Develop Reference

React: Tell child component to "reinitialize," even when the passed props are the same

I have a MyComponent that renders a Timer component. My current setup is like this:
MyComponent.render:
render () {
return <Timer time={this.state.time} lag={this.lag || 0} />
}
Timer:
class Timer extends Component {
constructor(props) {
super(props);
this.state = {
time: this.props.time,
};
}
startTimer = (duration) => {
if (duration > 0){
this.on = true;
let timer = duration * 1000 + this.props.lag;
var s = setInterval(() => {
this.setState({time: Math.round(timer/1000)});
timer = timer - 500;
if (timer <= 0) {
this.on = false;
clearInterval(s);
}
}, 500);
}
}
componentDidMount = () => {
this.startTimer(this.props.time);
}
render() {
return (
<div className="Timer-container">
<div className="Timer-value">{this.state.time}</div>
</div>
);
}
}
As you can see, when the Timer is initialized, it immediately starts counting down. On subsequent renders of MyComponent, I want to restart the Timer, even if the time prop doesn't change. In other words, I want it to "reinitialize" on every render. How do I achieve this?

First of all, to reset the counter, you need to store something in the state,
either the interval (so you can clear it)
or the current time (so you can set it to the initial value).
As you want to do something if the parent re-rendered (but the props didn't change), basically what you need to check is why your component updated. An answer to that would be "Trace why a React component is re-rendering"
A quick way for your example would be to check if the state has changed (not recommended):
componentDidUpdate(prevProps, prevState, snapshot) {
if( prevState === this.state ){
clearInterval( this.state.interval );
this.startTimer( this.props.time );
}
}
Another quick solution would be (if it is an option for you) to pass a shouldRerender property to the component, and then check for this property inside the component:
// -- inside MyComponent
render () {
return <Timer
time={ this.state.time }
lag={ this.lag || 0 }
shouldRerender={ {/* just an empty object */} } />;
}
// -- inside Timer
componentDidUpdate(prevProps, prevState, snapshot) {
if( prevProps.shouldRerender !== this.props.shouldRerender ){
clearInterval( this.state.interval );
this.startTimer( this.props.time );
}
}
That looks a bit "dirty" to me. A cleaner way would be to pass some state to shouldRerender, which changes on every update (e.g. just an increasing number).
However, I think the approach to check if parent rendered is not the React way. I, personally, do consider if a component renders or not an implementation detail (I don't know if that's correct to say), that is, I don't care when React decides to render, I only care for props and state (basically).
I would recommend to think about what actually is "cause and effect", what is the reason why you want to reset the timer. Probably the re-render of the parent is only the effect of some other cause, which you might be able to use for your time reset, too.
Here some different concepts that might be useful for use cases I can imagine:
not use one Time instance, but destroy and create inside parent when needed, maybe also using a key prop.
use a HOC (like withTimer) or custom hook (like useTimer), injecting a reset() function (plus create a separate TimerView component)
keep the time state in MyComponent, passing time and onChange down to the Timer component (<Timer time={ this.state.time } onChange={ time => { this.setState({ time: time }); } } />), then both MyComponent and Timer can set / reset the time.

How to change the state from two different function call?

How can i modify state from two different function call? Following code gives me error'Maximum update depth exceeded.'
class App extends Component {
// fires before component is mounted
constructor(props) {
// makes this refer to this component
super(props);
// set local state
this.state = {
date: new Date(),
myQuestions:myQuestion,
counter :0,
activeQuestion:-1,
};
// This binding is necessary to make `this` work in the callback
this.handleClick = this.handleClick.bind(this);
this.handleClick = this.begin.bind(this)
}
begin(){
this.setState({activeQuestion:1})
}
handleClick() {
if(this.state.activeQuestion <= myQuestion.length){
this.setState(prevState => ({
counter: this.state.counter + 1,
activeQuestion:this.state.activeQuestion+1
}));
}
render() {
return (
<div className="App">
<div id = "myQuiz">
<div class ="intro {{ (activeQuestion > -1)? 'inactive':'active' }}">
<h2>Welcome</h2>
<p>Click begin to test your knowledge</p>
<p class = "btn" onClick={this.begin('begin')}>Begin</p>
</div>
<div>
<button onClick={this.handleClick}></button>
</div>
What will be the right way to change state from different function call?

