How to build a xcode 9 project with Swift 4.0 using Pods in Swift 3?

If you are using some pods written in Swift 4, but some are Swift 3.2, here is the way you can specify the SWIFT_VERSION value for them:

swift_32 = ['Pod1', 'Pod2', 'Pod3'] # if these pods are in Swift 3.2
swift4 = ['Pod4', 'Pod5', 'Pod6'] # if these pods are in Swift 4

post_install do |installer|

    installer.pods_project.targets.each do |target|
        swift_version = nil

        if swift_32.include?(target.name)
            swift_version = '3.2'
        end

        if swift4.include?(target.name)
            swift_version = '4.0'
        end

        if swift_version
            target.build_configurations.each do |config|
                config.build_settings['SWIFT_VERSION'] = swift_version
            end
        end

    end

end

Here is a far less verbose way to set the pods you need to 3.2 and leave all others at 4.0

post_install do |installer|
  installer.pods_project.targets.each do |target|
      if ['AirMapSDK', 'PhoneNumberKit', 'Lock', 'RxSwift', 'RxSwiftExt', 'RxCocoa', 'RxDataSources', 'ProtocolBuffers-Swift'].include? target.name
          target.build_configurations.each do |config|
              config.build_settings['SWIFT_VERSION'] = '3.2'
          end
      end
  end
end

Just modify the array in the if statement. everything else will default to 4.0

Project Navigator > Choose 'Pods' > Choose the Swift 3.2 Pod > 'Build Settings' > Scroll down and then set Swift Language Version to 3.2 in 'Swift Compiler - Language section'.

On doing this, Xcode will show one Buildtime issue. It will ask you to convert the source code of pods to Swift 4. Don't do that. Click on that issue > Uncheck 'Remind Me' > Click 'Convert Later'.

Project Navigator

Build Settings