The open-source library react-native-gesture-handler
is a great way to add gestures to cross-platform React Native apps. Two of the main reasons I find this useful because it uses native support to handle gestures and the other reason being it is better at performing on each native platform than React Native’s built-in touch system Gesture Responder system.
Setting up react-native-gesture-handler
yarn add react-native-gesture-handler
OR
npm install react-native-gesture-handler
Go to following link to see installation process
https://www.npmjs.com/package/react-native-gesture-handler
For iOS users, navigate inside ios/
directory from the terminal and run pod install
.
Everything is set up, all you have to do is run the build command again, such as for iOS: react-native run-ios
and for Android: react-native run-android
.
Set up App component to display an image
In this section, let us quickly set up the App
component to display a placeholder image. You can use any image as a placeholder. Here is the snippet for the App.js
file to get started.
import React from 'react'
import { Image, View, Dimensions } from 'react-native'
const { width } = Dimensions.get('window')
const App = () => {
return (
<View style={{ flex: 1, justifyContent: 'center', alignItems: 'center' }}>
<Image
source={{
uri: 'picsum.photos/536/354'
}}
style={{
width: width,
height: 300
}}
resizeMode="contain"
/>
</View>
)
}
export default App
It uses the width
of the device’s screen to calculate the width of the image using Dimensions
from react-native
. To run this demo for the first time build the app for the platform you are using:
- for iOS, run:
react-native run-ios
- for Android, run:
react-native run-android
Using dynamic Image component with Animated API
Animated.Image
is going to serve the purpose of displaying an image as well as perform scale animations.
Animated
API uses declarative relationships between input and output values. For single values, you can use Animated.Value()
. It is required since it is going to be a style property initially.
Start by importing Animated
from react-native
and replace the Image
with Animated.Image
.
import { View, Dimensions, Animated } from 'react-native'
// in return statement
return (
<View style={{ flex: 1, justifyContent: 'center', alignItems: 'center' }}>
<Animated.Image
source={{
uri: 'https://miro.medium.com/max/1080/1*7SYuZvH2pZnM0H79V4ttPg.jpeg'
}}
style={{
width: width,
height: 300,
transform: [{ scale: 1 }]
}}
resizeMode="contain"
/>
</View>
)
Also, by mentioning the value of the scale
to one, it is going to display the image as usual.
Now, wrap the Animated.Image
with PinchGestureHandler
. Ths wrapper component is going to have two props.
return (
<View style={{ flex: 1, justifyContent: 'center', alignItems: 'center' }}>
<PinchGestureHandler
onGestureEvent={this.onZoomEvent}
onHandlerStateChange={this.onZoomStateChange}
>
<Animated.Image
source={{
uri: 'https://miro.medium.com/max/1080/1*7SYuZvH2pZnM0H79V4ttPg.jpeg'
}}
style={{
width: width,
height: 300,
transform: [{ scale: this.scale }]
}}
resizeMode="contain"
/>
</PinchGestureHandler>
</View>
)
Adding Animated event and state change handler
Let us define the onZoomEvent
first, before the return
statement. This event is going to be an Animated event. This way gestures can directly map to animated values. The animated value to be used here is scale
.
Passing useNativeDriver
as boolean true allows the animations to happen on the native thread instead of JavaScript thread. This helps with performance.
scale = new Animated.Value(1)
onZoomEvent = Animated.event(
[
{
nativeEvent: { scale: this.scale }
}
],
{
useNativeDriver: true
}
)
Now define the handler method onZoomStateChange
that handles the state change when the gesture is over. Each gesture handler has is assigned a state that changes when a new touch event occurs.
There are different possible states for every handler but for the current gesture handler, ACTIVE
is used to check whether the event is still active or not. To access these states, the object is required to import from the library itself.
The Animated.spring
on scale
property has toValue
set to 1
which is the initial scale value when the animation is done.
onZoomStateChange = event => {
if (event.nativeEvent.oldState === State.ACTIVE) {
Animated.spring(this.scale, {
toValue: 1,
useNativeDriver: true
}).start()
}
}