主线程检查器报错：UIImagePickerController后台线程调用UI API

点击导航栏按钮打开UIImagePickerController时，调试器弹出主线程检查器错误：UI API在后台线程被调用-[UIImagePickerController didDisplayPhotoPickerSourceType:]，找不到该方法的官方文档。已确保在主线程present图片选择器，但错误依然出现。

相关代码片段 #

override func viewDidLoad() {
    super.viewDidLoad()
    collectionView.backgroundColor = .blue
    register()
    setUp()
    collectionView.reloadData()
}

private func setUp() {
    navigationController?.isNavigationBarHidden = false
    let layout = collectionView.collectionViewLayout as! UICollectionViewFlowLayout
    layout.minimumInteritemSpacing = 2
    layout.minimumLineSpacing = 2
    collectionView.collectionViewLayout = layout
    
    navigationItem.leftBarButtonItem = UIBarButtonItem(barButtonSystemItem: .add, target: self, action: #selector(addNewPerson))
}

@objc private func addNewPerson() {
    DispatchQueue.main.async { [weak self] in
        let picker = UIImagePickerController()
        picker.allowsEditing = true
        picker.delegate = self
        self?.present(picker, animated: false)
    }
}

调试日志 #

Main Thread Checker: UI API called on a background thread: -[UIImagePickerController didDisplayPhotoPickerSourceType:]
PID: 85527, TID: 2803975, Thread name: (none), Queue name: com.apple.NSXPCConnection.user.anonymous.85796, QoS: 0
Backtrace:
4   PhotosUI                            0x00000001ad0380c8 -[PUPhotoPickerExtensionHostContext didDisplayPhotoPickerSourceType:] + 96
5   Foundation                          0x0000000180fb6c10 __NSXPCCONNECTION_IS_CALLING_OUT_TO_EXPORTED_OBJECT_S1__ + 12
6   Foundation                          0x0000000180fb59e0 -[NSXPCConnection _decodeAndInvokeMessageWithEvent:reply:flags:] + 1664
7   Foundation                          0x0000000180fb79ec message_handler_message + 84
8   Foundation                          0x0000000180fb6f80 message_handler + 148
9   libxpc.dylib                        0x00000001800cca0c _xpc_connection_call_event_handler + 68
10  libxpc.dylib                        0x00000001800ce10c _xpc_connection_mach_event + 1048
11  libdispatch.dylib                   0x0000000105b3f904 _dispatch_client_callout4 + 12
12  libdispatch.dylib                   0x0000000105b42a90 _dispatch_mach_msg_invoke + 672
13  libdispatch.dylib                   0x0000000105b2d75c _dispatch_lane_serial_drain + 324
14  libdispatch.dylib                   0x0000000105b43d50 _dispatch_mach_invoke + 504
15  libdispatch.dylib                   0x0000000105b2d75c _dispatch_lane_serial_drain + 324
16  libdispatch.dylib                   0x0000000105b2e708 _dispatch_lane_invoke + 420
17  libdispatch.dylib                   0x0000000105b3b3ac _dispatch_root_queue_drain_deferred_wlh + 324
18  libdispatch.dylib                   0x0000000105b3a918 _dispatch_workloop_worker_thread + 732
19  libsystem_pthread.dylib             0x0000000247927b74 _pthread_wqthread + 284
20  libsystem_pthread.dylib             0x0000000247926934 start_wqthread + 8
Main Thread Checker: UI API called on a background thread: -[UIImagePickerController didDisplayPhotoPickerSourceType:]
PID: 85527, TID: 2803975, Thread name: (none), Queue name: com.apple.NSXPCConnection.user.anonymous.85796, QoS: 0
Backtrace:
4   PhotosUI                            0x00000001ad0380c8 -[PUPhotoPickerExtensionHostContext didDisplayPhotoPickerSourceType:] + 96
5   Foundation                          0x0000000180fb6c10 __NSXPCCONNECTION_IS_CALLING_OUT_TO_EXPORTED_OBJECT_S1__ + 12
6   Foundation                          0x0000000180fb59e0 -[NSXPCConnection _decodeAndInvokeMessageWithEvent:reply:flags:] + 1664
7   Foundation                          0x0000000180fb79ec message_handler_message + 84
8   Foundation                          0x0000000180fb6f80 message_handler + 148
9   libxpc.dylib                        0x00000001800cca0c _xpc_connection_call_event_handler + 68
10  libxpc.dylib                        0x00000001800ce10c _xpc_connection_mach_event + 1048
11  libdispatch.dylib                   0x0000000105b3f904 _dispatch_client_callout4 + 12
12  libdispatch.dylib                   0x0000000105b42a90 _dispatch_mach_msg_invoke + 672
13  libdispatch.dylib                   0x0000000105b2d75c _dispatch_lane_serial_drain + 324
14  libdispatch.dylib                   0x0000000105b43d50 _dispatch_mach_invoke + 504
15  libdispatch.dylib                   0x0000000105b2d75c _dispatch_lane_serial_drain + 324
16  libdispatch.dylib                   0x0000000105b2e708 _dispatch_lane_invoke + 420
17  libdispatch.dylib                   0x0000000105b3b3ac _dispatch_root_queue_drain_deferred_wlh + 324
18  libdispatch.dylib                   0x0000000105b3a918 _dispatch_workloop_worker_thread + 732
19  libsystem_pthread.dylib             0x0000000247927b74 _pthread_wqthread + 284
20  libsystem_pthread.dylib             0x0000000247926934 start_wqthread + 8

解决方案 #

这个错误属于Apple PhotosUI框架内部问题，和你的代码无关。从堆栈日志能看到，错误来自PUPhotoPickerExtensionHostContext的内部NSXPC通信回调，该回调被分发到后台线程，触发了主线程检查器警告。

可以按以下方式处理：

• 直接忽略警告：该警告不会影响App功能，只是框架内部线程调用不符合规范，发布版本中主线程检查器默认关闭，不会出现这个提示。
• 临时禁用主线程检查器：开发时如果不想看到这个警告，可在Scheme设置中关闭"Main Thread Checker"（路径：Edit Scheme -> Run -> Diagnostics -> 取消勾选Main Thread Checker）。
• 改用PHPickerViewController：如果想彻底避免这类问题，推荐使用iOS 14+推出的PHPickerViewController，它是UIImagePickerController的现代替代方案，设计更合理，能避免很多旧框架的内部问题。示例代码如下：

import PhotosUI

@objc private func addNewPerson() {
    DispatchQueue.main.async { [weak self] in
        var config = PHPickerConfiguration()
        config.filter = .images
        config.selectionLimit = 1
        config.allowsEditing = true
        
        let picker = PHPickerViewController(configuration: config)
        picker.delegate = self
        self?.present(picker, animated: true)
    }
}

// 实现PHPickerViewControllerDelegate
extension YourViewController: PHPickerViewControllerDelegate {
    func picker(_ picker: PHPickerViewController, didFinishPicking results: [PHPickerResult]) {
        picker.dismiss(animated: true)
        
        guard let result = results.first else { return }
        if result.itemProvider.canLoadObject(ofClass: UIImage.self) {
            result.itemProvider.loadObject(ofClass: UIImage.self) { image, error in
                DispatchQueue.main.async {
                    // 处理选中的图片
                    if let selectedImage = image as? UIImage {
                        // 执行你的业务逻辑
                    }
                }
            }
        }
    }
}

内容的提问来源于stack exchange，提问作者brunshte