How to use composition API to create a new component in vue3? - vuejs2

Question

When we use vue2 to create API, we just follow options API like below:
data are in data
methods are in methods
<script>
export default {
name: 'demo',
components: {},
filter:{},
mixins:{},
props: {},
data(){
return{
}
},
computed:{},
watch:{},
methods: {},
}
</script>
But the vue3 changed, how should I build a component with vue3 composition API?
Some example say that I should import reactive etc. From vue first and put all codes in setup(){}?
Some example show that I can add setup to <script>?
Please give me an example.

Answer 1

ok bro , Composition Api works like that:
<script>
import { fetchTodoRepo } from '#/api/repos'
import {ref,onMounted} from 'vue'
export default {
setup(props){
const arr = ref([]) // Reactive Reference `arr`
const getTodoRepo = async () => {
arr.value = await fetchTodoRepo(props.todo)
}
onMounted(getUserRepo) // on `mounted` call `getUserRepo`
return{
arr,
getTodoRepo
}
}
}
</script>

Answer 2

There are two ways to create a component in vue3.
One：<script> + setup(){},such as this:
<script>
import { reactive, onMounted, computed } from 'vue'
export default {
props: {
title: String
},
setup (props, { emit }) {
const state = reactive({
username: '',
password: '',
lowerCaseUsername: computed(() => state.username.toLowerCase())
})
onMounted(() => {
console.log('title: ' + props.title)
})
const login = () => {
emit('login', {
username: state.username,
password: state.password
})
}
return {
login,
state
}
}
}
</script>
Two：use <script setup="props">
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