Optimize fastlane snapshot by more than 50%

Snapshot is a great tool for generating screenshots. However when you want to generate for so many devices and languages, it becomes a very long build process.

For example in this case:

lane :generate_screenshots do |options|
  scheme = "GenerateScreenshots-UITests"
  path = "~/Library/Developer/Xcode/DerivedData/AppDevelopment"
  devices = [
      "iPhone X",
      "iPhone Xs Max"
    ]
  snapshot(
    devices: devices,
    derived_data_path: path,
    languages: [
      "en-US",
      "nb-NO"
    ],
    output_directory: "fastlane/screenshots",
    scheme: scheme
  )
end

Snapshot will call this method 4 times!

set -o pipefail && xcodebuild -scheme GenerateScreenshots-UITests -project ./My.xcodeproj -derivedDataPath '~/Library/Developer/Xcode/DerivedData/' -destination 'platform=iOS Simulator,name=iPhone Xs Max,OS=12.4' FASTLANE_SNAPSHOT=YES clean build test

That means your server builds your app 4 times and runs the tests 4 times

Our proposed solution is to build one time and test 4 times.

lane :generate_screenshots do |options|
  scheme = "GenerateScreenshots-UITests"
  path = "~/Library/Developer/Xcode/DerivedData/AppDevelopment"
  xcversion
  devices = [
      "iPhone X",
      "iPhone Xs Max"
    ]
# building before running tests is faster because snapshot is building for each device
  run_tests(
    build_for_testing: true,
    code_coverage: false,
    derived_data_path: path,
    devices: devices,
    scheme: scheme
  )
  screenshots_output_directory = "fastlane/screenshots"
  snapshot(
    devices: devices,
    derived_data_path: path,
    languages: [
      "en-US",
      "nb-NO"
    ],
    output_directory: screenshots_output_directory,
    scheme: scheme,
    test_without_building: true
  )
end

For this example, 18 min was reduced to 9 min.