Imagine I have some "page" component, which needs to ask for data from a server. The data it requests will depend on whether or not the current user is authenticated. Further, in the event of a login, the page will want to reload the data. My question is, how can I accomplish something like this using HOCs rather than inheritance?
To illustrate the problem, I'll demonstrate a solution using inheritance. The program will have the following objects. I'll leave out the boilerplate code.
session: an EventEmitter that emits start when the session changes (either a login or a log out).
Page: the superclass that all pages inherit from
MyPage: the subclass of Page in this example
API: will be an API class for retrieving data from the server
Here's the code:
// Page superclass
class Page extends React.Component {
componentWillMount() {
session.on("start", this.loadData);
this.loadData();
}
loadData() {
// this method is overwritten in subclasses
}
}
// MyPage subclass
class MyPage extends Page {
loadData() {
if(session.isAuthenticated()) {
API.loadPrivateData();
} else {
API.loadPublicData();
}
}
}
Here's a solution that uses an HOC, but seems less elegant than inheritance. It still requires that every "subclass" page have a method loadData, and it requires that method to be called in every "subclass's" componentWillMount.
// Page HOC
function Page(WrappedComponent) {
return class EnhancedPage extends React.Component {
componentWillMount() {
session.on("start", this.loadData);
// this._page.loadData() will fail here
// since this._page is undefined until rendering finishes
}
loadData() {
this._page.loadData();
}
render() {
return <WrappedComponent {...props} ref={(e) => { this._page = e; }} />
}
}
}
// MyPage
class MyPage extends React.Component {
componentWillMount() {
this.loadData();
}
loadData() {
if(session.isAuthenticated()) {
API.loadPrivateData();
} else {
API.loadPublicData();
}
}
}
const component = Page(MyPage)
// what would make sense here is to have a method something like
// const component = Page(MyPage, () => MyPage.loadData())
// but then the MyPage.loadData logic would need to be defined
// elsewhere
This pattern will happen often: I'll want to load some data, then reload when the session changes. I'd like to understand the "react" way of accomplishing the same.
EDIT: I am not trying to pass a username or "loggedIn" flag through the HOC. That is to say something like <WrappedComponent isLoggedIn={session.isAuthenticated()} {...props} /> won't cut it here. Tying the API logic to props requires that I check for changes in MyPage.componentWillUpdate().
When using a HOC you shouldn't place the loadData function on the wrapped component. Instead pass the function as a parameter to the HOC constructor.
Something like this might work for you. The sessionHoc function takes a callback function which'll be called every time the session state changes. Its result will be passed to WrappedComponent as a data prop.
function sessionHoc(onSessionChange) {
return function (WrappedComponent) {
return class extends React.Component {
constructor(props) {
super(props);
this.state = {
data: null,
};
session.on('start', this.handleSessionChange.bind(this));
}
handleSessionChange() {
this.setState({
data: onSessionChange(),
});
}
render() {
return <WrappedComponent data={data} {...this.props} />
}
};
};
}
class MyPage extends React.Component {
render() {
// Just access this.props.data here!
}
}
const EnhancedPage = sessionHoc(function () {
if (session.isAuthenticated()) {
return API.loadPrivateData();
} else {
return API.loadPublicData();
}
})(MyPage);
Hopefully this helped! :)
Related
Could you please have a look on the following code. I need to get some value from another class. This works asynchronous, so I provided a function handleGameDeserialization.
