show loading spinner until all child components inside of a Page have been rendered - Nativescript

Question

I'm trying to show an activity indicator, when I go from one page to another. The target page contains many components within it, and it takes time to load. that's why I need some way to listen when all the child components are loaded, and at that moment tell my variable isBussy to be false

<template>
<StackLayout>
  <ActivityIndicator :busy="isBussy" v-if="isBussy" />
  <StackLayout v-else>
    <Component1 />
    <Component2 />
    <Component3 />
    <Component4 />
  </StackLayout>
<StackLayout>
</template>

<script>

import Component1 from '~/components/Component1'
import Component2 from '~/components/Component2'
import Component3 from '~/components/Component3'
import Component4 from '~/components/Component4'

export default {

  data() {
    return {
      isBussy: true
    }
  },
  mounted() {
    this.$nextTick(function() {
      // Code that will run only after the
      // entire view has been re-rendered
      this.isBussy = false
    })
  }

}
</script>

this code does not work, since once the navigation is indicated from the previous page with: @tap="$goto('otherPage', { props: { foo: bar } })"

it remains stuck on the initial page, and all the components begin to load in the background of the destination page, but without displaying the parent page, changing to this, only when the whole process ends, and never show/hide the activity indicator as expected.

By the way this expected behavior works perfectly when i do request and process them with Promises, then I turn on or off a variable in the state and it works. but I can not replicate that behavior in the navigation between pages and listen to load all the components

EDIT

Finally I achieved the desired behavior with a little trick I found on the internet

  mounted() {
    setTimeout(() => {
      this.isBussy = false
    }, 500)
  },

this causes that the rendering of all the children components is delayed only a little, so that the activity indicator is shown, but not too much to produce that none of the components contained in the else block is detected and begin to rendering

Answer 1

There are two main ideas to understand here I think. I'll describe both.

1. General technique to Fetch Data without blocking render

It sounds like you understand this concept at the parent component level but then are asking how to do something very similar for the child components that this page contains.

The way I handle this, is in my component, I have my data default to an isLoading state. Then, in beforeMount() or mounted(), I perform my asynchronous actions and make necessary changes to my page's data.

The problem becomes entirely recursive when we look at child components. You want to make sure your child components are rendering and that any long running data fetching that needs to occur within their implementation will simply cause them to re-render once that fetching is complete.

Here is a working example: https://codesandbox.io/embed/r4o56o3olp This example uses Nuxt. Aside from the addition fetch() and asyncData() methods, the rest of the Vue lifecycle hooks are the same here.

I use new Promise and setTimeout to demonstrate an operation that would use promises and be asynchronous. (e.g. axios.get(..))

The About page loads, and the beforeMount() lifecycle hook performs the asynchronous fetching in a way that doesn't block the page from rendering.

I use the beforeMount() hook because, according to here ( https://alligator.io/vuejs/component-lifecycle/ ), it is the first lifecycle hook that we have access to once the page's data is reactive. (So modifying this.myDataProp would trigger a re-render if {{ myDataProp }} was used in the template).

I also included a child component where I purposely made its data take twice as long to load. Since I again, am letting the component render immediately, and then I handle the fetching/updating of data in an appropriate lifecycle hook, I can manage when the end-user perceives a page to be loaded.

In my working example, the LongLoadingComponent did the same exact technique as the About page.

Once you see how to use beforeMount() or mounted() to fetch data and then update state, I think the trick is to take a moment and really think about the default state of your component. When it first renders, what should the user see before any of it's data fetching/long-running operations are completed?

Once you determine what your default (not yet loaded) component should look like, try getting that to render on your screen, and secondarily add in the logic that fetches and updates state data.

2. Listening for when a Child Component is finished rendering from a parent component

This makes use of the above technique, but includes the usage of the updated() hook and emitting a custom event ( https://v2.vuejs.org/v2/guide/components-custom-events.html )

If you really want to listen for when your child components are finished rendering, you can $emit a custom event in your updated() hook. Perhaps something like this (in one of your child components)

if (this.dataLoaded) { this.$emit('loadedAndRendered') }

So when the child's async operations are done, it can flip it's dataLoaded property to true. If dataLoaded is used in the child's <template> somewhere, then the component should re-render (for it's "finished" state). When the child re-renders, the updated() hook should trigger. (again, see: https://alligator.io/vuejs/component-lifecycle/ ) I included the if (this.dataLoaded) part just to handle case where updated() hook might be called during intermediate data updates. (We only want to emit loadedAndRendered event if child is finished loading data/updating.)

3. Other caveats about universal nuxt applications

It wasn't until after I wrote this answer that I realized you aren't using Nuxt. However I'm adding this in case other Nuxt users happen to come across this.

I'm adding this section just because it took some focused hands-on time for me to wrap my head around. A Nuxt Universal Application does both server-side and client-side rendering. Understanding when something renders on the client vs when it was rendered on the server was a little difficult for me at first. In the working example I linked above, when you visit the about page you can also see if that component was fetched from the server or if it was just rendered by the client.

I'd recommend playing with a Page's fetch() and asyncData() methods and see how it impacts when certain things render on your screen. ( https://nuxtjs.org/api/pages-fetch/ ) ( https://nuxtjs.org/api/ ). Seeing what these methods are useful for helps me also identify what they are not useful for.

If you're using a Vuex store, I'd recommend seeing what happens when you refresh a page or use instead of a to navigate between pages. (Seeing something like the SSR schema diagram can be helpful here: https://nuxtjs.org/guide#schema )

..I have yet to fully appreciate the details of the bundling and delivery behavior that Webpack provides for a Universal Nuxt app (See right side of diagram here: https://medium.freecodecamp.org/universal-application-code-structure-in-nuxt-js-4cd014cc0baa )