You are not passing function to cllick handler. Instead you are calling the function like
onClick={this.begin('begin')}
This line is causing an infinite loop because calling this function is updating the state, which in turn is calling the render function. Change this to
onClick={this.begin}
If you want to pass the parameter to handler then
onClick={() => this.begin('begin')}

You have multiple issues in your code.
You override this.handleClick with the bound version of begin() in your constructor and this.begin will still not be bound to this.
Where does myQuestion come from in the constructor? Is it a globally scoped variable?
In your handleClick() you do not use the prevState to calculate the next state. Instead you use this.state which may lead to bad behavior if react batches multiple calls to setState().
Why do you use an onClick handler on a paragraph? Shouldn't this be a button?
You have to use className instead of class in jsx because class is a reserved keyword in javascript.
Also the class you are trying to set will be literally the text intro {{ (activeQuestion > -1)? 'inactive':'active' }}. This is for sure not what you where trying to achieve.

Proper usage of the extendability of "this" in React component

We could briefly describe that this.props is data flow from parent component and this.state is for keeping the current state of the component, and the mechanism we massively depend when we develop in React is re-rendering after setState().
If my understanding of the usage of these two are not wrong,
except holding function object, is it proper to utilize the extendability of this to hold some values considered as global variables?
For example, if I want to make 'swipe' manner available on my component, I may could do something like:
class Slider extends React.Component {
constructor(props) {
super(props);
this.state = {
movement: 0,
touchStartX: 0,
prevTouchX: 0,
beingTouched: false
};
this.handleTouchStart = this.handleTouchStart.bind(this);
this.handleTouchMove = this.handleTouchMove.bind(this);
this.handleTouchEnd = this.handleTouchEnd.bind(this);
}
handleTouchStart(e) {
this.setState({
touchStartX: e.targetTouches[0].clientX,
beingTouched: true
});
}
handleTouchMove(e) {
if (this.state.beingTouched) {
let deltaX = e.targetTouches[0].clientX - this.state.touchStartX;
this.setState({
movement: deltaX,
prevTouchX: e.targetTouches[0].clientX
});
}
}
handleTouchEnd(e) {
// handle the sliding and set state touchStartX and beingTouched to 0 and false.
}
render() {
return (<div className = 'sliderBox'
onTouchStart = {e => this.handleTouchStart(e)}
onTouchMove = {e => this.handleTouchMove(e)}
onTouchEnd = {e => this.handleTouchEnd(e)}></div>);
}
}
export default Slider;
This is a part of my built application, it just works well. But I still wonder if it's a good way to use state property.
Or it's just OK to do something like:
class Slider extends React.Component {
constructor(props) {
super(props);
this.movement = 0;
this.touchStartX = 0;
this.prevTouchX = 0;
this.beingTouched = false;
this.handleTouchStart = this.handleTouchStart.bind(this);
this.handleTouchMove = this.handleTouchMove.bind(this);
this.handleTouchEnd = this.handleTouchEnd.bind(this);
}
handleTouchStart(e) {
this.touchStartX = e.targetTouches[0].clientX;
this.beingTouched = true;
}
handleTouchMove(e) {
if (this.beingTouched) {
let deltaX = e.targetTouches[0].clientX - this.state.touchStartX;
this.movement = deltaX;
this.prevTouchX = e.targetTouches[0].clientX;
}
}
handleTouchEnd(e) {
// handle the sliding and set state touchStartX and beingTouched to 0 and false.
}
render() {
return (<div className = 'sliderBox'
onTouchStart = {e => this.handleTouchStart(e)}
onTouchMove = {e => this.handleTouchMove(e)}
onTouchEnd = {e => this.handleTouchEnd(e)}></div>);
}
}
export default Slider;
But it seems that the utilization of the extendability of this above is rarely seen?
Sorry if my question is meaningless, I just wonder if is there any spirit or principle to utilize the extendability of this? Props and cons?