The function gets the right value (as I tested with the alert), however the setState function has no impact. Could that be a "this-context" issue?
export default class GameplayScreen extends React.Component {
constructor(props) {
super(props);
this.fbGame = new FBGame();
global.currentScreenIndex = 'Gameplay';
this.state = {
currentGame: 'N/A'
}
// this.handleGameDeserialization = this.handleGameDeserialization.bind(this);
if (this.props.route.params != null) {
this.gameKey = this.props.route.params.gameKey;
this.game = this.fbGame.deserializeGame(this.gameKey, this.handleGameDeserialization);
}
}
handleGameDeserialization = (game) => {
// alert('yeah'+game); // here comes the expected output
this.setState({
currentGame: game
});
}
render() {
return (
<View>
<Text>{this.state.currentGame}</Text>
</View>
/*<Board game={this.state.game}/>*/
)
}
}
I call that function when the component GameplayScreen is navigated to. As you can see above, there is a class FBGame, which does the deserialization (read the game from firebase database)
export default class FBGame {
...
deserializeGame(key, handleGameDeserialization) {
var gameRef = firebase.database().ref("games/"+key).child("serialized");
gameRef.on("value", snapshot => {
//console.log('deserialized: ' + );
handleGameDeserialization(snapshot.val().serialized);
});
}
...
}
edit:
When I use componentDidMount like below, it works fine. But this seems to be an anti-pattern. I still don't understand, why it doesn't work, when callded in the constructor and how I am supposed to solve this.
componentDidMount() {
this.game = this.fbGame.deserializeGame(this.gameKey, this.handleGameDeserialization);
}
For things like subscriptions that will update the state and other side-effects, you should put the logic out in componentDidMount() which will fire immediately after the component is mounted and won’t give you any trouble if you update the state inside of it.
You can't but things that call this.setState in the constructor.
When you open reactjs.org, under "Declarative" header, there is a sentence: React will efficiently update and render just the right components when your data changes.
For a couple of my apps, I'm using the following structure:
App
| AppContainer(all the app logic, protected before login)
| Login(Login form)
This structure works well if you return 2 different components inside App's render, according to the user's credentials.
render(){
if(isUserLoggedIn()){
return <AppContainer />;
}
return <Login />;
}
Inside the Login component, I'm refreshing the page with window.location.reload so the App's render will be triggered, and I'll get the AppContainer component.
But it feels a little like jQuery + Angular. Is there a better (more React) way to trigger render function, or is this how things should be?
Is there a better(more React) way to trigger render function...
The usual way is to have state, in this case at minimum a boolean for whether the user is logged in, and update that state when the user logs in successfully or logs out. Updating state triggers rendering.
In your case, since you're using Redux, you'd probably have your state there.
I don't use Redux (yet?), this is vaguely what it would look like without, roughly (if you're using a class component as you seem to be):
class App extends Component {
constructor(props) {
super(props);
this.state = {
loggedIn: /*...initial value, perhaps from web storage or cookie...*/;
};
this.onLogin = this.onLogin.bind(this);
this.onLogout = this.onLogout.bind(this);
}
onLogin() {
// ...probably stuff here, then:
this.setState({loggedIn: true});
}
onLogout() {
// ...probably stuff here, then:
this.setState({loggedIn: false});
}
render() {
if (this.state.logedIn) {
return <AppComponent onLogout={this.onLogout}/>;
}
return <Login onLogin={this.onLogin}/>;
}
}
or with hooks:
const App = () => {
const [loggedIn, setLoggedIn] = useState(/*...initial value, perhaps from web storage or cookie...*/);
const onLogin = useCallback(() => {
// ...probably stuff here, then:
setLoggedIn(true);
}, [loggedIn]);
const onLogout = useCallback(() => {
// ...probably stuff here, then:
setLoggedIn(false);
}, [loggedIn]);
if (this.state.logedIn) {
return <AppComponent onLogout={onLogout}/>;
}
return <Login onLogin={onLogin}/>;
}
(again, roughly)
If you need to update the component state, then you can pass an observable and listen for changes or use some state management library.
