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Context

作用：給整個(gè)組件樹共享全局的數(shù)據(jù)

最適合的場(chǎng)景：雜亂無章的組件都需要同一些數(shù)據(jù)

如果單純?yōu)榱瞬粚訉觽鬟f屬性的話， Context 是不合適的，因?yàn)?Context 會(huì)弱化及污染組件的純度，導(dǎo)致組件復(fù)用性降低

Context API

Context.Provider 數(shù)據(jù)提供者
通過 React.createContext 創(chuàng)建的一個(gè)上下文對(duì)象里的組件，Provider 組件中可以插入其他組件，來訂閱這個(gè) Context ；
通過 Provider 的 value 將數(shù)據(jù)傳遞給 Consumer 組件；
Context.Consumer 數(shù)據(jù)消費(fèi)者
訂閱 Context 的變更；
組件會(huì)找離自己最近的 Provider，獲取其 value；
value 變化，插入 Provider 的組件都會(huì)重新渲染；
沒有匹配到 Provider，使用默認(rèn)值；其他情況均不使用默認(rèn)參數(shù)；

新舊 value 值對(duì)比，用的是和 Object.is 相同的算法

案例1：更改主題

/* Header.css */
.header{
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  height: 50px;
  line-height: 50px;
  background-color: black;
  color: #fff;
}
.header.red{
  background-color: red;
}

.header.pink{
  background-color: pink;
}

// context.js
const ThemeContext = React.createContext('black')

export {
  ThemeContext
}

// Header.jsx
import "./Header.css";
import { ThemeContext } from "./context";

export default class Header extends React.Component {
  render() {
    return (
      <ThemeContext.Consumer>
        {(theme) => (
          <header className={`header ${theme}`}>{this.props.children}</header>
        )}
      </ThemeContext.Consumer>
    );
  }
}

// index.jsx
import Header from "./context_comps/Header";
import { ThemeContext } from "./context_comps/context";

class App extends React.Component {
  state = {
    theme: "black",
  };
  render() {
    return (
      <div>
        <ThemeContext.Provider value={this.state.theme}>
          <Header>This is a title</Header>
        </ThemeContext.Provider>
      </div>
    );
  }
}

案例2：使用組合方式解決數(shù)據(jù)傳遞問題

class App extends React.Component {
  state = {
    headerTitle: "This is a title",
    cityInfo: {
      name: "shanghai",
      text: "上海",
    },
    cityData: [
      {
        name: "beijing",
        text: "北京",
      },
      {
        name: "shanghai",
        text: "上海",
      },
      {
        name: "guangzhou",
        text: "廣州",
      },
    ],
  };
  changeCity(cityInfo) {
    this.setState({
      cityInfo,
    });
  }
  render() {
    return (
      <>
        <Header
          text={this.state.headerTitle}
          citySelector={
            <Selector
              cityData={this.state.cityData}
              cityInfo={this.state.cityInfo}
              changeCity={this.changeCity.bind(this)}
            />
          }
        ></Header>
        <span>{this.state.cityInfo.text}</span>
      </>
    );
  }
}

class Header extends React.Component {
  render() {
    return (
      <header>
        <h1>{this.props.text}</h1>
        <div>{this.props.citySelector}</div>
      </header>
    );
  }
}

class Selector extends React.Component {
  render() {
    return (
      <select
        defaultValue={this.props.cityInfo.name}
        onChange={(e) => this.props.changeCity({
          name: e.target.value,
          text: e.target[e.target.selectedIndex].text
        })}
      >
        {this.props.cityData.map((item, index) => {
          return <option value={item.name} key={index}>{item.text}</option>;
        })}
      </select>
    );
  }
}

contextType

class 的靜態(tài)屬性
contextType -> React.createContext() -> Context 對(duì)象
this -> context -> CityContext
在生命周期函數(shù)和 render 函數(shù)中都可以訪問
語義不太好

static contextType = CityContext;

componentDidMount() {
  console.log(this.context);
}

render() {
  console.log(this.context);
}

案例3

// index.jsx
import Home from "./views/Home";
import { btnStyle } from "./config";
import { BtnStyeContext, LoginStatusContext } from "./context_comps/context";

class App extends React.Component {
  state = {
    style: btnStyle.success,
    loginStatus: false,
  };

  doClick(e) {
    console.log(e.target.textContent);
  }

  login() {
    this.setState({
      loginStatus: !this.state.loginStatus,
    });
  }
  
  render() {
    return (
      <div>
        <BtnStyeContext.Provider
          value={{
            style: this.state.style,
            doClick: this.doClick,
          }}
        >
          <LoginStatusContext.Provider
            value={{
              status: this.state.loginStatus,
              login: this.login.bind(this),
            }}
          >
            <Home />
          </LoginStatusContext.Provider>
        </BtnStyeContext.Provider>
      </div>
    );
  }
}

ReactDOM.render(<App />, document.querySelector("#app"));


