I have made some sandbox code of my problem here:

https://codesandbox.io/s/clever-zeh-kdff1z

<template>
  <div v-if="started">
    <HelloWorld :msg="msg" @exit="exit" @remake="remake" />
  </div>
  <button v-if="!started" @click="started = !started">start</button>
</template>

<script>
import HelloWorldVue from "./components/HelloWorld.vue";
export default {
  name: "App",
  components: {
    HelloWorld: HelloWorldVue,
  },
  data() {
    return {
      started: false,
      msg: "Hello Vue 3 in CodeSandbox!",
    };
  },
  methods: {
    exit() {
      this.started = false;
    },
    remake() {
      this.msg = this.msg   1;
      //this code should recreate our child but...
      this.exit();
      this.started = true;
      // setTimeout(() => {
      //   this.started = true;
      // });
    },
  },
};
</script>

So! We have 2 components parent and child. The idea is simple - we have a flag variable in our parent. We have a v-if statement for this - hide / show an element depend on the flag value "false" or "true". After we toggle the flag - the child component should be recreated. This is the idea. Simple.

In our parent we have a button which will set the flag variable to "true" and our child will be created and will appear on our page.

Ok. Now we have 2 buttons inside our child. One button is "exit" which is emit an event so the flag variable of parent will set to "false" and the elemint will disappear from our page(It will be destroyed btw). Works as charm. Ok.

The second button "remake". It emit event so the flag variable will be just toggled (off then on). Simple. We set to "false", we set to "true". So the current child should dissapear, and then imediatly will be created new one.

But here we are facing the problem! Ok, current child is still here, there is no any recreation, it just updates current one... So in child I have checked our lifecycle hooks - created and unmounted via console.log function. And the second button dont trigger them. Start->Exit->Start != Start->Remake.

So can anyone please explain me why this is happening? I cant figure it out.

Interesting thing, if you can see there is some asynchronous code commented in my demo. If we set our flag to "true" inside the async function the child will be recreated and we will see the created hook message but it seems like crutch. We also can add a :key to our component and update it to force rerender, but it also seems like a crutch.

Any explanations on this topic how things work would be nice.

CodePudding user response：

Vue re-uses elements and components whenever it can. It will also only rerender once per tick. The length of a 'tick' is not something you should worry yourself about too much, other than that it exists. In your case the this.exit() and this.started = true statements are executed within the same tick. The data stored in this.started is both true in the last tick and the current tick as it does not end the tick in between the statements, and so nothing happens to your component.

In general you should think in states in Vue rather than in lifecycles. Or in other words: What are the different situations this component must be able to handle and how do you switch between those states. Rather than determining what to do in which point in time. Using :key="keyName" is indeed generally a crutch, as is using import { nextTick } from 'vue'; and using that to get some cadence of states to happen, as is using a setTimeout to get some code to execute after the current tick. The nasty part of setTimeout is also that it can execute code on a component that is already destroyed. It can sometimes help with animations though.

In my experience when people try to use lifecycle hooks they would rather have something happen when one of the props change. For example when a prop id on the child component changes you want to load data from the api to populate some fields. To get this to work use an immediate watcher instead:

watch: {
  id: {
    handler(newId, oldId) {
      this.populateFromApi(newId);
    },
    immediate: true
  }
}

Now it will call the watcher on component creation, and call it afterwards when you pass a different id. It will also help you gracefully handle cases where the component is created with a undefined or null value in one of the props you expect. Instead of throwing an error you just render nothing until the prop is valid.