RxSwift 4/RxCocoa 4中Variable将被弃用：BehaviorRelay与Variable的优劣对比

Great question! When RxSwift 4 rolled out, Variable was marked for deprecation, and BehaviorRelay (part of the new Relay class in RxCocoa 4) was positioned as its replacement. Let's dive into their respective strengths and weaknesses, especially focusing on how they fit (or clash with) reactive programming principles.

Variable: The Convenient But Imperative Choice #

Pros #

• Dead-simple imperative assignment: You can update its value directly with variable.value = newValue—no extra methods or ceremony required. This makes it super approachable for developers new to RxSwift, as it feels familiar to traditional state management patterns.
• Built-in observable conversion: It comes with an asObservable() method out of the box, so you can easily pass it to subscribers without extra setup.

Cons #

• Violates reactive programming's declarative ethos: Assigning values directly is an imperative action, which clashes with the reactive paradigm of describing how data flows rather than manually triggering changes. This often leads to messy, mixed-style code that's harder to reason about.
• Uncontrolled state mutations: Since value is read-write, any part of your code can modify it at any time. Tracking down where a state change originated becomes a nightmare, especially in larger projects.
• Deprecated status: The RxSwift team has officially phased out Variable, so it won't receive updates or bug fixes. Using it means risking compatibility issues with future RxSwift versions, and community support for it is dwindling.

BehaviorRelay: The Declarative, Future-Proof Replacement #

Pros #

• Strictly declarative state updates: Instead of direct assignment, you use relay.accept(newValue) to send new values. This enforces reactive best practices, ensuring all state changes are explicit and traceable.
• Safe, error-free streams: Relays are designed to never send Error or Completed events. Unlike Variable (which wraps a BehaviorSubject that can terminate), BehaviorRelay keeps subscriptions alive indefinitely—perfect for managing UI state that shouldn't unexpectedly stop updating.
• Read-only public value: The value property is read-only externally, so only the relay itself can trigger state changes. This prevents accidental or unauthorized mutations, making your codebase more predictable.
• Official support: As the recommended replacement for Variable, it's actively maintained by the RxSwift team and has strong community backing. You'll find up-to-date documentation and examples, and it's compatible with all modern RxSwift versions.

Cons #

• Slightly less "convenient" syntax: For developers used to Variable's direct assignment, switching to accept(_:) requires a small adjustment. It's a minor hurdle, though, and quickly becomes second nature once you embrace reactive principles.
• No "shortcut" for imperative habits: While this is technically a feature, it can feel restrictive if you're still leaning on imperative patterns. But this constraint is exactly what pushes you to write cleaner, more idiomatic reactive code.

内容的提问来源于stack exchange，提问作者Yi-shen Chen