How to setup spotless for a git pre commit hook that only check changed files

[Zkffkah]

I'm using spotless with an android kotlin project now. And I have spotless set up like this

apply plugin: 'com.diffplug.gradle.spotless'
spotless {
    kotlin {
        target "**/*.kt"
        ktlint(versions.ktlint)
    }
}

What I want to do is set up a git pre commit hook that apply gradlew spotlessCheck before the commit but only on the commited files. It's something like this eslint-pre-commit-check
I have search around the repo but can't figure out how to achieve this. Is there any way to do this? Thanks.

[nedtwigg]

The spotless gradle plugin has fast up-to-date checking built-in, so performance should be very good even without limiting to changed files. If your objective is to enforce the check only on changed files, that is a good idea, but it is not supported at this time. EDIT: now supported via ratchetFrom.

(bad advice from me deleted here)

[Zkffkah]

Understood. Following is my workaround now. Hope it helps someone else who want to achieve this too.

#!/bin/sh
# From gist at https://gist.github.com/chadmaughan/5889802

echo '[git hook] executing gradle spotlessCheck before commit'

# stash any unstaged changes
git stash -q --keep-index

# run the spotlessCheck with the gradle wrapper
./gradlew spotlessCheck --daemon

# store the last exit code in a variable
RESULT=$?

# unstash the unstashed changes
git stash pop -q

# return the './gradlew spotlessCheck' exit code
exit $RESULT

[nedtwigg]

Our latest versions (4.2.1 as of writing) have a feature called ratchetFrom. It's not exactly the same as this, but very close, and should perform much better. I'm closing this issue, but I might be wrong. If ratchetFrom does not make this (excellent!) workaround obsolete for you, please let me know!

[ehuisman]

ratchetFrom is very useful, but it does not, in fact make this obsolete, since it does not differentiate between changes staged for the commit and changes not staged for the commit. In order for a pre-commit hook to be useful, it should preferably only consider staged changes. Otherwise running spotlessApply in a hook makes it possible to make commits which do not in fact contain the formatting fixes.

(Also, if there is a way to make spotlessApply return a non-zero value if it does in fact change any files, that would be extremely useful as well. Now I'm running spotlessCheck first to find out if the hook should fail or not, then running spotlessApply to fix the problems automatically before failing. This probably does warrant opening a separate issue.)