Gitlab/CI job rule evaluation - gitlab-ci

Question

I am trying to skip a GitLab ci job based on the results of the previous job, however, as a result, the job never runs. I have the impression that rules are evaluated at the beginning of the Pipeline not at the beginning of the Job. Is there any way to make it work?
cache:
paths:
- .images
stages:
- prepare
- build
dirs:
stage: prepare
image:
name: docker.image.me/run:latest
script:
- rm -rf .images/*
- [ $(($RANDOM % 2)) -eq 1 ] && touch .images/DESKTOP
desktop:
stage: build
needs: ["dirs"]
image:
name: docker.image.me/run:latest
rules:
- exists:
- .images/DESKTOP
when: always
script:
- echo "Why is this never launched?"

Answer 1

Dynamically created jobs could be a solution (https://docs.gitlab.com/ee/ci/parent_child_pipelines.html#dynamic-child-pipelines).
You could create a yml-file with a your "desktop"-job in section "script" in your "dirs"-job if ".images/DESKTOP" is created.
Else your created yml-file should be empty.
The created yml-file can be triggered in a seperat job after "dirs"-job.
I'm using for creating dynamic child pipelines jsonnet (https://jsonnet.org/).

Answer 2

The rules evaluation is happening at the beginning of a Gitlab pipeline
Quoting from Gitlab docs https://docs.gitlab.com/ee/ci/yaml/#rules
Rules are evaluated when the pipeline is created, and evaluated in order until the first match. When a match is found, the job is either included or excluded from the pipeline, depending on the configuration.
Here the problem seems to be with the usage of exists keyword
Quoting from Gitlab docs https://docs.gitlab.com/ee/ci/yaml/#rulesexists
Use exists to run a job when certain files exist in the repository
But here it seems that .images/DESKTOP is in Gitlab's runner cache, not in your repository.
cache:
paths:
- .images