Doctor summary (to see all details, run flutter doctor -v):

[✓] Flutter (Channel unknown, v1.12.13+hotfix.9, on Mac OS X 10.15.6 19G2021, locale zh-Hans-CN)

[✓] Android toolchain - develop for Android devices (Android SDK version 30.0.2)

[✓] Xcode - develop for iOS and macOS (Xcode 11.7)

[✓] Android Studio (version 4.0)

[✓] VS Code (version 1.50.1)

[☠] Connected device (the doctor check crashed)

    ✗ Due to an error, the doctor check did not complete. If the error message below is not helpful, please let us

      know about this issue at https://github.com/flutter/flutter/issues.

    ✗ Exception: idevice_id returned an error:

      ERROR: Unable to retrieve device list!

解决办法:

输入 sudo pkill usbmuxd

os x上，苹果有一个服务，叫usbmuxd，这个服务主要用于在USB协议上实现多路TCP连接，将USB通信抽象为TCP通信。苹果的iTunes, XCode，都直接或者间接地用到了这个服务。