Here is one possible solution:
Create observable class
declare type IObserverHandler = (event: any) => void;
export class Observable {
private observers: IObserverHandler[] = [];
public subscribe(observer: IObserverHandler) {
if (!this.observers.includes(observer)) {
this.observers.push(observer);
}
}
public unsubscribe(observer: IObserverHandler) {
this.observers = this.observers.filter(o => o !== observer);
}
public publish(event: any) {
for (const observer of this.observers) {
observer(event);
}
}
}
Create Login class that will publish events on actions such as login or logout
class Login extends Observable {
public login() {
this.publish({ value: true });
}
public logout() {
this.publish({ value: false });
}
}
In component subscribe to observer and update component state using event value
export abstract class Component extends React.Component<any, any> {
private observer: IObserverHandler;
private observable: Login;
constructor(props: any) {
super(props);
this.observable = this.props.observable;
this.state = { isAuthenticated: false }
this.observer = (event) => {
this.setState({ isAuthenticated: event.value })
}
this.observable.subscribe(this.observer);
}
componentWillUnmount() {
this.observable.unsubscribe(this.observer);
}
}
You can use useNavigate and navigate to the same url you are on. For example, instead of window.location.reload(), you can say navigate("/...your current url....")
window.location.reload() is not the best option everytime. It works on localhost, but for example on when you deploy it to the internet by using services such as "Netlify", it can can cause "not found url" error
Creating some extra state and tracking them for re-rendering your page might unnecessarily complicate your code.
I have a scenario where I'm trying to update a React/Redux state from a function that's placed on the Window. The function on the window is unable to access the function that's in the React component. Any idea how to bind that function in this kind of setup? This snippet just has a console log where the Redux call would go.
class MyComponent extends Component {
updateRedux = a => {
console.log(a)
}
componentDidMount() {
window.windowFunction = function(a) {
this.updateRedux(a)
}
}
render() {
return (
<Stuff />
)
}
}
this is not accessible inside your function, you need to bind it.
Try with:
class MyComponent extends Component {
updateRedux = a => {
console.log(a)
}
componentDidMount() {
window.windowFunction = function(a) {
this.updateRedux(a)
}.bind(this)
}
render() {
return (
<Stuff />
)
}
}
if you meant that you want to update Redux state with some action (this is the only way to update Redux state by design), then you need to make this action and its functions available to your Component with connect(mapStateToProps, mapDispatchToProps)(Component)
One of the comments above about converting the windowFunction to an arrow function resolved the issue. Thanks!
class MyComponent extends Component {
updateRedux = a => {
console.log(a)
}
componentDidMount() {
window.windowFunction = a => {
this.updateRedux(a)
}.bind(this)
}
render() {
return (
<Stuff />
)
}
}
What you could do is separate the concerns using a presenter and a connected
component, using react-redux. I am assuming you know of this library, comment
if you need more details.
// Simple "presenter", the getComponentData is used to get the data for the
// redux store.
class MyComponentPresenter extends Component {
// returns data for redux
getComponentData () {}
componentDidMount() {
this.props.updateRedux(this); // update Redux
}
render() {
return (
<Stuff />
)
}
}
// This component has the exact same interface, but comes with a updateRedux
// props which automatically dispatches an action
export const MyComponent = connect(null, {
updateRedux(componentInstance) {
return {
type: "updateRedux"
};
}
});
// in the reducer
//
function reducer (state, action) {
switch (action.type) {
case "updateRedux":
return ...
}
}
No more need for globally available function (which in your example is redefined for each instance of MyComponents which is probably not what you want).
I want to call a promise based function before dispatching an action to the store.
The problem is that I only want to call the function when the component is going to be displayed. I use a toggle action that turns the component on and off.
Here is a sample of my code:
if ( /*component is going to be displayed*/) {
init().then(function() {
store.dispatch(toggleSomething());
});
}
else {
store.dispatch(toggleSomething());
}
Action:
export const SomethingActions = {
TOGGLE_SOMETHING: 'TOGGLE_SOMETHING'
};
export function toggleSomething() {
return {
type: SomethingActions.TOGGLE_SOMETHING
};
}
Reducer:
export default function somethingState(state = defaultState, action) {
switch (action.type) {
case somethingActions.TOGGLE_SOMETHING
return Object.assign({}, state, { open: !state.open});
default:
return state;
}
}
part of the React component:
Something.propTypes = {
display: React.PropTypes.bool.isRequired
};
function mapStateToProps(state, ownProps) {
return {
display: state.something.open
};
}
I basically want to know the value of open/display of the component above or another way to know whether the component is being displayed or not.
