vue2和vue3的区别是什么

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<template>
  <div class='form-element'>
  <h2> {{ title }} </h2>
  </div>
</template>

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<template>
  <div class='form-element'>
  </div>
   <h2> {{ title }} </h2>
</template>

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export default {
  props: {
    title: String
  },
  data () {
    return {
      username: '',
      password: ''
    }
  },
  methods: {
    login () {
      // 登陆方法
    }
  },
  components:{
            "buttonComponent":btnComponent
        },
  computed:{
	  fullName(){
	    return this.firstName+" "+this.lastName;     
	  }
}
 
}

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export default {
  props: {
    title: String
  },
  
  setup () {
    const state = reactive({ //数据
      username: '',
      password: '',
      lowerCaseUsername: computed(() => state.username.toLowerCase()) //计算属性
    })
     //方法
    const login = () => {
      // 登陆方法
    }
    return { 
      login,
      state
    }
  }
}

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export default {
  props: {
    title: String
  },
  data () {
    return {
      username: '',
      password: ''
    }
  }
}

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import { reactive } from 'vue'

export default {
  props: {
    title: String
  },
  setup () {
    const state = reactive({
      username: '',
      password: ''
    })

    return { state }
  }
}

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<template>
  <div>
    <h2> {{ state.username }} </h2>
  </div>
</template>

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Vue2--------------vue3
beforeCreate  -> setup()
created       -> setup()
beforeMount   -> onBeforeMount
mounted       -> onMounted
beforeUpdate  -> onBeforeUpdate
updated       -> onUpdated
beforeDestroy -> onBeforeUnmount
destroyed     -> onUnmounted
activated     -> onActivated
deactivated   -> onDeactivated

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import { toRefs } from 'vue'
 
setup(props) {
	const { title } = toRefs(props)
 
	console.log(title.value)
	onMounted(() => {
      console.log('title: ' + props.title)
    })

}

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login () {
      this.$emit('login', {
        username: this.username,
        password: this.password
      })
 }

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setup (props, { attrs, slots, emit }) {
    // ...
    const login = () => {
      emit('login', {
        username: state.username,
        password: state.password
      })
    }

    // ...
}

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import { ref } from 'vue'
 
const count = ref(0)
console.log(count.value) // 0

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<template>
<teleport to="#modal">
  <div id="center" v-if="isOpen">
    <h2><slot>this is a modal</slot></h2>
    <button @click="buttonClick">Close</button>
  </div>
</teleport>
</template>
<script>

export default {
  props: {
    isOpen: Boolean,
  },
  emits: {
    'close-modal': null
  },
  setup(props, context) {
    const buttonClick = () => {
      context.emit('close-modal')
    }
    return {
      buttonClick
    }
  }
}
</script>
<style>
  #center {
    width: 200px;
    height: 200px;
    border: 2px solid black;
    background: white;
    position: fixed;
    left: 50%;
    top: 50%;
    margin-left: -100px;
    margin-top: -100px;
  }
</style>

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<template>
<div id="app">
  <img alt="Vue logo" src="./assets/logo.png">
  <HelloWorld msg="Welcome to Your Vue.js App"/>
  <HooksDemo></HooksDemo>
  <button @click="openModal">Open Modal</button><br/>
<modal :isOpen="modalIsOpen" @close-modal="onModalClose"> My Modal !!!!</modal>
</div>
  
</template>
<script>
import HelloWorld from './components/HelloWorld.vue'
import HooksDemo from './components/HooksDemo.vue'
import Modal from './components/Modal.vue'
import{ref} from 'vue'
export default {
  name: 'App',
  components: {
    HelloWorld,
    HooksDemo,
    Modal
  },
  setup() {
    const modalIsOpen = ref(false)
    const openModal = () => {
      modalIsOpen.value = true
    }
    const onModalClose = () => {
      modalIsOpen.value = false
    }
    return {
      modalIsOpen,
      openModal,
      onModalClose
    }
  }
}
</script>

<style>
#app {
  font-family: Avenir, Helvetica, Arial, sans-serif;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  text-align: center;
  color: #2c3e50;
  margin-top: 60px;
}
</style>

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<template>
  <teleport to="#modal">
    <div id="center">
      <h2>柏特better</h2>
    </div>
  </teleport>
</template>

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<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width,initial-scale=1.0">
    <link rel="icon" href="<%= BASE_URL %>favicon.ico">
    <title><%= htmlWebpackPlugin.options.title %></title>
  </head>
  <body>
    <noscript>
      <strong>We're sorry but <%= htmlWebpackPlugin.options.title %> doesn't work properly without JavaScript enabled. Please enable it to continue.</strong>
    </noscript>
    <div id="app"></div>
    <div id="modal"></div>
    <!-- built files will be auto injected -->
  </body>
</html>