为何iOS 10及以上需在应用启动完成前设置消息与通知代理？

Great question! This requirement ties directly to how iOS 10+ handles notification workflows and FCM's integration with the system's messaging pipeline. Let's break down the key reasons:

• Early system notification pipeline initialization
  iOS 10 introduced the UNUserNotificationCenter framework, which manages all notification handling for apps. The system spins up this center very early in your app's launch sequence. If you don't assign its delegate in applicationWillFinishLaunching: or applicationDidFinishLaunching:, the system won't have a valid object to route notification events to when they arrive—this means notifications might not show up in the user's notification center, or your app will miss critical callbacks like willPresentNotification: or didReceiveNotificationResponse:.

• FCM data message delivery timing
  For FCM data messages (payloads without a visible notification component), the FIRMessaging delegate is responsible for receiving and processing them. These messages can arrive the moment your app starts up—even if the app was woken up by a push notification. If you delay setting the FIRMessaging delegate, you risk missing these early data messages, since FCM will have already attempted to deliver them before your delegate is ready.

• Strict app launch sequence constraints
  iOS enforces a tight launch order, and the applicationWillFinishLaunching: and applicationDidFinishLaunching: methods are the earliest safe points to configure core services like notification handling. If you set the delegates later (e.g., in a view controller's viewDidLoad), you might miss the system's initial notification registration or message dispatch—this is especially critical when your app is launched directly from a push notification. In that scenario, the system tries to deliver the notification immediately after launching the app, and an unassigned delegate means the notification goes unhandled.

• Preventing race conditions
  Delaying delegate setup creates race conditions: a notification could arrive before your delegate is assigned, leading to unexpected behavior or lost messages. By setting the delegates during launch, you ensure your app is ready to handle notifications and data messages as soon as the system is ready to send them, eliminating this risk entirely.

内容的提问来源于stack exchange，提问作者zeus