Allow lookup template attributes from globalTemplateAttributes

[vikage]

This PR allow xcodegen to lookup template attributes from globalTemplateAttributes spec to share attributes accross all yaml files.
For example here, we have a shared today-extension-target.yml between multi project specs: legacy-project.yml and new-project.yml
We need to configure different config files between legacy and old. With this PR we just declare a version attribute from project spec and today-extension-target.yml can access it

Example
today-extension-target.yml

targetTemplates:
  TodayExtTemplate:
    platform: iOS
    type: app-extension
    settings:
      CODE_SIGN_IDENTITY: Apple Development
    configFiles:
      Debug: Config/${version}/Debug.xcconfig
      Release: Config/${version}/Release.xcconfig
    sources:
      - TodayExtension

targets:
  TodayExt:
    templates:
      - TodayExtTemplate

legacy-project.yml

name: Demo

options:
  createIntermediateGroups: True
  
globalTemplateAttributes:
  version: legacy

include:
  - today-extension-target.yml

targets:
  Demo:
    type: application
    platform: iOS
    deploymentTarget: "10.0"
    sources:
      - DemoXcodeGenGlobalTemplateAttribute

new-project.yml

name: Demo

options:
  createIntermediateGroups: True
  
globalTemplateAttributes:
  version: new

include:
  - today-extension-target.yml

targets:
  Demo:
    type: application
    platform: iOS
    deploymentTarget: "10.0"
    sources:
      - DemoXcodeGenGlobalTemplateAttribute

[vikage]

@yonaskolb @brentleyjones Please help me review this PR

[vikage]

@yonaskolb Please help me review this PR. Thanks

[yonaskolb]

I can see the utility in this @vikage. Got a few notes:

• does the existing functionality of referencing environment variables in the spec also solve your issue? eg Config/${VERSION}/Debug.xcconfig and VERSION=new xcodegen generate
• If this were added I can see this being useful just as globally accessible values, as opposed to restricting to just target or scheme templates. What do you think?
• If this were moved forward, it would definitely need some unit tests, and an example in Tests/Fixtures/TestProject/project.yml

[vikage]

I can see the utility in this @vikage. Got a few notes:

• does the existing functionality of referencing environment variables in the spec also solve your issue? eg Config/${VERSION}/Debug.xcconfig and VERSION=new xcodegen generate
• If this were added I can see this being useful just as globally accessible values, as opposed to restricting to just target or scheme templates. What do you think?
• If this were moved forward, it would definitely need some unit tests, and an example in Tests/Fixtures/TestProject/project.yml

@yonaskolb

• Currently, exporting a variable on the command line could fix my issue. But I think it will be great if we have it in the yml config file.
• If this was added, we could access variables globally without any restrictions.

If you are agree with this feature, I will implement an example and some unit tests

[yonaskolb]

Yes happy for you to move it forward, with the ability to reference these values anywhere and not just within templates

[vikage]

@yonaskolb I added unit test and example. Please help me review.
Thanks

[vikage]

@yonaskolb Please help me take a look

[vikage]

@yonaskolb Please review, thanks