[info.plist] Xcode 10 Error: Multiple commands produce

Read this answer if error message references Core Data files

Synopsis: You may have both automatically-generated and manually-generated Core Data managed object class files.

This answer applies if the first line of the error refers to a Foo+CoreDataProperties.o or Foo+CoreDataClass.o file. Example:

error: Multiple commands produce '/Users/me/Library/Developer/Xcode/DerivedData/MyApp-uebslaqdwgldkjemijpdqmizgyzc/Build/Intermediates.noindex/ MyApp /Debug-iphonesimulator/ MyApp.build/Objects-normal/x86_64/Foo+CoreDataProperties.o':

1) Target ' MyApp ' (project ' MyApp ') has compile command for Swift source files

2) Target ' MyApp ' (project ' MyApp ') has compile command for Swift source files

The root cause can be seen by expanding the Compile Swift Source Files section of the Build Transcript. For example:

<unknown>:0: error: filename "Address+CoreDataClass.swift" used twice: '/Users/myUserName/Projects/Jnky/Foo+CoreDataProperties' and '/Users/jk/myUserName/Developer/Xcode/DerivedData/MyApp-uebslaqdwgldkjemijpdqmizgyzc/Build/Intermediates.noindex/MyApp.build/Debug/MyApp.build/DerivedSources/CoreDataGenerated/Jnky/Foo+CoreDataProperties.swift'

The first file mentioned there is a source file in your project directory, which someone generated by selecting your data model in the Project Navigator and clicking in the menu Editor > Create Managed Object Subclass. This feature was added in Xcode 7 or so.

The second file is a file of the same name but which is buried in Xcode's DerivedData. This file is generated automatically by Xcode during every build if the data model (.xcdatamodeld) file is included in the target's Compile Sources build phase. This feature was added in Xcode 9 or so. Zero, one or two files are generated for each entity/class, depending on the setting of the Codegen popup. That popup is in the Data Model Inspector when you select an entity while editing your data model…

The settings are:

• Manual/None No files are generated
• Category/Extension One file, Foo+CoreDataProperties.m or .swift is generated, containing an Objective-C category or Swift extension.
• Class Definition That same Category/Extension file is generated, and in addition a Foo+CoreDataClass.m or .swift is generated, containing class declaration and definition.

So you see the problem occurs when a developer (like me) who is accustomed to the older Xcode begins a project in a newer Xcode. We think that we need to use the Create Managed Object Subclass menu item, which we do, to create the files we can see in the Project Navigator while not realizing that our settings in the Codegen popup are causing Xcode to create duplicate files, which Apple "cleverly" does not show in the Project Navigator, because they don't trust developers to read and heed the comment in the header // This file was automatically generated and should not be edited.

Solution 1 - Use the Older Way

You can disable all automatic Codegen for a data model with just one setting:

• Open the problem Target's Build Phases (In Project Navigator, select project, then in list of TARGETS which appears, select the problem target, then tab Build Phases).
• Expand the Compile Sources entry and find the problem data model (.xcdatamodeld file).
• Delete it from the compile list
• Ensure the data model is included in the Copy Bundle Resources list.

Solution 2 - Core Data Magic For Beginners

Here, you go all in on the newer way.

• Leave your data model as is in that Compile Sources.
• In each Entity Inspector in your data model, set Codegen to Class Definition.
• In the Project Navigator, delete and trash any Foo+CoreDataClass files, and rename any Foo+CoreDataProperties.m or .swift files to something like Foo+MyProperties.
• In each Foo+MyProperties.m or .swift file, if there are properties generated by Xcode, delete these properties because they will be in the hidden files created by Codegen.

With this solution, your class definitions are generated automatically from the data model on each build. You can't even see them. It is Core Data Magic, nice and simple for beginners.

Solution 3 - For Most Real-World Apps

But Solution 2 is no good if you really want to add non-managed properties. (Objective-C does not allow properties to be added in categories, and Swift does not allow stored properties to be added in extensions.) So in most real-world apps, you probably want to go halfway between Solutions 1 and 2…

• Leave your data model in the list of Compile Sources
• In each Entity Inspector in your data model, set Codegen to Category/Extension.
• In the Project Navigator, delete and trash any Foo+CoreDataClass.m or .swift files, and, to reduce future confusion, rename any Foo+CoreDataProperties.m or .swift files to maybe just Foo.m or .swift.
• Ensure that each Foo.m or .swift file contains the class definition, to which you can add your own non-managed properties.

(Acknowledgments to the answer by Positron. My answer here explains why Positron's answer (my Solution 1) works, and adds Solution 2 and Solution 3.)