BehaviorSubject与PublishSubject选型指南：何时分别使用二者？

Great question! There isn't a universal "golden rule" for choosing between BehaviorSubject, PublishSubject, and other Subject types, but their differences are clear, and we can map them directly to specific needs. Let's break this down:

Core Subject Type Recap #

First, a quick reminder: Subjects are a special type of Observable that can both emit events and subscribe to other Observables. The key distinction between common types boils down to how they handle past events for new subscribers.

Key Use Cases for Each Subject #

• PublishSubject: Ideal when you only care about events emitted after a subscriber connects.

  Example: Suppose Stage (B) has already sent events 1, 2, 3. When Subscriber (A) reconnects later, PublishSubject will only deliver events 4, 5, 6 to (A). This is perfect if you don't need to catch up on what happened before joining.

• ReplaySubject: Use this when you need new subscribers to receive a specific history of past events (you can configure how many events to replay).

  Example: In the same scenario above, ReplaySubject would first send the historical events [1,2,3] to (A), then follow with the new events 4, 5, 6.

• BehaviorSubject: This is the middle ground—it immediately sends the most recent event to new subscribers, then continues with all new events after that.

  Example: If Stage (B) last emitted event 3 before (A) reconnects, BehaviorSubject will first send 3 to (A), then deliver 4, 5, 6 as they come in. This is super useful for state management, where you need a subscriber to know the current state right away.

Quick Cheat Sheet for Selection #

To sum it up without overcomplicating:

• Pick PublishSubject if you don't need any past events—subscribers only get what happens after they join.
• Go with BehaviorSubject when subscribers need the latest state/value immediately upon connecting (think app UI that needs the current user session or settings right away).
• Choose ReplaySubject when you need to replay a specific number of past events (like showing the last 10 chat messages to a new user).

内容的提问来源于stack exchange，提问作者Human