Loading states pattern in Vue - Step-by-Step Execution

Concept Flow - Loading states pattern

Component mounts

↓

Set loading = true

↓

Fetch data async

↓

Data received?

No→Wait

Yes↓

Set loading = false

↓

Render data or loading indicator

This flow shows how a Vue component sets a loading flag, fetches data asynchronously, then updates the UI when data arrives.

Execution Sample

Vue

import { ref, onMounted } from 'vue';

const loading = ref(true);
const data = ref(null);

onMounted(async () => {
  data.value = await fetchData();
  loading.value = false;
});

This Vue code sets loading true, fetches data on mount, then sets loading false to show data.

Execution Table

Step	Action	loading.value	data.value	UI Rendered
1	Component mounts, loading set true	true	null	Show loading spinner
2	Start fetchData async call	true	null	Show loading spinner
3	fetchData resolves with data	true	{...data...}	Show loading spinner
4	Set loading to false	false	{...data...}	Show data content
5	Render completes with data	false	{...data...}	Show data content

💡 Loading is false and data is loaded, so UI shows data instead of spinner.

Variable Tracker

Variable	Start	After Step 1	After Step 3	After Step 4	Final
loading.value	true	true	true	false	false
data.value	null	null	{...data...}	{...data...}	{...data...}

Key Moments - 3 Insights

Why does the UI show a spinner before data arrives?

Because loading.value is true at steps 1-3, the UI renders the loading spinner as shown in the execution_table rows 1-3.

When does the UI switch from spinner to data?

At step 4, loading.value changes to false, triggering the UI to render the data content instead of the spinner.

What happens if fetchData takes a long time?

The loading.value stays true, so the spinner remains visible until data arrives, preventing empty or broken UI.

Visual Quiz - 3 Questions

Look at the execution table, what is loading.value at Step 3?

A true

B false

C null

D undefined

💡 Hint

Check the loading.value column at Step 3 in the execution_table.

At which step does the UI start showing the actual data instead of the spinner?

A Step 2

B Step 4

C Step 3

D Step 1

💡 Hint

Look at the UI Rendered column in the execution_table to find when data content appears.

If loading.value was never set to false, what would the UI show?

A Data content

B Empty screen

C Loading spinner forever

D Error message

💡 Hint

Refer to the key_moments explanation about loading.value controlling the spinner visibility.

Concept Snapshot

Loading states pattern in Vue:
- Use a reactive loading flag (ref) set true initially
- Fetch data asynchronously onMounted
- When data arrives, set loading false
- UI shows spinner if loading true, else shows data
- Keeps UI clear and responsive during async fetch

Full Transcript

This visual execution shows how a Vue component uses a loading state pattern. When the component mounts, it sets a loading flag to true and starts fetching data asynchronously. While waiting, the UI shows a loading spinner. Once the data arrives, the loading flag is set to false, and the UI updates to show the fetched data. This pattern helps keep the user informed and prevents showing empty or broken content during data loading.