Vue3 - after provide/inject object inside component is loosing data - vue-router

Question

I'm having small issue with provide/inject in my project.
In App.vue, I'm pulling data from DB and pushing it into object. With console log I checked and all data it's there.
<template>
<router-view />
</template>
<script>
export default {
provide() {
return {
user: this.user,
};
},
data() {
return {
user: '',
};
},
methods: {
///pulling data from DB
func() {
fetch("url")
.then((response) => {
if (response.ok) {
return response.json();
}
})
.then((data) => {
const user = [];
for (const id in data) {
user.push({
id: data[id].user_id,
firstName: data[id].user_firstname,
lastName: data[id].user_lastname,
email: data[id].user_email,
phone: data[id].user_phone,
address1: data[id].user_address_1,
address2: data[id].user_address_2,
address3: data[id].user_address_3,
address4: data[id].user_address_4,
group: data[id].user_group,
});
}
this.user = user;
})
.catch((error) => {
console.log(error);
});
},
},
created() {
this.func();
},
};
</script>
Console log of object user App.vue
Object { id: "3", firstName: "test", lastName: "test", … }
Next I'm injecting it into component. Object inside component exists, but empty - all data cease to exist.
<script>
export default {
inject: ["user"],
};
</script>
console log of object user in component
<empty string>
While in App.vue data is still there, in any components object appears to be empty, but it is there. Any idea why?
Thanks for help.

Answer 1

In short, this happens because you are reassigning user rather than changing user.
Let's say you have a Child component that consumes your inject data and renders the users in a list:
<template>
<div> Child </div>
<ul>
<li v-for="user in users" :key="user.id"> {{user.name}} </li>
</ul>
</template>
<script>
import {inject} from "vue";
export default {
name: "Child",
setup() {
const users = inject("users");
return {users};
}
}
</script>
To provide the users from parent component, all you need to ensure is that users itself is a reactive object, and you keep changing it from the parent rather than reassigning it.
I am going to use the composition api to illustrate what I mean. Compared to options api, everything in composition api is just plain javascript hence there is a lot less behind-the-scene magic. At the end I will tell you how options api is related to the composition api.
<template>
<button #click=generateUsers>
Generate Users
</button>
<Child/>
</template>
<script>
import {reactive, provide, toRefs} from "vue";
import Child from "./Child.vue";
export default {
name: "App",
components: {
Child
},
setup() {
const data = reactive({users: ""});
const generateUsers = () => {
// notice here you are REASSIGNING the users
data.users = [
{id: 1, name: "Alice"}, {id: 2, name: "Bob"}
];
console.log(data.users);
}
// this way of provide will NOT work
provide("users", data.users);
// this way works because of toRefs
const {users} = toRefs(data);
provide("users", users);
return {generateUsers};
}
}
</script>
A few things to note:
the data options in the options api is exactly the same as const data = reactive({users: ""}). Vue will run your data() method, from where you have to return a plain object. And then Vue will automatically call reactive to add reactivity to it.
provide, on the other hand, is not doing any magic - neither in options api, nor in the composition api. It just passes whatever it is given to the consuming component without any massaging.
the reason provide("users", data.users) does not work as you would expect is that the way you populate the users is not a change to the same data.users object (which actually is reactive), but a reassign all together.
the reason toRefs works is because toRefs links to the original parent.
With this understanding in mind, to fix your original code, you just need to ensure you change, instead of reassigning, the users. The simplest way is to define user as an array and push into it when you load data. (in contrast to defining it initially as a string and reassigning it later)
P.S. what also works in composition api, and is a lot simpler, is to:
<template>
<button #click=generateUsers>
Generate Users
</button>
<Child/>
</template>
<script>
import {ref, provide} from "vue";
import Child from "./Child.vue";
export default {
name: "App",
components: {
Child
},
setup() {
const users = ref();
const generateUsers = () => {
// notice here you are not reassigning the users
// but CHANGING its value
users.value = [
{id: 1, name: "Alice"}, {id: 2, name: "Bob"}
];
console.log(users.value);
}
provide("users", users);
return {generateUsers};
}
}
</script>