iOS中关于NotificationCenter通知线程问题

在iOS开发中，NotificationCenter是经常会用到的，我一般发通知会自己封装一个方法，在主线程中进行发送。

func postNotification(name: NSNotification.Name, object: Any?, _ userInfo: [AnyHashable : Any]? = nil){
        DispatchQueue.main.async {
            NotificationCenter.default.post(name: name, object: object, userInfo: userInfo)
        }
    }

因为我的想法是在接收到通知后有可能会操作UI，如果某次发送通知是在子线程(例如网络请求，请求到特定数据后发出通知)，接收到通知更新UI，会造成程序崩溃。为了验证这个想法，我做了一个简单的测试。

测试

首先在子线程中发送通知，在接收到通知后更新界面中label的显示

运行程序，结果是程序不会崩溃，但是界面并不会更新任何信息，控制台打印信息如下：

=================================================================
Main Thread Checker: UI API called on a background thread: -[UILabel setText:]
PID: 1248, TID: 222437, Thread name: (none), Queue name: com.apple.root.default-qos, QoS: 0
Backtrace:
4   testnoti                            0x000000010420bc68 _T08testnoti14ViewControllerC8updateUIyyF + 200
5   testnoti                            0x000000010420be44 _T08testnoti14ViewControllerC8updateUIyyFTo + 40
6   CoreFoundation                      0x00000001c3a8b068 <redacted> + 20
7   CoreFoundation                      0x00000001c3a8b034 <redacted> + 64
8   CoreFoundation                      0x00000001c3a8a720 <redacted> + 392
9   CoreFoundation                      0x00000001c3a8a3cc <redacted> + 96
10  CoreFoundation                      0x00000001c3afe060 <redacted> + 1404
11  CoreFoundation                      0x00000001c39da164 _CFXNotificationPost + 384
12  Foundation                          0x00000001c44583b4 <redacted> + 68
13  testnoti                            0x000000010420b9d8 _T08testnoti14ViewControllerC11viewDidLoadyyFyycfU_ + 512
14  testnoti                            0x000000010420ba2c _T0Ix_IyB_TR + 48
15  libdispatch.dylib                   0x0000000105021338 _dispatch_client_callout + 16
16  libdispatch.dylib                   0x000000010502c5e4 _dispatch_continuation_pop + 528
17  libdispatch.dylib                   0x000000010502314c _dispatch_source_invoke + 1880
18  libdispatch.dylib                   0x000000010502bef4 _dispatch_queue_override_invoke + 448
19  libdispatch.dylib                   0x00000001050393ac _dispatch_root_queue_drain + 376
20  libdispatch.dylib                   0x0000000105039db0 _dispatch_worker_thread2 + 156
21  libsystem_pthread.dylib             0x00000001c37155c0 _pthread_wqthread + 480
22  libsystem_pthread.dylib             0x00000001c371536c start_wqthread + 4
2018-07-03 14:14:48.830508+0800 testnoti[1248:222437] [reports] Main Thread Checker: UI API called on a background thread: -[UILabel setText:]
PID: 1248, TID: 222437, Thread name: (none), Queue name: com.apple.root.default-qos, QoS: 0
Backtrace:
4   testnoti                            0x000000010420bc68 _T08testnoti14ViewControllerC8updateUIyyF + 200
5   testnoti                            0x000000010420be44 _T08testnoti14ViewControllerC8updateUIyyFTo + 40
6   CoreFoundation                      0x00000001c3a8b068 <redacted> + 20
7   CoreFoundation                      0x00000001c3a8b034 <redacted> + 64
8   CoreFoundation                      0x00000001c3a8a720 <redacted> + 392
9   CoreFoundation                      0x00000001c3a8a3cc <redacted> + 96
10  CoreFoundation                      0x00000001c3afe060 <redacted> + 1404
11  CoreFoundation                      0x00000001c39da164 _CFXNotificationPost + 384
12  Foundation                          0x00000001c44583b4 <redacted> + 68
13  testnoti                            0x000000010420b9d8 _T08testnoti14ViewControllerC11viewDidLoadyyFyycfU_ + 512
14  testnoti                            0x000000010420ba2c _T0Ix_IyB_TR + 48
15  libdispatch.dylib                   0x0000000105021338 _dispatch_client_callout + 16
16  libdispatch.dylib                   0x000000010502c5e4 _dispatch_continuation_pop + 528
17  libdispatch.dylib                   0x000000010502314c _dispatch_source_invoke + 1880
18  libdispatch.dylib                   0x000000010502bef4 _dispatch_queue_override_invoke + 448
19  libdispatch.dylib                   0x00000001050393ac _dispatch_root_queue_drain + 376
20  libdispatch.dylib                   0x0000000105039db0 _dispatch_worker_thread2 + 156
21  libsystem_pthread.dylib             0x00000001c37155c0 _pthread_wqthread + 480
22  libsystem_pthread.dylib             0x00000001c371536c start_wqthread + 4
2018-07-03 14:14:56.519094+0800 testnoti[1248:222437] [Animation] +[UIView setAnimationsEnabled:] being called from a background thread. Performing any operation from a background thread on UIView or a subclass is not supported and may result in unexpected and insidious behavior. trace=(
    0   UIKitCore                           0x00000001efeaebbc <redacted> + 136
    1   libdispatch.dylib                   0x0000000105021338 _dispatch_client_callout + 16
    2   libdispatch.dylib                   0x0000000105022150 dispatch_once_f + 120
    3   UIKitCore                           0x00000001efeaeb30 <redacted> + 100
    4   UIKitCore                           0x00000001efeaecb8 <redacted> + 92
    5   UIKitCore                           0x00000001f01278f0 <redacted> + 372
    6   UIKitCore                           0x00000001f01279e4 <redacted> + 32
    7   QuartzCore                          0x00000001c8272c60 <redacted> + 332
    8   QuartzCore                          0x00000001c81d34fc <redacted> + 328
    9   QuartzCore                          0x00000001c8200d74 <redacted> + 596
    10  QuartzCore                          0x00000001c8201514 <redacted> + 252
    11  libsystem_pthread.dylib             0x00000001c3715a58 <redacted> + 584
    12  libsystem_pthread.dylib             0x00000001c37157e8 <redacted> + 80
    13  libsystem_pthread.dylib             0x00000001c3721058 <redacted> + 80
    14  libsystem_pthread.dylib             0x00000001c3715550 _pthread_wqthread + 368
    15  libsystem_pthread.dylib             0x00000001c371536c start_wqthread + 4
)

很明显是在告诉我们，操作UI线程错误，必须在主线程操作，在代码中Xcode也给出了提示：

现在改动一下代码，使通知在主线程发送

运行程序，5秒后界面得到及时更新，所以在使用通知的时候，需要确定接收通知是否会进行UI操作，如果需要操作UI，则在主线程发送通知(封装后比较方便)或者接收通知后在主线程更新UI。