android: log through android.util.Log, and unbreak release builds

[hpoul]

android/build.gradle declared slf4j-api and kotlin-logging-jvm. Both are
facades, and there was no provider — no slf4j-android, no logback-android,
no slf4j-simple. SLF4J 2.x with no provider reports No SLF4J providers were found once and discards every call after that, so the plugin's Android
logging has produced no output at all unless a consuming app happened to ship
a binding of its own.

It also broke release builds. kotlin-logging names every backend it can bind
to, so R8 walked references to ch.qos.logback.* classes that were genuinely
missing and refused to complete:

Missing class ch.qos.logback.classic.Logger (referenced from: ...)
Execution failed for task ':app:minifyReleaseWithR8'
> Compilation failed to complete

Minification runs in release and nowhere else, so every app that added
6.0.0-dev.4 hit this at the last possible moment — on the way to Play.

The CI job you'd expect to catch this already existed

This is the part worth reading, because it changes what the durable fix is.

.github/workflows/dart.yml already builds appbundle, and the example app
already has minifyEnabled true. R8 has been running in CI on every push.
It stayed green because the example app was not a realistic consumer — it
carried its own slf4j-api, logback-android and kotlin-logging, plus an
assets/logback.xml wiring logback's LogcatAppender at TRACE.

That provider did two things. It supplied the very ch.qos.logback.* classes
R8 could not find, so the build completed here while breaking everywhere else.
And it made the plugin's logging appear to work during development, which is
presumably why nobody noticed consumers were getting nothing.

So the fix isn't a new CI job, it's deleting a dependency from the example. With
no provider there, the existing appbundle build now fails exactly the way a
consumer's does.

The three commits

1. android: fill in the consumer proguard rules — consumerProguardFiles 'proguard.pro' was already declared and proguard.pro was zero bytes; the
   mechanism was wired up and never filled in. -dontwarn rather than -keep,
   since keeping them would ask R8 to preserve classes that do not exist. This
   commit stands alone as the unblock.
2. android: log through android.util.Log instead of slf4j — a drop-in
   PluginLog object, so all 35 call sites compile untouched and the change is
   an import and a factory line, three times. Both dependencies removed.
3. example: stop shipping a logging backend — the durable half.

The two decisions you asked me to record

Tag: one constant, BiometricStorage — not derived from the class name.
minSdk is 23, and below API 24 Log.isLoggable throws
IllegalArgumentException for a tag over 23 characters. The derived name for
the largest file here would be BiometricStoragePluginKt, which is 24. A
truncated tag would also be unguessable for setprop; one tag means one command
turns the whole plugin on.

Level: BuildConfig.DEBUG || Log.isLoggable(TAG, level) — isLoggable
alone answers false below INFO, which would leave trace and debug (most of
the call sites) silent until someone ran setprop, and silent by default is
precisely the behavior being fixed. Debug builds now log everything, matching
what the example's logback.xml used to configure; release stays quiet unless
asked. Needs buildFeatures.buildConfig, since AGP 8 stopped generating
BuildConfig for library modules by default.

Verification

Red before, green after — the failure was made reachable first:

build	proguard.pro	plugin deps	example provider	result
appbundle --release	empty	slf4j + kotlin-logging	none	fails, the 4 missing logback classes
appbundle --release	rules	slf4j + kotlin-logging	none	passes
appbundle --release	rules	none	none	passes
appbundle --release	empty	none	none	passes

Logging confirmed on an emulator, which is the claim that has never been true
before. Debug build, no provider on the classpath:

V BiometricStorage: onMethodCall(canAuthenticate)
V BiometricStorage: Initialized BiometricStorageFile(masterKeyName='default_unauthenticated_master_key', ...) with InitOptions(...)

And on a minified release APK: zero plugin lines by default, then after
adb shell setprop log.tag.BiometricStorage VERBOSE, the same lines appear —
which also shows R8 did not strip PluginLog.

Also flutter test (9 passing, including the win32 guard), flutter analyze --fatal-infos clean in both packages, cd example && flutter test.

Two things for you to decide

The consumer rules are now redundant, and that last table row is the
evidence — with the dependencies gone, the release build passes with
proguard.pro empty. I kept them because they're the standalone unblock and
they'd also cover a consumer who brings kotlin-logging themselves. But
consumer rules apply to the consuming app's entire R8 run, so
-dontwarn org.slf4j.** shipping inside our AAR would silence missing-class
warnings in their code too. Say the word and I'll drop the file.

dart format --set-exit-if-changed is already failing on main — four
.dart files this branch never touches (biometric_storage.dart is 93 lines
of drift), from a newer formatter in Dart 3.13. CI's "Verify formatting" step
will be red on this PR for that reason alone. I left it per CLAUDE.md's "do not
fix unrelated existing violations"; happy to do it as its own PR.

🤖 Generated with Claude Code

[hpoul]

Added a fourth commit, 179f150, adding BiometricStorageLogging — a settable level and a hook for a custom logging framework. Both optional and null by default, so nothing changes for an app that ignores them.

The motivation turned out to be concrete rather than hypothetical. authpass binds slf4j to uk.uuid.slf4j:slf4j-android with level=VERBOSE in config.properties, deliberately, in every build type — so it is one of the few consumers whose plugin logs actually worked, and moving off slf4j would have silently cost it those logs in release builds. (It has no logback: the logback.xml in its assets is dead, with no dependency to read it.)

// keep verbose logging in a release build, no setprop needed
BiometricStorageLogging.level = Log.VERBOSE

// or hand every record to your own framework
BiometricStorageLogging.sink =
    BiometricStorageLogging.Sink { priority, tag, message, throwable ->
        // slf4j, Timber, a file appender, a crash reporter
    }

throwable is passed alongside message rather than flattened into it, so an implementation can report the real exception; priority is an android.util.Log constant so it maps onto most frameworks directly.

