All 7 State Management with Provider questions
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What is the Provider package, and how does it let you properly share a given piece of state across several separate different widgets without needing to manually pass it down through every single intermediate widget constructor? What is ChangeNotifier, and how does calling notifyListeners let your interface automatically rebuild itself the moment a given piece of shared state actually genuinely changes? What is the difference between context.watch, context.read, and context.select, and how does correctly choosing the genuinely appropriate one help meaningfully avoid unnecessary, wasteful widget rebuilds? How does MultiProvider let you properly and cleanly combine several genuinely separate individual providers together, avoiding a genuinely awkward, deeply nested pyramid of several separate individually nested Provider widgets? How should you properly structure a genuinely large app's own overall Provider architecture, deciding exactly where within the widget tree each individual specific provider should genuinely be scoped and properly placed? How do you properly test a widget that genuinely depends on Provider, using a properly mocked or fake provided value to reliably verify that widget's own behavior in complete isolation?
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Flutter › State Management with Provider

How does ProxyProvider let you properly create a given provider whose own value genuinely depends on the current value of a completely separate other already existing provider?

Intermediate

Answer

ProxyProvider lets you properly build a given provider's own value based directly on one or more other separate providers that already genuinely exist further up within the widget tree, automatically and properly recreating that particular dependent value whenever any one of those specific separate upstream providers it genuinely depends on happens to actually change, which is especially useful for a scenario like a genuine ApiClient that needs to be properly reconstructed whenever a separate AuthProvider's own current authentication token actually changes, ensuring that ApiClient always genuinely uses the exact correct, most current up to date token.
ProxyProvider<AuthModel, ApiClient>(
  update: (context, auth, previousApiClient) => ApiClient(token: auth.token),
)
Real-world example An app properly reconstructs its shared ApiClient using ProxyProvider whenever a separate AuthModel's own current token value actually changes, correctly ensuring every single subsequent API call consistently uses the exact correct, most current up to date authentication token.

Common follow-ups: What genuinely happens to the previous ApiClient instance once ProxyProvider properly creates a genuinely new updated one?;How many genuinely separate distinct provider dependencies can a single given ProxyProvider actually genuinely depend on together at once?

Firebase Authentication in Flutter;Dependency Injection in Flutter (GetIt & Service Locator)
All 7 State Management with Provider questions