatting. In practice, this is very tedious. For React Native, we decided to use web paradigm for this where you can nest text to achieve the same effect.

<Text style={{fontWeight: 'bold'}}>
  I am bold
  <Text style={{color: 'red'}}>
    and red
  </Text>
</Text>

Behind the scenes, this is going to be converted to a flat NSAttributedString that contains the following information

"I am bold and red"
0-9: bold
9-17: bold, red

Containers

The <Text> element is special relative to layout, everything inside is no longer using the flexbox layout but using text layout. This means that elements inside of a <Text> are no longer rectangles but wrap when they see the end of the line.

<Text>
  <Text>First part and </Text>
  <Text>second part</Text>
</Text>
// Text container: all the text flows as if it was one
// |First part |
// |and second |
// |part       |

<View>
  <Text>First part and </Text>
  <Text>second part</Text>
</View>
// View container: each text is its own block
// |First part |
// |and        |
// |second part|

Limited Style Inheritance

On the web, the usual way to set a font family and size for the entire document is to write

/* CSS, *not* React Native */
html {
  font-family: 'lucida grande', tahoma, verdana, arial, sans-serif;
  font-size: 11px;
  color: #141823;
}

When the browser is trying to render a text node, it's going to go all the way up to the root element of the tree and find an element with a font-size attribute. An unexpected property with this system is that any node can have font-size attribute, including a <div>. The reason why it was designed this way is that it is convenient, even though not really semantically correct.

In React Native, we are more strict about it. The first place where it'll show up is that you have to wrap all the text nodes inside of a <Text> component. It is not allowed to have a text node directly under a <View>.

// BAD: will fatal, can't have a text node as child of a <View>
<View>
  Some text
</View>

// GOOD
<View>
  <Text>
    Some text
  </Text>
</View>

You also lose the ability to setup a default font for an entire subtree. The recommended way to use consistent fonts and sizes across your application is to create a component MyAppText that's going to set them and use this component all across your app. You can also make other components such as MyAppHeaderText for other kind of texts.

<View>
  <MyAppText>Text styled with the default font for the entire application</MyAppText>
  <MyAppHeaderText>Text styled as a header</MyAppHeaderText>
</View>

React Native still has the concept of style inheritance, but limited to text subtrees. In this case, the second part will be both bold and red.

<Text style={{fontWeight: 'bold'}}>
  I am bold
  <Text style={{color: 'red'}}>
    and red
  </Text>
</Text>

We believe that this more constrained way to style text will yield better apps.

• (Developper) React components are designed with strong isolation properties in mind, you should be able to drop a component anywhere in your application and it will look and behave the same way, as long as the props are the same. Having text properties be inherited from outside of the props breaks isolation.

• (Implementor) The implementation of React Native is also simplified. We do not need to have a fontFamily field on every single element and we do not need to potentially traverse the tree up to the root every time we display a text node. The style inheritance is only encoded inside of the native Text component and doesn't leak to other components or the system itself.