watchOS 后台任务在 setTaskCompleted 上崩溃

Question

在我的手表扩展委托init 函数中，我在WCSession 上设置了KVO 观察者：

if WCSession.isSupported() {
  let defaultSession = WCSession.default
  defaultSession.addObserver(self, forKeyPath: "activationState", 
                             options: [.old, .new], 
                             context: &ExtensionDelegate.wcSessionKVOcontext)
  defaultSession.addObserver(self, forKeyPath: "hasContentPending", 
                             options: [.old, .new], 
                             context: &ExtensionDelegate.wcSessionKVOcontext)
}

为了完成所有watch后台任务，这里调用了函数

  override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
    if context == &ExtensionDelegate.wcSessionKVOcontext {
      printStatusChanges(keyPath: keyPath, change: change)
      DispatchQueue.main.async {
        self.completeAllTasksIfReady()
      }
    }
  }

与

  private func completeAllTasksIfReady() {
    let session = WCSession.default
    // the session's properties only have valid values if the session is activated, so check that first
    if session.activationState == .activated && !session.hasContentPending {
            if wcBackgroundTasks.isEmpty {
                print("No background tasks")
            } else {
                wcBackgroundTasks.forEach { $0.setTaskCompleted() }
                print("\(wcBackgroundTasks.count) connectivity background tasks completed")
            }
      wcBackgroundTasks.removeAll()
    }
  }

通常，这可以正常工作。
但是我在以下日志中崩溃了：

Thread 0 name:  Dispatch queue: com.apple.main-thread
Thread 0 Crashed:
0   libsystem_kernel.dylib          0x1db2eda8 __abort_with_payload + 24
1   libsystem_kernel.dylib          0x1db2ae60 abort_with_payload_wrapper_internal + 60
2   libsystem_kernel.dylib          0x1db2ae24 abort_with_payload_wrapper_internal + 0
3   WatchKit                        0x2ddfce86 -[WKRefreshBackgroundTask setTaskCompleted] + 414
4   Watch Extension                 0x00037258 closure #1 in ExtensionDelegate.completeAllTasksIfReady() + 209496 (ExtensionDelegate.swift:196)
5   Watch Extension                 0x000372b0 thunk for @callee_guaranteed (@owned WKRefreshBackgroundTask) -> (@error @owned Error) + 209584 (ExtensionDelegate.swift:0)
6   Watch Extension                 0x0003af08 partial apply for thunk for @callee_guaranteed (@owned WKRefreshBackgroundTask) -> (@error @owned Error) + 225032 (ExtensionDelegate.swift:0)
7   libswiftCore.dylib              0x00525350 0x2fc000 + 2265936
8   libswiftCore.dylib              0x00433b7c 0x2fc000 + 1276796
9   libswiftCore.dylib              0x00304060 0x2fc000 + 32864
10  Watch Extension                 0x00036eec ExtensionDelegate.completeAllTasksIfReady() + 208620 (ExtensionDelegate.swift:196)
11  Watch Extension                 0x000350d4 closure #1 in ExtensionDelegate.observeValue(forKeyPath:of:change:context:) + 200916 (ExtensionDelegate.swift:108)
12  Watch Extension                 0x0000993c _T0Ieg_IeyB_TR + 22844 (AlertManager.swift:0)
13  libdispatch.dylib               0x1d9d3456 _dispatch_call_block_and_release + 10
14  libdispatch.dylib               0x1d9d3432 _dispatch_client_callout + 6
15  libdispatch.dylib               0x1d9e3604 _dispatch_main_queue_callback_4CF$VARIANT$mp + 858
16  CoreFoundation                  0x1df7db1e __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 10
17  CoreFoundation                  0x1df7b73c __CFRunLoopRun + 932
18  CoreFoundation                  0x1dec7660 CFRunLoopRunSpecific + 534
19  GraphicsServices                0x1fb7ab3e GSEventRunModal + 94
20  UIKit                           0x24746604 UIApplicationMain + 156
21  libxpc.dylib                    0x1dcf7b14 _xpc_objc_main + 586
22  libxpc.dylib                    0x1dcf94a8 xpc_main + 154
23  Foundation                      0x1e9b2cf2 service_connection_handler + 0
24  PlugInKit                       0x2525c06e -[PKService run] + 676
25  WatchKit                        0x2de1b036 main + 162
26  libdyld.dylib                   0x1da2e782 start + 2  

很明显，当在WKRefreshBackgroundTask 任务之一中调用setTaskCompleted 时会发生崩溃。

但是可能是什么原因，或者如何调试呢？

Answer 1

我的错。 Apple 回复了我的错误报告：

此问题的行为与预期基于以下几点：
- 这是一个断言，以确保同一任务不会多次完成
- 这由“调用WKWatchConnectivityRefreshBackgroundTask: 0x16e7b230 表示，这已经 已完成”
- 这可能指向项目代码中的一个（或多个）错误，而不是 SDK/API

因此，我仔细检查了我的代码中是否有可能多次调用WKWatchConnectivityRefreshBackgroundTask 函数setTaskCompleted。确实是这样：

当我开始开发我的手表扩展时，我复制了Apple demo code。它包含以下功能：

func handle(_ backgroundTasks: Set<WKRefreshBackgroundTask>) {
    for backgroundTask in backgroundTasks {
        if let wcBackgroundTask = backgroundTask as? WKWatchConnectivityRefreshBackgroundTask {
            // store a reference to the task objects as we might have to wait to complete them
            self.wcBackgroundTasks.append(wcBackgroundTask)
        } else {
            // immediately complete all other task types as we have not added support for them
            backgroundTask.setTaskCompleted()
        }
    }
    completeAllTasksIfReady()
}

这里，应该处理的WKWatchConnectivityRefreshBackgroundTask 存储在数组wcBackgroundTasks 中。稍后在completeAllTasksIfReady() 这个数组中的所有任务都由wcBackgroundTasks.forEach { $0.setTaskCompleted() } 完成。

我的错是我将我的WKWatchConnectivityRefreshBackgroundTask 存储在这个数组中（后来为它调用了setTaskCompleted）并立即处理它并设置了setTaskCompleted。所以这被调用了两次。
感谢发现这个的 Apple 工程师。