I have a component with render and onPress methods described below...
onCardPressed(event) {
console.log(this.props);
const { data } = this.props;
console.log(event, data);
}
render() {
const { data } = this.props;
return (
<TouchableOpacity
onPress={this.onCardPressed}
>
<Container style={{ elevation: 5 }}>
<SectionTitle>
This is a
{` ${data.city} `}
card
</SectionTitle>
</Container>
</TouchableOpacity>
);
}
In this example, the card will properly display This is a London card, but in the onPress method this.props returns undefined.
How can I access the this.props object for evaluation?
You can fix this by two way. The argument in favour of adding these lines to the constructor is so that the new bound functions are only created once per instance of the class. You could also use
onPress={this.onCardPressed.bind(this)}
or (ES6):
onPress={() => this.onCardPressed()}
Related
i have broken my head trying to understand a problem with my code. I'm new with React Native so there may be a standard behavior that i am unaware of. This is my problem:
In my component i have a useEffect() to load my data like "componentDidMount":
useEffect( () => {
async function loadDadosLista(){
let listaRecebida = await getListaByID(route.params.idLista);
setLista(listaRecebida);
};
loadDadosLista();
}, []);
My function works correctly, the function getListaById accesses my realm.db and return my object lista. After that I can access the data and associate it with components of type TextInput. My real problem is that any change the i do in any component using properties of lista, overwrites all data leaving only the one that has been modified. I'm using spread operator but apparently it doesn't work. Below is my complete code for better understanding.
function ListConfig(){
const [lista, setLista] = useState({});
useEffect( () => {
async function loadDadosLista(){
let listaRecebida = await getListaByID(route.params.idLista);
setLista(listaRecebida);
};
loadDadosLista();
}, []);
return(
<View style={styles.container}>
<View style={[styles.containerLinha, styles.linha2]}>
<View style={styles.inputLocal}>
<TextInput
name='estabelecimento'
placeholder='Venda do seu Francisco'
placeholderTextColor={theme.colors.cinzaPrimario}
style={styles.textInputLocal(theme)}
value={lista.estabelecimento}
maxLength={25}
onChangeText={ (value) => {
setLista({
...lista,
estabelecimento: value
})
}}
textAlignVertical='bottom'
/>
<IconLocation width={20} height={24} />
</View>
</View>
<View style={styles.containerNotif}>
<Text style={styles.textoNotif(theme)}>
Me notifique 20 minutos antes
</Text>
<ToggleSwitch
isOn={lista.notificacaoAtiva}
onColor={theme.colors.cinzaSecundario}
offColor={theme.colors.cinzaSecundario}
thumbOnStyle={{
backgroundColor: theme.colors.destaque
}}
size="medium"
onToggle={(isOn) => {
setLista({
...lista,
notificacaoAtiva: isOn
});
}}
/>
</View>
</View>
);
}
export default ListConfig;
My object lista have this properties:
{
estabelecimento: 'nameOfEstabelecimento',
notificacaoAtiva: true
}
I am doing a loan calculation app and i run into the trouble since i am new to react native and previously i have been manipulating the DOM using querySelector or getElementById functions. However this does not work in react, and i am using state to store the value from the user, but i just can't seem to get it right, What am i doing wrong?
I've inserted the calculation element that is later rendered in app.js. All elements are showing up with no error, but the problem is to get user input data and then be able to use that data and do calculations.
