You can use control logic (if statements) to specify whether or not a component should be rendered.
function ConditionalPanel(props) {
//assign element to show to variable
let thingToRender = null; //null element will not render
if(conditionOne){ //based on props or state
thingToRender = <OptionA />
} else if(conditionTwo) {
thingToRender = <OptionB />
} else if(conditionThree) {
return null; //show nothing!
}
//keep return statement as simple as possible!
return (<div>{thingToRender}</div>);
}
function ConditionPanel(props) {
//can use inline expressions via shortcutting. Use with caution
return (
<div>
{conditionOne == true && <OptionA />}
</div>
)
}
We add user interaction in React the same way as with the DOM: by listening for events and executing callback functions when they occur.
function MyButton() {
//A function that will be called when clicked
//The name is conventional, but arbitrary.
//The callback will be passed the DOM event as usual
const handleClick = function(event) {
console.log("clicky clicky");
}
//make a button with an `onClick` attribute!
//this "registers" the listener and sets the callback
return <button onClick={handleClick}>Click me!</button>;
}
special React prop
can only put listeners on HTML
elements, not Components!
function MyButton(props) {
//A function that will be called when clicked
//The name is conventional, but arbitrary.
//The callback will be passed the DOM event as usual
const handleClick = (event) => {
console.log("clicky clicky");
}
//make a button with an `onClick` attribute!
//this "registers" the listener and sets the callback
return <button onClick={handleClick}>Click me!</button>;
}
In addition to the props, React components can also track their internal state. This keeps track of information about the Component that may change due to user interaction.
State is reserved only for interactivity, that is, data that changes over time
Some examples of state data:
//The current "state"
const state = {
data: [ {}, {}, {} ],
...
}
//define presentation - lots of these kinds of functions
function renderData() {
//render all the data
//...
}
//define user interaction
button.addEventListener('click', function() {
//MODIFY THE STATE
state.data[i] = ...;
//CLEAR OLD VIEW AND RE-RENDER CONTENT
document.querySelector('#main').innerHTML = '';
renderData(); //RE-RENDER CONTENT
})
changeable data lives out here
1. modify the state data
2. re-render the view
2. re-render the view
On button click, do 2 things:
You add state to a component by using a state hook. The hook defines a "state variable" which will retain its value across Component function calls, as well as a function to update that variable.
//import the state hook function `useState()` to define state
import React, { useState } from 'react';
function CountingButton(props) {
const [count, setCount] = useState(0);
const handleClick = (event) => {
setCount(count+1); //update the state to be a new value
//and RE-RENDER the Component!
}
return (
<button onClick={handleClick}>Clicked {count} times</button>
);
}
state variable
update function
initial value for variable
In order to write correct React (that can be understood
and debugged by you and others), you need to follow the naming conventions:
React state is changed asynchronously.
Calling a "state-setter" function automatically re-renders the Component (by calling the function again).
function CountingButton(props) {
const [count, setCount] = useState(3) //initial value of 3
console.log(count); //will have "current" value of state
//3 first render, 4 after clicking
const handleClick = (event) => {
setCount(4); //request to change `count` to 4 AND re-render!
console.log(count); //will output "3";
//state has not changed yet!
}
return (
<button onClick={handleClick}>Clicked {count} times</button>
);
}
Because state changes are asynchronous, you can only "see" them after the component has re-rendered. Use console logs at the "rendering" step to debug
function CountingButton(props) {
const [count, setCount] = useState(3) //initial value of 3
console.log("DEBUG: count", count); //debug! variable here,
//after re-render
const handleClick = (event) => {
setCount(count + 1); //incremenet count AND re-render!
//do not debug variable here!
}
return (
<button onClick={handleClick}>Clicked {count} times</button>
);
}
Components can (and often do) contain multiple state variables.
//Example from React documentation
function ExampleWithManyStates(props) {
//Declare multiple state variables!
const [age, setAge] = useState(42);
const [fruit, setFruit] = useState('banana');
const [todos, setTodos] = useState([{ text: 'Learn Hooks' }]);
//...
}
state variable is an array of objects!
State variables will only be updated if a different value is passed to the setter function. For arrays and objects, pass a copy of the element with an updated element or property.
function TodoListWithError(props) {
//a state value that is an array of objects
const [todos, setTodos] = useState([{ text: 'Learn Hooks' }]);
const handleClick = (event) => {
todos[0].text = "Fix bugs"; //modify the object
//but don't make a new one
setTodos(todos) //This won't work! Not "changing"
}
//...
}
function TodoList(props) {
//a state value that is an array of objects
const [todos, setTodos] = useState([{ text: 'Learn Hooks' }]);
const handleClick = (event) => {
//create a copy of the array using the `map()` function
const todosCopy = todos.map((todoObject, index) => {
const objCopy = {...todoObject}; //copy object as well
if(index == 0) { //transform objects if needed
objCopy.text = "Fix bugs"
}
return objCopy; //return object to go into new array
})
setTodos(todosCopy) //This works!
}
//...
}
props are for information that doesn’t change from the Component’s perspective, including “initial” data. state is for information that will change, usually due to user interaction (see React FAQ).
props are for information that doesn’t change from the Component’s perspective, including “initial” data. state is for information that will change, usually due to user interaction (see React FAQ).
To access the
value
in an
<input>
, save that value in the
state
(and update it
onChange
). This is called a controlled form.
use DOM event to refer to which input element
function MyInput(props) {
const [inputValue, setInputValue] = useState('')
const handleChange = (event) => {
let newValue = event.target.value
setInputValue(newValue);
}
return (
<div>
<input type="text" onChange={handleChange} value={inputValue} />
You typed: {value}
</div>);
)
}
function App(props) {
const [data, setData] = useState([]);
const addItemToData = (newItem) => {
const newData = [...data, newItem]; //copy via spread
setData(newData); //update state
}
return (
<FruitButton callback={addItemToData} text={"Apple"} />
<FruitButton callback={addItemToData} text={"Banana"} />
)
}
function FruitButton(props) {
//what to do when clicked
const handleClick = (event) => {
//call given callback, passing in given text
props.callback(props.text);
}
return (
<button onClick={handleClick}>{props.text}</button>
)
}
To allow child components to "update" the parent's state, pass them a callback function as a prop.
Style Guide: do not pass a state setter function directly.
If multiple components rely on the same
data (variable), you should "lift up" that
state
to a shared parent, who can pass the information back down as
props
.
ChildA
ChildB
Parent
Has Data
(state)
Needs Data
<ChildA data={data} />
Has Data (prop)
Has Data (prop)
<ChildA data={data} />
Has Data (state)
Read/Review Ch 16-17: React & Interactive React
Problem Set 07 due Wednesday May 17
Get it done this week/weekend!
Project Draft 2 due Sunday May 21
Add one interactive feature
Next time: more interactive React!