What is a difference betwee...

Here is a concise answer to your question about when to use ephemeral state vs app state in Flutter:

Ephemeral State

Use ephemeral state when the state is:

• Specific to a single widget or a small, localized part of the UI[1][2]
• Not needed by other parts of the app
• Simple enough to manage with setState()
• Doesn't need to persist across app sessions

Examples of ephemeral state include:

• Current page in a PageView
• Progress of a complex animation
• Selected tab in a BottomNavigationBar[1]

For ephemeral state, use StatefulWidget and setState() to rebuild the widget tree when the state changes[1][2].

App State

Use app state when the state:

• Needs to be shared across multiple widgets or screens
• Persists across app sessions
• Is complex enough that setState() becomes unwieldy

Examples of app state include:

• User login ...