viewWillAppear与viewWillDisappear不匹配问题

由一个线上crash引起， 崩溃信息Cannot remove an observer because it is not registered as an observer.

崩溃代码如下所示：

class ObserverViewController: UIViewController {
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        print("viewWillAppear - ADD Observer")
        FakeNetworkManager.shared.addObserver(self, forKeyPath: "networkStatus", options: [.new, .old], context: nil)
    }

    override func viewWillDisappear(_ animated: Bool) {
        super.viewWillDisappear(animated)
        print("viewWillDisappear - REMOVE Observer")
        FakeNetworkManager.shared.removeObserver(self, forKeyPath: "networkStatus")
    }

    override func observeValue(forKeyPath keyPath: String?, of _: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        print("Observed network status changed.")
    }
}

• 崩溃原因：账号被挤下线时系统执行viewWillDisappear，通过removeObserver移除observer时，发现此时尚未执行addObserver，所以crash。具体细节描述如下
• 当用户在某个业务场景下，被其他设备登录相同账号挤掉时
• 此时ObserverViewController已经在页面层级中，而且已经经过了viewWillAppear和viewWillDisappear过程，并且不在页面层级最上层了
• 当前设备中需要关掉所有页面回到登录页面，此时系统主动触发了viewWillDisappear，进而导致crash

解决办法

1. try-catch方案

如果是Objective C语言的话，可以通过trycatch捕获异常

- (void)safeRemoveObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath {
    @try {
        [self removeObserver:observer forKeyPath:keyPath];
    } @catch (NSException *exception) {
        DDLogError(@"exception:%@", exception);
    }
}

2. NSKeyValueObservation（Swift方案）

private func addNetworkStatusObserver() {
    networkStatusObservation = MCNetworkStatusManager.sharedInstance().observe(.networkStatus, options: [.new, .old]) { [weak self] manager, _ in
        guard let self else { return }
        let currentStatus = manager.networkStatus.rawValue
         if currentStatus > MCNetworkStatus.notReachable.rawValue {
            self.reloadSafeWithFetchTop50PrivateChatUserFullinfo(true)
        }
    }
}

private func removeNetworkStatusObserver() {
    networkStatusObservation?.invalidate()
    networkStatusObservation = nil
}

注意：NSKeyValueObservation存在监听RawRepresentable类型数据时change结果异常的未修复bug

3. 第三方KVO组件

比如facebook的KVOController

总结

• 核心结论：线上复杂场景中，viewWillAppear和viewWillDisappear不一定配对出现

• 推荐方案：

  • Swift项目优先使用NSKeyValueObservation
  • Objective-C项目使用try-catch安全移除