76 lines
2.1 KiB
Markdown
76 lines
2.1 KiB
Markdown
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---
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title: Component State
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---
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## Component State
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In `Class` components, there is a way to store and manage state built in to React Native.
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```javascript
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class App extends Component {
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constructor () {
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super();
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this.state = {
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counter: 0
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};
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}
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incrementCount () {
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this.setState({
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counter: this.state.counter + 1
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});
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}
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decrementCount () {
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this.setState({
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counter: this.state.counter - 1
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});
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}
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render () {
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return (
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<View>
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<Text>Count: {this.state.counter}</Text>
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<Button onPress={this.decrementCount.bind(this)}>-</Button>
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<Button onPress={this.incrementCount.bind(this)}>+</Button>
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</View>
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);
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}
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}
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```
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State is similar to props, but it is private and fully controlled by the component. Here, the `constructor()` method is calling the parent class' constructor with `super();` - **`Component`** is the parent class of `App` because we are using the `extends` keyword. The `constructor()` method also initializes the component's state object:
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```
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this.state = {
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counter: 0
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};
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```
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The state can be displayed within the component:
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```js
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{this.state.counter}
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```
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Or updated by calling:
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```js
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this.setState({});
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```
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**Note:** Aside from its initial creation in your component's `constructor()` method, you should never directly modify the component's state with `this.state = `. You must use `this.setState` as can be seen in the `incrementCount` and `decrementCount` functions above.
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The count is incremented and decremented by calling the functions passed to the `onPress` handlers just like they would be if you called a click handler from JavaScript on the web.
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*ASIDE: In the first example, `<Button>` is a custom component; it's a combination of `<TouchableOpacity>` and `<Text>` from the React Native API:*
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```js
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const Button = ({ onPress, children, buttonProps, textProps }) => {
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const { buttonStyle, textStyle } = styles;
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return (
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<TouchableOpacity onPress={onPress} style={[buttonStyle, buttonProps]}>
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<Text style={[textStyle, textProps]}>
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{children}
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</Text>
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</TouchableOpacity>
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);
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};
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```
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