63 lines
1.6 KiB
Markdown
63 lines
1.6 KiB
Markdown
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---
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title: Optimize Re-Renders with shouldComponentUpdate
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---
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## Optimize Re-Renders with shouldComponentUpdate
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## Hint:
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Check to see if the value of ```nextProps``` is even.
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## Solution:
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For this solution, you will use an ```if/then``` statement to check whether the value of ```nextProps``` is even. ```nextProps``` differs from ```props``` in that it is a value that has not been rendered in the UI yet so in the ```shouldComponentUpdate()``` method, you are essentially asking permission to update the UI with the ```nextProps``` value.
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```jsx
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class OnlyEvens extends React.Component {
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constructor(props) {
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super(props);
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}
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shouldComponentUpdate(nextProps, nextState) {
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console.log('Should I update?');
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// change code below this line
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if (nextProps.value % 2 == 0) {
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return true;
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}
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return false;
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// change code above this line
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}
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componentWillReceiveProps(nextProps) {
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console.log('Receiving new props...');
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}
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componentDidUpdate() {
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console.log('Component re-rendered.');
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}
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render() {
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return <h1>{this.props.value}</h1>
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}
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};
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class Controller extends React.Component {
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constructor(props) {
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super(props);
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this.state = {
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value: 0
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};
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this.addValue = this.addValue.bind(this);
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}
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addValue() {
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this.setState({
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value: this.state.value + 1
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});
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}
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render() {
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return (
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<div>
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<button onClick={this.addValue}>Add</button>
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<OnlyEvens value={this.state.value}/>
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</div>
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);
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}
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};
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```
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