Auth state change listeners in Supabase - Step-by-Step Execution

Process Flow - Auth state change listeners

Start app

↓

Register auth listener

↓

User signs in/out

↓

Listener detects change

↓

Update app state

↓

Reflect changes in UI

The app starts and sets up a listener for authentication changes. When the user signs in or out, the listener detects it and updates the app state and UI accordingly.

Execution Sample

Supabase

const { data: authListener } = supabase.auth.onAuthStateChange((event, session) => {
  console.log('Auth event:', event);
  console.log('Session:', session);
});

This code sets up a listener that runs a function whenever the authentication state changes, logging the event and session info.

Process Table

Step	Event Triggered	Listener Action	Session State	App State Update
1	App starts	Listener runs callback (initial)	null	No user logged in
2	User signs in	Listener runs callback	{ user: 'user1' }	App updates to logged in
3	User signs out	Listener runs callback	null	App updates to logged out
4	User session refresh	Listener runs callback	{ user: 'user1' }	App keeps logged in state

💡 No more auth events; listener waits for next change

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 4
event	SIGNED_OUT	SIGNED_IN	SIGNED_OUT	TOKEN_REFRESHED
session	null	{ user: 'user1' }	null	{ user: 'user1' }
appState	logged out	logged in	logged out	logged in

Key Moments - 3 Insights

Why does the listener run when the app starts even if the user is not signed in?

The listener callback runs immediately upon registration (step 1) with the initial auth state: event='SIGNED_OUT', session=null, updating app state to logged out.

What happens if the user signs out and then signs in quickly?

The listener runs twice, once for SIGNED_OUT and once for SIGNED_IN events, updating the session and app state accordingly as shown in steps 3 and 2.

Why is the session sometimes null and sometimes an object?

Session is null when no user is signed in (steps 1 and 3), and contains user info when signed in or refreshed (steps 2 and 4).

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what is the appState?

A logged in

B logged out

C null

D session expired

💡 Hint

Check the 'App State Update' column at step 2 in the execution_table

At which step does the listener detect the user signing out?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Event Triggered' column for SIGNED_OUT event in the execution_table

If the session is null, what is the likely appState?

A logged in

B logged out

C refreshing

D error

💡 Hint

Refer to the variable_tracker for session and appState values

Concept Snapshot

Auth state change listeners track user sign-in and sign-out events.
Register a listener with supabase.auth.onAuthStateChange(callback).
Callback receives event type and session info.
Update app state and UI based on session presence.
Listener runs on every auth event including sign-in, sign-out, and token refresh.

Full Transcript

When you start your app, you set up a listener to watch for authentication changes. This listener waits for events like user sign-in or sign-out. When the user signs in, the listener runs a function that receives the event type and session details. The app then updates its state to show the user is logged in. If the user signs out, the listener runs again, session becomes null, and the app updates to logged out. The listener also runs on token refresh events to keep the session current. This way, your app always knows the user's auth status and can update the interface accordingly.