Here is my Class
class LanKalkylElement extends React.Component {
constructor(props) {
super(props);
this.state = {
loanAmount: 20000,
loanInterest: 2.5,
loanYear: 10,
};
}
changeAmount(loanAmount) {
this.setState(() => {
return {
loanAmount: parseFloat(loanAmount),
};
});
}
changeInterest(loanInterest) {
this.setState(() => {
return {
loanInterest: parseFloat(loanInterest),
};
});
}
changeYear(loanYear) {
this.setState(() => {
return {
loanYear: parseFloat(loanYear),
};
});
}
calcButton() {
Alert.alert(this.props.loanAmount);
}
buttonHomeFunc() {
this.props.navigation.navigate('Start');
}
render() {
const {loanAmount, loanInterest, loanYear} = this.state;
return(
<View style={styles.contentStyle}>
<Text style={styles.text}> Lånebelopp </Text>
<TextInput style={styles.numericInput}
onBlur={Keyboard.dismiss}
keyboardType={'numeric'}
value={loanAmount}
onValueChange={this.changeAmount.bind(this)} />
<Text style={styles.text}> Ränta </Text>
<TextInput style={styles.numericInput}
onBlur={Keyboard.dismiss}
keyboardType={'numeric'}
value={loanInterest}
onValueChange={this.changeInterest.bind(this)} />
<Text style={styles.text}> Antal år: {String(loanYear)}</Text>
<Slider step={1}
maximumValue={15}
value={loanYear}
onValueChange={this.changeYear.bind(this)} />
<Button title='Kalkylera' onPress={() => this.calcButton()}/>
<Text style={styles.textResult}>Total summa att återbetala:</Text>
<Text style={styles.textResult}>varav räntekostnad:</Text>
<Button title='Tillbaka' onPress={() => this.buttonHomeFunc()}/>
</View>
)
}
}
export default withNavigation(LanKalkylElement);
When a user changes a value in a text input, onValueChange is called. You have bound this prop to functions that modify the state for this component.
This means the value in the text input will always match the value in the state. Therefore, if you need to access the value in a text input you would simply retrieve it from the state, like this:
const loanAmount = this.state.loanAmount;
doSomethingWithLoanAmount(loanAmount);
I have two screens, one list (Flatlist) and one filter screen where I want to be able to set some filters for the list. the list screen has the states "data" and "usedFilters". When I am switching to the filters screen, the states are set as navigation parameters for react navigation and then passed via navigation.navigate, together with the onChange function, as props to the filter screen. There they are read, and the filters screen class' state is set (usually with passed filters from the list screen, if no valid filters has been passed, some are initialized).
After that the filters can be changed. If that happens, the state of the filter screen gets updated.
If then the apply button is clicked the filter screens' state is passed to the onChange function and via that back to the list screen, the onChange function updates the state "usedFilters" state of the list screen. If the cancel button is pressed null is passed to the onChange function and there is no setState call.
Setting new states for the list screen works perfectly fine. the problem is, that when i press the cancel button (or the back button automatically rendered by react navigation) the changes are kept nevertheless. That only happens if the state has been changed before. So if there has never been applied a change and hence the "usedFitlers" state of the list screen is null, this behavior does not occur. Only if I already made some changes and hence the "usedFitlers" state of the list screen has a valid value which is passed to the filters screen the cancel or go back buttons won't work as expected.
I am using expo-cli 3 and tried on my android smartphone as well as the iOS simulator. Same behavior. I looked into it with chrome dev tools as well but i simply couldn't figure out where the "usedFitlers" state was updated.
I am using react native 0.60 and react navigation 3.11.0
My best guess is that for some reason the two states share the same memory or one is pointer to the other or sth like that. (Had problems like that with python some time ago, not knowing the it uses pointers when assigning variables).
Anyone got an idea?
