onDispose abstract method

Adds a listener to perform an operation right before the provider is destroyed.

This includes:

• when the provider will rebuild (such as when using watch or refresh).
• when an autoDispose provider is no longer used
• when the associated ProviderContainer/ProviderScope is disposed`.

Prefer having multiple onDispose, for every disposable object created, instead of a single large onDispose:

Good:

final disposable1 = Disposable(...);
ref.onDispose(disposable1.dispose);

final disposable2 = Disposable(...);
ref.onDispose(disposable2.dispose);

Bad:

final disposable1 = Disposable(...);
final disposable2 = Disposable(...);

ref.onDispose(() {
  disposable1.dispose();
  disposable2.dispose();
});

This is preferable for multiple reasons:

• It is easier for readers to know if a "dispose" is missing for a given object. That is because the dispose call is directly next to the object creation.
• It prevents memory leaks in cases of an exception. If an exception happens inside a dispose() call, or if an exception happens before onDispose is called, then some of your objects may not be disposed.

See also:

• Provider.autoDispose, a modifier which tell a provider that it should destroy its state when no longer listened to.
• ProviderContainer.dispose, to destroy all providers associated with a ProviderContainer at once.
• onCancel, a life-cycle for when all listeners of a provider are removed.