1、升级Xcode到Version 10.0 (10A255)后，运行已有项目，报如下错误：
error: Multiple commands produce '/Users/galahad/Library/Developer/Xcode/DerivedData/yoowei-drnrntneloepunakcqbcdycudqeh/Build/Products/Debug-iphoneos/yoowei.app':
1) Target 'yoowei' has create directory command with output '/Users/galahad/Library/Developer/Xcode/DerivedData/yoowei-drnrntneloepunakcqbcdycudqeh/Build/Products/Debug-iphoneos/yoowei.app'
2) That command depends on command in Target 'yoowei': script phase “[CP] Copy Pods Resources”
原因：Xcode 10 默认使用的build system是New build system，与Xcode9不同导致。
解决：2种方法
1）第一种方法 不修改build system
根据error 日志，script phase “[CP] Copy Pods Resources”，而且与output有关，应该是使用了cocoapods导致的，尝试删除该项目target-Copy Pods Resources-Output Files，成功解决问题。选中项目target -> Build phase -> Copy Pods Resources -> Output Files -> 移除${TARGET_BUILD_DIR}/${UNLOCALIZED_RESOURCES_FOLDER_PATH} 然后重新编译，OK 。
error: Cycle in dependencies between targets 'yooweiExtension' and 'yoowei'; building could produce unreliable results.
Cycle path: yooweiExtension → yoowei → yooweiExtension
Cycle details:
→ Target 'yooweiExtension': CodeSign /Users/galahad/Library/Developer/Xcode/DerivedData/yoowei-drnrntneloepunakcqbcdycudqeh/Build/Products/Debug-iphoneos/yooweiExtension.appex
○ Target 'yooweiExtension': ProcessProductPackaging /Users/galahad/Library/Developer/Xcode/DerivedData/yoowei-drnrntneloepunakcqbcdycudqeh/Build/Intermediates.noindex/yoowei.build/Debug-iphoneos/yooweiExtension.build/yooweiExtension.appex.xcent
○ Target 'yooweiExtension' has target dependency on Target 'yoowei'
→ Target 'yoowei' has target dependency on Target 'yooweiExtension'
○ That command depends on command in Target 'yooweiExtension': script phase “[CP] Check Pods Manifest.lock”
对应的解决方案：选中项目target -> Build phase -> Target Dependencies 去掉相互的依赖即可
2）第二种方法 修改build system
在Xcode菜单栏 -> File -> Workspace Setting，将build system修改为legacy build system，然后clean后编译。
2、典型问题
ld: library not found for -lstdc++.6.0.9
clang: error: linker command failed with exit code 1 (use -v to see invocation)
解决方案：（我一般升级xcode 之前都会将lib 给备份一份。具体路径是：/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/usr/lib ）
1、提前在xcode 9中 根据路径 /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/usr/lib 找到 libstdc++.6.0.9.tbd 备份
2、复制刚才的libstdc++.6.0.9.tbd 文件，手动添加到 升级后的xcode 10 /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/usr/lib 文件夹中，运行即可。
需要的话，可以找我要。注意分真机和模拟器环境：
1.真机环境：/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/usr/lib/
2.模拟器环境：/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator.sdk/usr/lib/
20190313遇到一种情况：编译报错 library not found for -l"stdc++.6.0.9"
比较奇怪就是，我已经按照上面将libstdc++.6.0.9.tbd 手动添加过了。但是为什么还会报错呢？
解决办法：
打开Build Phases--Link Binary With Libraries，删除6.0.9依赖 添加Libc++.tdb 即可。
如果依赖c++6.0.9的SDK是第三方SDK等待第三方更新解决。