Installing a sink also turns every level on unless level says otherwise — a sink is an explicit statement that something wants the records, and the automatic default describes logcat rather than somebody else's framework. Without that rule, a sink installed in a release build would silently receive almost nothing.

The inline short-circuit survives: the level is still decided before the message lambda runs, so a filtered-out logger.debug { "expensive $x" } builds no string whether or not a sink is installed.

Verified on a minified release APK of the example, where the plugin is otherwise silent:

D SinkProof: [BiometricStorage] Attached to new activity.
V SinkProof: [BiometricStorage] onMethodCall(canAuthenticate)

Records arrive tagged by the sink and not by the plugin, so they are diverted rather than duplicated; and they arrive at VERBOSE with no level set, confirming the sink rule. R8 keeps the API and the SAM conversion.

The example app itself is left clean — it is the R8 canary, and adding a logging framework back to it is exactly what hid this bug. The wiring above was a throwaway edit for the test. Usage is documented in the README's new "Logging" section instead.

[hpoul]

Dropped the consumer rules in 3fd53e9 — android/proguard.pro is deleted and consumerProguardFiles is gone with it, rather than left pointing at an empty file. An empty rules file reads as "considered, nothing needed", which is exactly the wrong signal: the previous one had never been filled in, and that is how this shipped in the first place.

Verified: flutter build appbundle --release in example/ still completes minifyReleaseWithR8 with the file deleted.

I left commits 3fbdd13 (adds the rules) and 3fd53e9 (removes them) as a pair rather than rewriting history, since this PR already has review comments pointing at commits. The add/remove churn is deliberate and the messages explain it: 3fbdd13 was the standalone unblock for anyone on 6.0.0-dev.4, and it stopped being load-bearing once the dependencies went. Happy to squash if you would rather the PR not contain the round trip.

The CHANGELOG's "fixed at both ends" line is updated accordingly — it now just says the dependencies are gone, and tells anyone who added -dontwarn ch.qos.logback.** to their own proguard-rules.pro that they can drop it.

Correction on the formatting

I was wrong earlier: dart format is not failing, and there is nothing to fix. CI's step passes.

What I measured the first time was an artifact of my own environment. I ran dart format in a fresh worktree before any flutter pub get had populated .dart_tool/, and without package_config.json the formatter cannot resolve the package's language version, so it falls back to the latest and reports four files as needing changes.

Confirmed by moving package_config.json aside and back:

.dart_tool/package_config.json	result
absent	Formatted 12 files (4 changed), exit 1
present (languageVersion 3.10)	Formatted 12 files (0 changed), exit 0

CI always runs flutter pub get before the format step, so it sees exit 0. The repo is correctly formatted and I have not touched any .dart file. Sorry for the noise — no separate formatting PR is needed.

[Copilot]

Pull request overview

This PR fixes Android release build failures caused by missing SLF4J backends during R8 minification, and replaces the plugin’s Android logging with a android.util.Log-based implementation that can be forwarded into an app’s preferred logging system.

Changes:

• Replace slf4j-api/kotlin-logging usage with an internal PluginLog wrapper over android.util.Log, plus a public BiometricStorageLogging hook for level/sink configuration.
• Remove logging backend dependencies from the example app (and delete its logback.xml) so CI’s existing release appbundle build actually exercises the “real consumer” case.
• Bump version to 6.0.0-dev.5 and document the new Android logging behavior in README/CHANGELOG.

Reviewed changes

Copilot reviewed 14 out of 16 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
README.md	Documents Android logging defaults and how to install a sink/level override.
pubspec.yaml	Version bump to 6.0.0-dev.5.
CHANGELOG.md	Release notes for the Android R8 fix and the new logging behavior/hooks.
android/build.gradle	Removes logging deps, enables buildConfig, and removes unused consumer ProGuard wiring.
android/src/main/kotlin/design/codeux/biometric_storage/PluginLog.kt	New internal logging wrapper over android.util.Log with lazy message construction.
android/src/main/kotlin/design/codeux/biometric_storage/BiometricStorageLogging.kt	New public Android logging configuration API (level + sink).
android/src/main/kotlin/design/codeux/biometric_storage/BiometricStoragePlugin.kt	Switches logger factory to PluginLog.
android/src/main/kotlin/design/codeux/biometric_storage/BiometricStorageFile.kt	Switches logger factory to PluginLog.
android/src/main/kotlin/design/codeux/biometric_storage/CryptographyManager.kt	Switches logger factory to PluginLog.
example/android/app/build.gradle	Removes SLF4J/logback/kotlin-logging dependencies and documents why.
example/android/app/src/main/assets/logback.xml	Removes the example’s logback configuration.
example/android/app/src/main/kotlin/design/codeux/biometric_storage_example/MainActivity.kt	Removes kotlin-logging usage and clarifies why FlutterFragmentActivity is required.
example/.gitignore	Ignores Kotlin’s per-build .kotlin directory.
example/pubspec.lock	Updates path dependency version to 6.0.0-dev.5.
.github/workflows/dart.yml	Adds clarification that the release appbundle job is the R8 guardrail.
android/proguard.pro	Removed in the PR branch (no longer present).

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[hpoul]

Fixed the Copilot finding in 7d5ddd7 — the slf4j snippet lists its imports, so a reader is entitled to assume the list is complete, and LoggerFactory had none. Added import org.slf4j.LoggerFactory.

That was the only actionable item in the review; the rest was a summary of the diff.