Styjun

I want to create a button component that will automatically have rounded corners, no matter its dimension.

As you know, to achieve rounded corners, one way to achieve it is to specify the border radius as half of the height of the button.

The way I implemented is that in the custom component I use the onLayout function like this:

 onLayout(event: LayoutChangeEvent) 
 const height = event.nativeEvent.layout;
 this.setState( borderRadius: height / 2 );
 

The problem is that the button will initially appear on screen as a rectangle and only after a millisecond, it will round the corners causing a flicker.

My guess is that onLayout is called after the component renders.

How would one go about implementing this? Thanks!

Before the borderRadius is calculated, you could return transparent button, this would prevent this flickering effect...

// you pass radius, and height from component state

const MyButton = ( radius, height ) => 
 if (radius === null) return <View style= backgroundColor: transparent >...</View>

else return <View style= borderRadius: radius, backgroundColor: 'red' >...</View>;
;

To do this precisely, you would need to know the size that the string would take up once rendered. I wasn't able to find a React Native API for this (and I'm assuming you couldn't either), but I know both Android and iOS have such APIs. So therefore the solution would be to create a native module (https://facebook.github.io/react-native/docs/native-modules-android.html) for in iOS and Android which exposes a method called "measureText" or something. Then in each native class you'd use the corresponding API:

• Android API mentioned in this answer should work: https://stackoverflow.com/a/7329169/3930970
  


• iOS API: https://developer.apple.com/documentation/foundation/nsstring/1531844-sizewithattributes
  

I haven't actually tried this so I'm curious if something like this ends up working. Cheers!

You can use the lifecycle method componentWillMount() to calculate the border radius:

componentWillMount() 
 const height = Dimensions.get('window');
 radius = height / 2;

This method is only called one time, which is before the initial
render. Since this method is called before render().

And then, you can style your button using the calculated radius:

<TouchableOpacity
 style=[styles.button, borderRadius: radius ]
 onPress=() => alert('Hello World!') 
>

Here is a working demo.