Yes, you can attach variables directly to the component's this. It's proper in your use case.
In React's Documentation itself, in the state and lifecycle section, it gives an example of storing a timer id directly in this:
componentDidMount() {
this.timerID = setInterval(
() => this.tick(),
1000
);
}
Note how we save the timer ID right on this.
While this.props is set up by React itself and this.state has a special meaning, you are free to add additional fields to the class manually if you need to store something that is not used for the visual output.
If you don’t use something in render(), it shouldn’t be in the state.
We will tear down the timer in the componentWillUnmount() lifecycle hook:
componentWillUnmount() {
clearInterval(this.timerID);
}
And as of Gleb Kost's answer, I agree that it's a normal practice.
You've nailed it. If it doesn't make sense to be in props, neither in state, feel free to attach it directly to this, if it also makes sense.

It seems that extendability of this in the react component is a normal practice, I've seen it being used in almost every more or less complex React project I have worked on.
As for deciding where to put the data in the state or on the component itself, I usually ask myself a question: does the component need to react to the change of those properties? If yes, they are going in the state, if not - on the component.
In your case, since you are using those properties only in event handlers and don't really need the component to re-render every time they change, I'd say optimal would be to utilise this, as you do.
On the other hand, if you want to use beingTouched property in the render method, for example, to change the background color of the component when it's touched than you need to put it in the state, otherwise the component would not react as expected, because it will be unaware that the property has changed.

Create function that can pass a parameter without making a new component

My question is to do with the issue React has for binding functions in the render function.
The following is not good practice:
render() {
<div onClick={this.callFunction.bind(this)}/>
}
as each re render would add a new function to the page, eventually causing the browser to run out of memory.
The solution is to do this:
constructor() {
this.callFunction = this.callFunction.bind(this);
}
render() {
<div onClick={this.callFunction}/>
}
The problem with this is when I want to pass a value into the function.
I know I can make the div a child component, and pass the parameter in through the callBack, but this does not seem sensible if the div is only being used once in the whole application. I accept I could make this work, but this is not the scope of this question.
I also know that this solution:
render() {
<div onClick={() => this.callFunction.call(this, param)}/>
}
Is no better, as it is still creating a new function.
So my question is, how can I create a function that I can pass a parameter into without making a new component, and without binding a new funciton on each render?

You can't avoid creating a second component as you need to pass a function reference as an event handler, this will be executed by the browser when the event triggers.
So the problem is not the binding but the fact that you need to pass a reference, and references can't receive parameters.
EDIT
By the way, if you don't like the syntax and noise of binding or anonymous arrow functions you can use currying.
I posted an example in a different question if you find it interesting. this won't solve the problem though, it's just another approach to pass a new reference (which i find it to be the most terse)

You can change the declaration of callFunction to be an arrow function, which implictly binds the scope, like so:
callFunction = () => {
console.log('hi');
};
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions
Then your original render function would work as expected!

Use Side effect
Side effect is something that a function use that comes from outside but not as argument. Now this mechanism is majorly used in Redux/Flux where the entire state is stored in a Store and every component fetches their state from it.
class MyComponent extends React.Component {
constructor(props) {
super(props);
this.state = {
handlerProps: {
onClick: { count: 0},
onChange: { count: 0}
}
}
}
onClickHandler = () => {
const state = this.state.handlerProps.onClick;
console.log('onClick', state.count);
}
onChangeHandler = (value) => {
const state = this.state.handlerProps.onChange;
console.log('onClick', state.count);
this.setState({value: value})
}
buttonClick = () => {
const random = Math.ceil(Math.random()* 10) % 2;
const handler = ['onClick', 'onChange'][random];
const state = this.state.handlerProps;
state[handler].count++;
console.log('Changing for event: ', handler);
this.setState({handlerProps: state});
}
render () {
return (
<div>
<input onClick={this.onClickHandler} onChange={this.onChangeHandler} />
<button onClick={ this.buttonClick }>Update Props</button>
</div>
)
}
}
ReactDOM.render(<MyComponent/>, document.querySelector('.content'))
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.0.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.0.0/react-dom.min.js"></script>
<div class='content' />

