Example usage:

string_flag(
    name = "ios_simulator_id",
    build_setting_default = "",
    visibility = ["//visibility:public"],
)

ios_test_runner(
    name = "custom_test_runner",
    simulator_id = "//:ios_simulator_id",
    visibility = ["//visibility:public"],
)

bazel test //... --//:ios_simulator_id="21461F1A-68DD-43E0-A795-D0329CB1E217"

I would have put this string_flag in rules_apple but it seems that you can not use it from external repos, so --@build_bazel_rules_apple//apple:ios_simulator_id=... doesn't work

Can you file a bug against bazel for rules sets not being able to define these and have them work? We'll get it raised internally.

I filed bazelbuild/bazel#10152 about this. I think in the meantime this would probably still be a fine way to handle this, but I guess it depends on how you feel about the public API changing if this starts working because I initial made simulator_id a private attr referencing the flag that was in rules_apple

Can you use test arguments for this? we've added support for these in our internal test runner:
“--destination=platform=ios_device,id={UUID}” targets on a local iOS real device.
“--destination=platform=ios_simulator,id={UUID}” targets on an existing iOS simulator.

and invoke it with blaze as such:
blaze test <target> --test_arg=--destination=id={UUID},platform=ios_simulator or you can also pass it in args of the ios_{unit,ui}_test targets in the BUILD file

I've submitted the --test_arg option here #653

#653 is the better approach