KVO: Swift's Built-In Observer Pattern
KVO lets an object subscribe to property changes on another object automatically. It shines when you need UI updates driven by model state, but forgetting to remove observers before deallocation crashes your app with an NSException.
KVO is Swift's runtime mechanism that notifies one object when a property on another object changes. You use it when you need loose coupling between a model and its consumers, such as syncing a progress bar to a download manager's bytesRead property without direct delegation. The system uses Objective-C runtime machinery under the hood, so forgetting to call removeObserver before either object deallocates triggers an immediate NSException and hard crash, a footgun that has pushed many teams toward Combine or observation tokens.
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