Wednesday, February 19, 2020

ReactNative: understanding alignment of Views and Texts.

why does the Text take up the full space of the View, instead of just the space for "Hello"?
Because of the View is a flex container and by default has flexDirection: 'column' and alignItems: 'stretch', which means that its children should be stretched out to fill its width.
(Note, per the docs, that all components in React Native are display: 'flex' by default and that display: 'inline' does not exist at all. In this way, the default behaviour of a Text within a View in React Native differs from the default behaviour of span within a div on the web; in the latter case, the span would not fill the width of the div because of a span is an inline element by default. There is no such concept in React Native.)
How can the Text be floated / aligned to the right?
The float the property doesn't exist in React Native, but there are loads of options available to you (with slightly different behaviours) that will let you right-align your text. Here are the ones I can think of:

1. Use textAlign: 'right' on the Text element

<View>
  <Text style={{textAlign: 'right'}}>Hello, World!</Text>
</View>
(This approach doesn't change the fact that the Text fills the entire width of the View; it just right-aligns the text within the Text.)

2. Use alignSelf: 'flex-end' on the Text

<View>
  <Text style={{alignSelf: 'flex-end'}}>Hello, World!</Text>
</View>
This shrinks the Text element to the size required to hold its content and puts it at the end of the cross direction (the horizontal direction, by default) of the View.

3. Use alignItems: 'flex-end' on the View

<View style={{alignItems: 'flex-end'}}>
  <Text>Hello, World!</Text>
</View>
This is equivalent to setting alignSelf: 'flex-end' on all the View's children.

4. Use flexDirection: 'row' and justifyContent: 'flex-end' on the View

<View style={{flexDirection: 'row', justifyContent: 'flex-end'}}>
  <Text>Hello, World!</Text>
</View>
flexDirection: 'row' sets the main direction of the layout to be horizontal instead of vertical; justifyContent is just like alignItems, but controls alignment in the main direction instead of the cross direction.

5. Use flexDirection: 'row' on the View and marginLeft: 'auto' on the Text

<View style={{flexDirection: 'row'}}>
  <Text style={{marginLeft: 'auto'}}>Hello, World!</Text>
</View>
This approach is demonstrated, in the context of the web and real CSS, at https://stackoverflow.com/a/34063808/1709587.

6. Use position: 'absolute' and right: 0 on the Text:

<View>
  <Text style={{position: 'absolute', right: 0}}>Hello, World!</Text>
</View>
Like in real CSS, this takes the Text "out of flow", meaning that its siblings will be able to overlap it and its vertical position will be at the top of the View by default (although you can explicitly set a distance from the top of the View using the top style property).
Naturally, which of these various approaches you want to use - and whether the choice between them even matters at all - will depend upon your precise circumstances.
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