Loading states and error patterns in Angular - Step-by-Step Execution

Concept Flow - Loading states and error patterns

Component Initialized

↓

Start Data Fetch

↓

Show Loading State

↓

Data Fetch Result

↓

Success

↓

Show Data

↓

User Action?

↓

Retry or End Process

This flow shows how an Angular component starts fetching data, shows a loading state, then either displays data on success or an error message on failure, allowing retry.

Execution Sample

Angular

const dataSignal = signal(null);
const loadingSignal = signal(false);
const errorSignal = signal(null);

function fetchData() {
  loadingSignal.set(true);
  errorSignal.set(null);
  fetch('/api/data')
    .then(res => res.json())
    .then(data => {
      dataSignal.set(data);
      loadingSignal.set(false);
    })
    .catch(err => {
      errorSignal.set('Failed to load');
      loadingSignal.set(false);
    });
}

This Angular code uses signals to track loading, data, and error states during a fetch call.

Execution Table

Step	Action	loadingSignal	errorSignal	dataSignal	UI State
1	Call fetchData()	true	null	null	Show loading spinner
2	Fetch starts	true	null	null	Show loading spinner
3	Fetch success response received	true	null	data object	Still loading spinner (waiting to update)
4	Set dataSignal with data	true	null	data object	Still loading spinner
5	Set loadingSignal to false	false	null	data object	Show data content
6	User triggers fetchData() again	true	null	data object	Show loading spinner
7	Fetch fails	true	null	data object	Show loading spinner
8	Set errorSignal to 'Failed to load'	true	'Failed to load'	data object	Show error message
9	Set loadingSignal to false	false	'Failed to load'	data object	Show error message
10	User retries fetchData()	true	null	data object	Show loading spinner

💡 Loading ends when loadingSignal is false; errorSignal shows error or null; dataSignal holds data or null.

Variable Tracker

Variable	Start	After Step 1	After Step 5	After Step 8	After Step 9	After Step 10
loadingSignal	false	true	false	true	false	true
errorSignal	null	null	null	'Failed to load'	'Failed to load'	null
dataSignal	null	null	data object	data object	data object	data object

Key Moments - 3 Insights

Why does the loading spinner show even after data is received but before loadingSignal is set to false?

Because loadingSignal remains true until explicitly set to false (see steps 3-5 in execution_table), the UI shows loading spinner until loadingSignal changes.

Why is errorSignal reset to null when fetchData is called again?

To clear previous errors and show fresh loading state, errorSignal is reset at fetch start (step 1 and 10), so old errors don't persist.

What happens if loadingSignal is never set to false after fetch?

The UI would keep showing the loading spinner indefinitely, blocking data or error display (see exit_note and step 5).

Visual Quiz - 3 Questions

Look at the execution_table at step 5, what is the UI state?

A Show data content

B Show loading spinner

C Show error message

D Show nothing

💡 Hint

Check the UI State column at step 5 in execution_table.

At which step does errorSignal get set to 'Failed to load'?

A Step 7

B Step 8

C Step 9

D Step 10

💡 Hint

Look at errorSignal column changes in execution_table rows.

If loadingSignal was not set to false after fetch success, what would the UI show?

A Show data content

B Show error message

C Show loading spinner

D Show nothing

💡 Hint

Refer to key_moments about loadingSignal controlling spinner visibility.

Concept Snapshot

Loading states and error patterns in Angular:
- Use signals to track loading, error, and data states.
- Set loading true before fetch, false after.
- Reset error before fetch.
- Show spinner when loading is true.
- Show error message if errorSignal is set.
- Show data when loading is false and no error.

Full Transcript

This Angular example shows how to manage loading and error states using signals. When fetchData is called, loadingSignal is set to true and errorSignal reset to null. The UI shows a loading spinner. On success, dataSignal is set with data and loadingSignal set to false, showing data content. On failure, errorSignal is set with an error message and loadingSignal set to false, showing the error message. Retrying resets errorSignal and loadingSignal to true again. This pattern helps users see clear feedback during data loading and errors.