// Home.jsx
import { BtnStyeContext, LoginStatusContext } from "../context_comps/context";
import Header from "../components/Header";
import Main from "../components/Main";
import Footer from "../components/Footer";

export default class Home extends React.Component {
  render() {
    return (
      <div className="page_home">
        <Header></Header>
        <hr />
        <Main></Main>
        <hr />
        <Footer></Footer>
        <hr />
      </div>
    );
  }
}


// Header.jsx
import { LoginStatusContext } from "../context_comps/context";
import StButton from "./StButton";

export default class Header extends React.Component {
  render() {
    return (
      <LoginStatusContext.Consumer>
        {({ status, login }) => (
          <div className="header">
            <h1>Header</h1>
            <StButton>Header({status ? " 已登錄" : " 未登錄"})</StButton>
            <button onClick={login}>登錄/注銷</button>
          </div>
        )}
      </LoginStatusContext.Consumer>
    );
  }
}

// Main.jsx
import { LoginStatusContext } from "../context_comps/context";
import StButton from "./StButton";

export default class Main extends React.Component{
  render() {
    return (
      <LoginStatusContext.Consumer>
        {({ status, login }) => (
          <div className="main">
            <h1>Main</h1>
            <StButton>Main({status ? " 已登錄" : " 未登錄"})</StButton>
            <button onClick={login}>登錄/注銷</button>
          </div>
        )}
      </LoginStatusContext.Consumer>
    );
  }
}

// Footer.jsx
import { LoginStatusContext } from "../context_comps/context";
import StButton from "./StButton";

export default class Footer extends React.Component {
  render() {
    return (
      <LoginStatusContext.Consumer>
        {({ status, login }) => (
          <div className="footer">
            <h1>Footer</h1>
            <StButton>Footer({status ? " 已登錄" : " 未登錄"})</StButton>
            <button onClick={login}>登錄/注銷</button>
          </div>
        )}
      </LoginStatusContext.Consumer>
    );
  }
}

// StButton.jsx
import { BtnStyeContext } from "../context_comps/context";
export default class StButton extends React.Component {
  render() {
    return (
      <BtnStyeContext.Consumer>
        // {({ style, doClick }) => (
        //   <button style={style} onClick={doClick} {...this.props}></button>
        // )}
      </BtnStyeContext.Consumer>
    );
  }
}

// context.js
// 提供 context
import { btnStyle } from "../config";

const BtnStyeContext = React.createContext({
  style: btnStyle.primary,
  doClick: () => {},
});

const LoginStatusContext = React.createContext({
  status: false,
  login: () => {},
});

export { BtnStyeContext, LoginStatusContext };

// btnStyle
export const btnStyle = {
  primary: {
    color: "#fff",
    backgroundColor: "blue",
  },
  success: {
    color: "#fff",
    backgroundColor: "green",
  },
  warning: {
    color: "#000",
    backgroundColor: "orange",
  },
  danger: {
    color: "#fff",
    backgroundColor: "red",
  },
}

Fragment 和短語法應(yīng)用

React.Fragment 組件，和 Document.createDocumentFragment() 類似，不會(huì)在原有 Dom 結(jié)構(gòu)中增加冗余標(biāo)簽

現(xiàn)階段，Fragment 組件除了 key 屬性，不支持其他任何屬性

短語法不支持 Key，和其他任何語法
<>xxx</>

class Table extends React.Component {
  render() {
    return (
      <table border={1}>
        <caption>Private Infomation</caption>
        <thead>
          <tr>
            <TableHeader />
          </tr>
        </thead>
        <tbody>
          <TableBody />
        </tbody>
      </table>
    );
  }
}

class TableHeader extends React.Component {
  state = {
    headers: ["Name", "ID", "Age"],
  };
  render() {
    return (
      <React.Fragment>
        {this.state.headers.map((item, index) => (
          <th key={index}>{item}</th>
        ))}
      </React.Fragment>
    );
  }
}

class TableBody extends React.Component {
  state = {
    info: [
      {
        name: "zsss",
        id: "897",
        age: "177",
      },
    ],
  };

  render() {
    return (
      <React.Fragment>
        {this.state.info.map((item) => (
          <tr>
            <td>{item.name}</td>
            <td>{item.id}</td>
            <td>{item.age}</td>
          </tr>
        ))}
      </React.Fragment>
    );
  }
}

class App extends React.Component {
  render() {
    return <Table></Table>;
  }
}

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React 中的 context（7）

React 中的 context（7）

Context

Context API

contextType

Fragment 和短語法應(yīng)用

相關(guān)閱讀更多精彩內(nèi)容

友情鏈接更多精彩內(nèi)容

色偷偷精品伊人,欧洲久久精品,欧美综合婷婷骚逼,国产AV主播,国产最新探花在线,九色在线视频一区,伊人大交九 欧美,1769亚洲,黄色成人av

React 中的 context（7）

Context

Context API

contextType

Fragment 和短語法應(yīng)用

相關(guān)閱讀更多精彩內(nèi)容

友情鏈接更多精彩內(nèi)容

色偷偷精品伊人,欧洲久久精品,欧美综合婷婷骚逼,国产AV主播,国产最新探花在线,九色在线视频一区,伊人大交九欧美,1769亚洲,黄色成人av