问题
I am adding a React Native Slider component to an existing app, and prefer to utilize its onValueChange prop so that as the user slides the knob left and right, a couple of Text components have their values updated in response to the Slider's current value.
On a first attempt this causes a significant amount of lag after letting go of the slider knob. StackOverflow isn't letting me embed the gif here so I'll leave the link instead: http://imgur.com/sEowpZt
When I comment out onValueChange and use onSlidingComplete instead, there is no lag at all, but the two text components will not have their values updated until the sliding stops, which reduces the effect. My suspicion is that the setState calls inside onValueChange are piling up at a faster rate than than at which they are being processed/completed and it has something to do with other parts of my app. To confirm this, I created a new react native app via react-native init, added that same _renderNameYourPriceModal() to the code, included a button on the screen to open it, and discovered there is no lag at all: http://imgur.com/iZnvDML
How can I determine what is slowing down the setState calls for my existing app? I discovered the React Perf tool and am using it like this to print in the Chrome dev console a summary table.
import Perf from 'react-native/Libraries/Renderer/shims/ReactPerf'
...
componentDidMount() {
setTimeout(() => {
Perf.start()
setTimeout(() => {
Perf.stop()
const measurements = Perf.getLastMeasurements()
Perf.printWasted(measurements)
}, 7000)
}, 1000)
}
According to the Perf docs for printWasted:
printWasted()
Perf.printWasted(measurements)The most useful part of the profiler.
"Wasted" time is spent on components that didn't actually render anything, e.g. the render stayed the same, so the DOM wasn't touched.
But I'm not sure how to interpret the results to make the necessary changes. For example, the first four rows of that table when running my app (which has 24 rows in total) looks like this:
I don't know which View the first line, "ItemDetailsScreen > View" inside the Owner > Component column, is referring to because there are 20+ Views on that screen alone. For further context, I'm using React-Navigation and this is a nested screen inside a StackNavigator, although I don't see how updates to this screen's state could cause re-renders in screens further up the hierarchy. Is it necessary to break this screen down further into more custom sub-components so that I override shouldComponentUpdate, or so that the printWasted results tell exactly which areas are at fault?
Here is the function I have for returning the Modal with the Slider inside:
_renderNameYourPriceModal() {
var likelihood = 'Possible'
var lowestVal = 5
var highestVal = 15
if (this.state.nypValue < 6) {
likelihood = 'Nearly Impossible'
} else if (this.state.nypValue < 8) {
likelihood = 'Highly Unlikely'
} else if (this.state.nypValue < 10) {
likelihood = 'Unlikely'
}
return (
<Modal
onRequestClose={() => { this.setState({nypModalVisible: false})}}
animationType={"fade"}
transparent={true}
visible={this.state.nypModalVisible}>
<View style={{paddingTop: 22, height: Dimensions.get('window').height, backgroundColor: 'rgba(252,84,102,0.9)', alignItems: 'center', justifyContent: 'center'}}>
<View
style={{
height: Dimensions.get('window').height * 0.5,
width: Dimensions.get('window').width * 0.9,
backgroundColor: 'white',
borderRadius: 10,
alignItems: 'center'
}}>
<View
style={{flex: 0.8, alignItems: 'center', justifyContent: 'center'}}>
<View style={{flex: 0.25, flexDirection: 'row', width: Dimensions.get('window').width * 0.97, top: -10, alignItems: 'flex-start', justifyContent: 'center'}}>
<View style={{flex: 0.1}}></View>
<View style={{flex: 0.8, alignSelf: 'center', alignItems: 'center', justifyContent: 'center'}}>
<Text style={{fontSize: 23}}>Name Your Price</Text>
</View>
<View style={{flex: 0.1, top: -5, height: 40, alignItems: 'flex-end', justifyContent: 'flex-end'}}>
<TouchableHighlight
underlayColor={'gray'}
style={{height: 40, width: 40, backgroundColor: 'gray', borderRadius: 20, alignItems: 'center', justifyContent: 'center'}}
onPress={() => {
// close
this.setState({nypModalVisible: false})
}}>
<Text style={{fontSize: 25, color: 'white'}}>X</Text>
</TouchableHighlight>
</View>
</View>
<View style={{flex: 0.25, width: Dimensions.get('window').width * 0.8, alignItems: 'center', justifyContent: 'flex-start'}}>
<View style={{flex: 0.5, flexDirection: 'row', alignItems: 'center', justifyContent: 'center'}}>
<View style={{flex: 0.2, alignItems: 'center', justifyContent: 'center'}}>
<Text style={{fontSize: 19}}>${lowestVal.toFixed(2)}</Text>
</View>
<View style={{flex: 0.6, alignItems: 'center', justifyContent: 'center'}}>
<Slider
style={{width: Dimensions.get('window').width * 0.5}}
maximumValue={15}
minimumValue={5}
step={0.5}
value={this.state.nypValue}
// onSlidingComplete={(val) => {
// this.setState({nypValue: val})
// }}
onValueChange={(val) => {
// change value here
this.setState({nypValue: val})
}}
/>
</View>
<View style={{flex: 0.2, alignItems: 'center', justifyContent: 'center'}}>
<Text style={{fontSize: 19}}>${highestVal.toFixed(2)}</Text>
</View>
</View>
<Text>${this.state.nypValue.toFixed(2)}</Text>
<Text>Likelihood: {likelihood}</Text>
</View>
<View style={{flex: 0.5, paddingTop: 20, alignItems: 'center', justifyContent: 'flex-start', paddingHorizontal: 10}}>
<Text style={{textAlign: 'center', top: 25, fontSize: 18}}>Let us know the price you'd like to see this item drop to, and we'll let YOU know when it does!</Text>
</View>
</View>
<View style={{flex: 0.2, alignItems: 'center', justifyContent: 'center'}}>
<TouchableHighlight
style={{height: 50, width: Dimensions.get('window').width * 0.8, alignItems: 'center', justifyContent: 'center', backgroundColor: '#70a8ff', borderRadius: 5}}
underlayColor={'#70a8ff'}
onPress={() => { }}>
<Text style={{fontSize: 20, color: 'white'}}>Set Price Alert</Text>
</TouchableHighlight>
</View>
</View>
</View>
</Modal>
)
}
来源：https://stackoverflow.com/questions/45805426/how-to-determine-js-bottlenecks-in-react-native-code