The only way I know of is to create a new React Component which takes the value and the event handler as props.
This way, the handler as a function remains static, and since the value is passed down separately (in its own prop) you don't have any functions being re-instanciated. Because you don't bind anything nor create a new function each time.
Here's an example:
We have two buttons. The first one prints the current state variable value and the other increments it by one.
Normally, if we had done this with onClick={() => this.print(this.state.value)} we would get a new instance of this function, each time the MyApp component would re-render. In this case, it would re-render each time we increment the value with the setState() inside this.increment.
However, in this example, no new instance of this.print happens because we are only passing its reference to the button. In other words, no fat arrow and no binding.
In the <Button /> component, we have a <button> to which event handler we pass a reference to a function - just like we did in <MyApp />. However, here we know exactly what to pass to the function. As such, we have myHandler trigger this.props.handler(this.props.value).
class MyApp extends React.Component {
constructor() {
super();
this.state = {
value: 0
};
}
print = (value) => {
console.log(value);
}
increment = () => {
// This will trigger a re-render, but none of the functions will be reinstanciated!
this.setState((prevState) => ({value: prevState.value + 1}));
}
render() {
// Note that both handlers below are passed just the references to functions. No "bind" and no fat arrow.
return(
<div>
<Button handler={this.print} value={this.state.value}>Print</Button>
<button onClick={this.increment}>Increment</button>
</div>
);
}
}
class Button extends React.Component {
// Clicking the button will trigger this function, which in turn triggers the function with the known argument - both of which were passed down via props.
myHandler = () => this.props.handler(this.props.value);
render() {
// Note again that the handler below is just given the reference to a function. Again, not "bind" nor fat arrow.
return(
<button onClick={this.myHandler}>{this.props.children}</button>
);
}
}
ReactDOM.render(<MyApp />, document.getElementById("app"));
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react-dom.min.js"></script>
<div id="app"></div>
Though quite tedious, it is an effective solution. That being said, even if you do create a new function each time you render, the performance implications are minimal. From the official docs:
The problem with this syntax is that a different callback is created each time the LoggingButton renders. In most cases, this is fine.

Refreshing a setInterval in React

This is based on the answer given
here:
I'm having trouble resetting a setInterval.
As of now the following works. I have a prop called mediaList which contains an object array of images. When changeActiveMedia is called, the object position is moved to the next one in the list.
class MyComponent extends React.Component {
constructor(props) {
super(props);
this.state = { activeMediaIndex: 0 };
}
componentDidMount() {
setInterval(this.changeActiveMedia.bind(this), 5000);
}
changeActiveMedia() {
const mediaListLength = this.props.mediaList.length;
let nextMediaIndex = this.state.activeMediaIndex + 1;
if(nextMediaIndex >= mediaListLength) {
nextMediaIndex = 0;
}
this.setState({ activeMediaIndex:nextMediaIndex });
}
renderSlideshow(){
const singlePhoto = this.props.mediaList[this.state.activeMediaIndex];
return(
<div>
<img src={singlePhoto.url} />
</div>
);
}
render(){
return(
<div>
{this.renderSlideshow()}
</div>
)
}
}
My problem arises here.
I have logic that can change the number of objects in the list, mediaList.
This becomes a problem because since the interval only updates once every 5000 seconds, if the nextMediaIndex within that time is 2, and then I all of a sudden update the mediaList to have only 1 item, I run into an error since mediaList[2] would not exist.
So my question is, is there a way to RESET and CLEAR the setInterval whenever this.props.mediaList is updated?

window.setInterval returns an id which identifies an Interval timer. You can use it in conjunction with clearInterval to cancel the interval.
this.interval = setInterval(...);
...
clearInterval(this.interval);
you can use componentWillReceiveProps as kind of a generic method of checking to see if mediaList has changed. for example:
componentWillReceiveProps(nextProps) {
if (nextProps.mediaList !== this.props.mediaList) {
clearInterval(this.interval);
}
}