I don't want to pollute the render function or store a bool that changes every time I call dispatch.
Is there a way to do that?
By the sounds of it, you'd want to take advantage of React's lifecycle methods. Particularly the componentWillMount and componentWillReceiveProps.
componentWillReceiveProps does not get triggered for the initial render, so you may want to extract out the logic into a separate function so that it can be reused for both hooks:
function trigger(isDisplayed) {
if (isDisplayed) {
init().then(function() {
store.dispatch(toggleSomething());
});
}
else {
store.dispatch(toggleSomething());
}
}
componentWillMount() {
trigger(this.props.display);
}
componentWillReceiveProps(nextProps) {
trigger(nextProps.display);
}
Q1: "The problem is that I only want to call the function when the component is going to be displayed"
A1: This is definitely a problem for react lifecycle methods, in particular, componentWillMount() & componentDidMount()
Q2: "I basically want to know the value of open/display of the component above or another way to know whether the component is being displayed or not."
A2: The componentDidMount() method will be called when the component is rendered. To prevent an infinite loop where the component calls your promise on render just to call the promise again when the state changes, avoid including the toggled state in your component. Dispatch actions on component mounting that toggle the state in the store, but don't use this state in this component. This way you know whether the component is rendered without having the UI update. I hope that helps!
import React from 'react';
class StackOverFlow extends React.Component {
constructor(props) {
super(props);
}
componentDidMount() {
toggleSomethingOn();
}
componentWillUnmount() {
toggleSomethingOff();
}
render() {
return (
<div>
The component has been rendered!
<br />
</div>
);
}
}
function toggleSomethingOn() {
//dispatches action to toggle state "open"
}
function toggleSomethingOff() {
//dispatches action to toggle state "closed"
}
export default StackOverFlow;
A2: If you are just looking to find out if a component has been rendered (outside of your code) you could go to your browser's developer tools and search the elements/DOM for your component html.
I am using React v0.14.8. I tried to call the fetchData function from another component. Here is my code:
export default class TagUtils extends React.Component {
deleteTag = () => {
Tags.deleteTag(this.props.tag).then(function(response){
if(response.message === 'tag successfully deleted')
Sidebar.fetchData();
});
}
// other codes
And:
export default class Sidebar extends React.Component {
fetchData() {
Tags.getTags().done((response) => {
this.setState({tags: response.tags || [], loaded: true});
});
}
//other codes
When I called deleteTag, I got this error in my console:
TypeError: _SidebarJsx2.default.fetchData is not a function
You can't call Sidebar.fetchData because fetchData is not a static member of Sidebar, it is an instance member. This means you need an instance of Sidebar to call fetchData on, for example new Sidebar().fetchData(). Of course, this is not how a React component is supposed to be used, and it would not set state on all other Sidebar instances, so it wouldn't be meaningful.
What you want to do is pass a callback to your TagUtils component:
export default class TagUtils extends React.Component {
deleteTag = () => {
Tags.deleteTag(this.props.tag).then((response) => {
if(response.message === 'tag successfully deleted')
this.props.onDeleteTag();
});
}
}
export default class Sidebar extends React.Component {
fetchData() {
Tags.getTags().done((response) => {
this.setState({tags: response.tags || [], loaded: true});
});
}
render() {
return (
{ this.state.tags.map((tag) =>
<TagUtils tag={tag} onDeleteTag={this.fetchData} />) }
);
}
}
If you have to thread this callback through several layers of components that's okay, that's typical in React. However, if you find yourself passing a lot of stuff down props through many component layers that seem out of place, or trying to reconcile changes across large horizontal spaces in your app, this is a primary use-case for things like Flux and Redux.