List Screen:
export default class ListScreen extends React.Component {
state = { data: [], usedFilters: null };
static navigationOptions = ({ navigation }) => {
let data = navigation.getParam('data')
let changefilter = navigation.getParam('changeFilter')
let currfilter = navigation.getParam('currFilter')
return {
headerTitle:
<Text style={Styles.headerTitle}>{strings('List')}</Text>,
headerRight: (
<TouchableOpacity
onPress={() => navigation.navigate('FilterScreen', {
dataset: data, onChange: changefilter, activeFilters:
currfilter })} >
<View paddingRight={16}>
<Icon name="settings" size={24} color=
{Colors.headerTintColor} />
</View>
</TouchableOpacity>
),
};
};
_onChangeFilter = (newFilter) => {
if (newFilter) {
this.setState({ usedFilters: newFilter })
this.props.navigation.setParams({ currFilter: newFilter });
} // added for debugging reasons
else {
this.forceUpdate();
let a = this.state.usedFilters;
}
}
_fetchData() {
this.setState({ data: fakedata.results },
() => this.props.navigation.setParams({ data: fakedata.results,
changeFilter: this._onChangeFilter }));
}
componentDidMount() {
this._fetchData();
}
render() {
return (
<ScrollView>
<FlatList/>
// Just data rendering, no problems here
</ScrollView>
);
}
}
Filter Screen:
export default class FilterScreen extends React.Component {
static navigationOptions = () => {
return {
headerTitle: <Text style={Styles.headerTitle}> {strings('filter')}
</Text>
};
};
state = { currentFilters: null }
_onChange = (filter, idx) => {
let tmp = this.state.currentFilters;
tmp[idx] = filter;
this.setState({ currentFilters: tmp })
}
_initFilterElems() {
const filters = this.props.navigation.getParam('activeFilters');
const dataset = this.props.navigation.getParam('dataset');
let filterA = [];
let filterB = [];
let filterC = [];
if (filters) {
// so some checks
} else {
// init filters
}
const filterElements = [filterA, filterB, filterC];
this.setState({ currentFilters: filterElements })
}
componentDidMount() {
this._initFilterElems()
}
render() {
const onChange = this.props.navigation.getParam('onChange');
return (
<ScrollView style={Styles.screenView}>
<FlatList
data={this.state.currentFilters} // Listeneinträge
keyExtractor={(item, index) => 'key' + index}
renderItem={({ item, index }) => (
<FilterCategory filter={item} name={filterNames[index]}
idx={index} onChange={this._onChange} />
)}
ItemSeparatorComponent={() => <View style=
{Styles.listSeperator} />}
/>
<View style={Layout.twoHorizontalButtons}>
<TouchableOpacity onPress={() => {
onChange(this.state.currentFilters);
this.setState({ currentFilters: null });
this.props.navigation.goBack();
}}>
<View style={Styles.smallButton}>
<Text style={Styles.buttonText}>{strings('apply')} </Text>
</View>
</TouchableOpacity>
<TouchableOpacity onPress={() => {
onChange(null);
this.setState({ currentFilters: null });
this.props.navigation.goBack();
}}>
<View style={Styles.smallButton}>
<Text style={Styles.buttonText}>{strings('cancel')}
</Text>
</View>
</TouchableOpacity>
</View>
</ScrollView >
);
}
}
So when I press the cancel button, null is returned to the _onChangeFilter function of the list screen. This part works, and according to console.log and the debugger, the setState is not called. But if i set a breakpoint within the else part, i can see that this.state.usedFilters has changed.
Ok after a while i figured it out. The problem was that the whole filters list was always just referenced since react native (js) seems to always use references, even when changing sub-parts of the lists.
fixed that by using lodash cloneDeep.
The question is almost similar to this one :
touchableopacity onpress function undefined (is not a function) React Native
But the problem is, I am getting the error despite the fact that I have bind the function. Here is my TouchableOpacity component:
<TouchableOpacity style={styles.eachChannelViewStyle} onPress={() => this.setModalVisible(true)}>
{item.item.thumbnail ?
<Image style={styles.everyVideoChannelThumbnailStyle} source={{uri: item.item.thumbnail}} />
: <ActivityIndicator style= {styles.loadingButton} size="large" color="#0000ff" />}
<Text numberOfLines={2} style={styles.everyVideoChannelVideoNameStyle}>
{item.item.title}
</Text>
</TouchableOpacity>
And this is my setModalVisible function:
setModalVisible(visible) {
console.error(" I am in set modal section ")
this.setState({youtubeModalVisible: visible});
}
Also, I have bind the function in constructor as follows:
this.setModalVisible = this.setModalVisible.bind(this);
But, I am still getting same error that undefined is not a function. Any help regarding this error?
