问题
I want to check if there are more than one screens are on stack when device back button is hit. If yes, I want to show previous screen and if no, I want to exit app.
I have checked number of examples but those use BackAndroid and Navigator. But both of them are deprecated. BackHandler is replacement for BackAndroid. And I can show previous screen by using props.navigation.goBack(null).
But I am unable to find code for finding screen count in stack. I don't want to use deprecated Navigator!
回答1:
This example will show you back navigation which is expected generally in most of the flows. You will have to add following code to every screen depending on expected behavior. There are 2 cases: 1. If there are more than 1 screen on stack, device back button will show previous screen. 2. If there is only 1 screen on stack, device back button will exit app.
Case 1: Show previous screen
import { BackHandler } from 'react-native';
constructor(props) {
super(props)
this.handleBackButtonClick = this.handleBackButtonClick.bind(this);
}
componentWillMount() {
BackHandler.addEventListener('hardwareBackPress', this.handleBackButtonClick);
}
componentWillUnmount() {
BackHandler.removeEventListener('hardwareBackPress', this.handleBackButtonClick);
}
handleBackButtonClick() {
this.props.navigation.goBack(null);
return true;
}
Important: Don't forget to bind method in constructor and to remove listener in componentWillUnmount.
Case 2: Exit App
In this case, no need to handle anything on that screen where you want to exit app.
Important: This should be only screen on stack.
回答2:
In a case where there are more than one screens stacked in the stack, the default back button behavior in react-native is to navigate back to the previous screen in the stack. Handling the device back button press when having only one screen to exit the app requires a custom setting. Yet this can be achieved without having to add back handling code to each and every screen by modifying the getStateForAction method of the particular StackNavigator's router.
Suppose you have the following StackNavigator used in the application
const ScreenStack = StackNavigator(
{
'Screen1': {
screen: Screen1
},
'Screen2': {
screen: Screen2
},
},
{
initialRouteName: 'Screen1'
}
);
The getStateForAction method of the stack navigator's router can be modified as follows to achieve the expected back behavior.
const defaultStackGetStateForAction =
ScreenStack.router.getStateForAction;
ScreenStack.router.getStateForAction = (action, state) => {
if(state.index === 0 && action.type === NavigationActions.BACK){
BackHandler.exitApp();
return null;
}
return defaultStackGetStateForAction(action, state);
};
the state.index becomes 0 only when there is one screen in the stack.
回答3:
try this react navigation
componentDidMount() {
BackHandler.addEventListener('hardwareBackPress', this.handleBackButton);
}
handleBackButton = () => {
const pushAction = StackActions.push({
routeName: 'DefaultSelections',
});
this.props.navigation.dispatch(pushAction);
}
current screen is "DefaultSelections" , on back button press, would be shifted on to the same and hence back button disabled work around, as disabling back button by
return true
for backButton ( as suggested by the official docs ) disables back button on all screens ; not wanted
回答4:
I am on v0.46.0 of react-native and had the same issue. I tracked the issue down to this file in the react-native code base
https://github.com/facebook/react-native/blob/master/Libraries/Utilities/BackHandler.android.js#L25
When running with the chrome debugger turned off the line
var subscriptions = Array.from(_backPressSubscriptions.values()).reverse()
always returns an empty array for subscriptions which in turn causes the invokeDefault variable to stay true and the .exitApp() function to be called.
After more investigation, I think the issue was discovered and discussed in the following PR #15182.
Even after copy/pasting the PR change in an older version of RN it did not work most likely caused by the issue described in the PR.
After some very slight modifications I got it working by changing to
RCTDeviceEventEmitter.addListener(DEVICE_BACK_EVENT, function() {
var invokeDefault = true;
var subscriptions = []
_backPressSubscriptions.forEach(sub => subscriptions.push(sub))
for (var i = 0; i < subscriptions.reverse().length; ++i) {
if (subscriptions[i]()) {
invokeDefault = false;
break;
}
}
if (invokeDefault) {
BackHandler.exitApp();
}
});
Simply using a .forEach which was the original implementation on the PR before the amended Array.from syntax works throughout.
So you could fork react-native and use a modified version, submit a PR though I imagine that will take a little while to be approved and merged upstream, or you can do something similar to what I did which was to override the RCTDeviceEventEmitter.addListener(...) for the hardwareBackPress event.
// other imports
import { BackHandler, DeviceEventEmitter } from 'react-native'
class MyApp extends Component {
constructor(props) {
super(props)
this.backPressSubscriptions = new Set()
}
componentDidMount = () => {
DeviceEventEmitter.removeAllListeners('hardwareBackPress')
DeviceEventEmitter.addListener('hardwareBackPress', () => {
let invokeDefault = true
const subscriptions = []
this.backPressSubscriptions.forEach(sub => subscriptions.push(sub))
for (let i = 0; i < subscriptions.reverse().length; i += 1) {
if (subscriptions[i]()) {
invokeDefault = false
break
}
}
if (invokeDefault) {
BackHandler.exitApp()
}
})
this.backPressSubscriptions.add(this.handleHardwareBack)
}
componentWillUnmount = () => {
DeviceEventEmitter.removeAllListeners('hardwareBackPress')
this.backPressSubscriptions.clear()
}
handleHardwareBack = () => { /* do your thing */ }
render() { return <YourApp /> }
}
回答5:
constructor(props){
super(props)
this.onBackPress = this.onBackPress.bind(this);
}
componentWillMount() {
BackHandler.addEventListener('hardwareBackPress', this.onBackPress);
}
componentWillUnmount(){
BackHandler.removeEventListener('hardwareBackPress', this.onBackPress);
}
onBackPress(){
const {dispatch, nav} = this.props;
if (nav.index < 0) {
return false;
}
dispatch(NavigationActions.back());
return true;
}
render(){
const {dispatch, nav} = this.props;
return(
<DrawerRouter
navigation= {
addNavigationHelpers({
dispatch,
state: nav,
addListener,
})
}
/>
);
}
回答6:
Here is how I implemented successfully using certain condition:
componentWillMount() {
BackHandler.addEventListener(
'hardwareBackPress',
this.handleBackButtonClick,
);
}
componentWillUnmount() {
BackHandler.removeEventListener(
'hardwareBackPress',
this.handleBackButtonClick,
);
}
handleBackButtonClick = () => {
//some condition
if (this.state.isSearchBarActive) {
this.setState({
isSearchBarActive: false,
});
this.props.navigation.goBack(null);
return true;
}
return false;
};
回答7:
I used flux for navigation.
const RouterComp = () => {
let backLoginScene=false;
return (
<Router
backAndroidHandler={() => {
const back_button_prohibited = ['login','userInfo','dashboard'];
if (back_button_prohibited.includes(Actions.currentScene) ) {
if (backLoginScene == false) {
ToastAndroid.show("Click back again to exit.", ToastAndroid.SHORT);
backLoginScene = !backLoginScene;
setTimeout(() => {
backLoginScene = false;
}, 2000);
return true;
} else {
backLoginScene = false;
BackHandler.exitApp();
}
return false;
}}}>
<Scene key='root' hideNavBar>
<Scene key='guest' hideNavBar >
<Scene key='login' component={Login} ></Scene>
<Scene key='userInfo' component={UserInfo}></Scene>
</Scene>
<Scene key='user' hideNavBar>
<Scene key='dashboard' component={Dashboard} title='Dashboard' initial />
<Scene key='newAd' component={NewAd} title='New Ad' />
</Scene>
</Scene>
</Router>
)
}
export default RouterComp;
来源：https://stackoverflow.com/questions/45031085/react-native-device-back-button-handling