The render method and your custom method must be under the same scope. In code below I have demonstrated the same. I hope you will modify your code accordingly as I assume you got the gist :)
class Demo extends Component {
onButtonPress() {
console.log("click");
}
render() {
return (
<View>
<TouchableOpacity onPress={this.onButtonPress.bind(this)}>
<Text> Click Me </Text>
</TouchableOpacity >
<View>
);
}
}
Alternatively binding method in constructor will also work
class Demo extends Component {
constructor(props){
super(props);
this.onButtonPress= this.onButtonPress.bind(this);
}
onButtonPress() {
console.log("click");
}
render() {
return (
<View>
<TouchableOpacity onPress={this.onButtonPress()}>
<Text> Click Me </Text>
</TouchableOpacity >
<View>
);
}
}
I'm not sure if this will help but I write my functions this way and haven't encountered this problem.
If I were you I'd try binding the function in the place where you declare it.
setModalVisible = (visible) => {
this.setState({ youtubeModalVisible: visible });
}
If you do this, you don't have to bind in the constructor.
constructor(props) {
...
// Comment this out to see it will still bind.
// this.setModalVisible = this.setModalVisible.bind(this);
...
}
Lastly, if this function will only set the modal's state to visible, you might want to remove the argument and pass it this way.
<TouchableOpacity style={styles.eachChannelViewStyle} onPress={this.setModalVisible}>
...
</TouchableOpacity>
// Refactored function declaration would look like this
setModalVisible = () => {
this.setState({ youtubeModalVisible: true });
}
I'm creating an APP that get some data from fetch function. No problem here. The array has the data correctly. I'm doing it like this:
constructor(){
super()
this.state = {
fetching: false,
api: []
}
}
componentWillMount(){
this.setState({ fetching: true })
api.getMonuments().then(res => {
this.setState({
api: res,
fetching: false
})
})
}
I got this data: an array of 4 objects
Then I want to pass that data to another scene. I'm doing it like this:
<View style={styles.contentContainer}>
<TouchableHighlight
style={[styles.button, {marginBottom:0}]}
onPress={() => navigate('Monumento', this.state.api)}
underlayColor='#000'
>
<View style={styles.buttonContent}>
<Animatable.Text
style={styles.buttonText}
animation="bounceInLeft"
duration={1500}
>
Click here!
</Animatable.Text>
</View>
</TouchableHighlight>
</View>
On the other scene I get that data with the navigation.state.params but the problem now is that there is no more an array with 4 objects in it, but instead there is an object that have 4 objects in it...if I console log the data that is what's appears
render(){
const api = this.props.navigation.state.params;
console.log('API:', api)
...
Now I want to use the map function but I can't because 'api' is not a function...How can I workaround this?
render(){
var api={"bar":"nihao"};
return(
<View>
{Object.entries(api).map(([key,v])=>{
return <View key={key}><Text>{v}</Text></View>
})}
</View>
)
}
api is a single object not array.
api is a array.
render(){
var api=[{"bar":"nihao"},{"bar":"nihao2"},{"bar":"nihao3"}];
return(
<View>
{api.map((v,index)=>{
return <View key={index}><Text>{v.bar}</Text></View>
})}
</View>
)
}
You can use Object.entries with RN for mapping the key/value pairs of an object. Eg:
const api = { 'foo': 'bar', 'foz': 'baz'};
...
render() {
return (
Object.entries(api).map(([key, value]) => {
return <View key={key}>{value}</View>
});
)
}
The issue is you are accessing params object, but what you want is api array. I guess you are using react navigation. If so, then your call to navigate function should be like this:
navigate('Monumento', {api: this.state.api}).
And you can retrieve it like this:
this.props.navigation.state.params.api.
Navigate function takes screen name and params object.
Read this: https://reactnavigation.org/docs/navigators/navigation-prop#navigate-Link-to-other-screens