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Make SignaturePadStateImpl safe to call from any thread - #30
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Apps can clear, resize, feed input to or export the state off the main thread while the pad takes input and draws on it. The state changed several plain fields and a SnapshotStateList in steps, so a clear() on another thread could interleave with a move, and the draw pass could take a subList of a list that had just shrunk. ConcurrencyTest hits IndexOutOfBoundsException and ConcurrentModificationException on the old code within a second. - Every public method holds a lock (atomicfu's SynchronizedObject). - The finished strokes and the stroke in progress are immutable lists, held together in one snapshot state and replaced on each change, so the draw pass reads them without the lock and without tearing. - The finished strokes also have a state of their own, replaced only when they change, which the pad's cache of them observes. This replaces strokeCurveCount and finishedStrokesVersion. - drawOnBitmap copies the curves and size under the lock and draws outside it, so exporting doesn't hold up input. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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Check name Status Explanation Description Check Check skipped - CodeRabbit’s high-level summary is enabled. Title check The title clearly and concisely describes the main change: making SignaturePadStateImpl safe to call from any thread. Linked Issues check Check skipped because no linked issues were found for this pull request. Out of Scope Changes check Check skipped because no linked issues were found for this pull request.
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Docstring coverage is 39.53% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 43 functions across 7 files. (2 skipped: 2 unsupported.)
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Review comments at
@signaturepad/src/jvmTest/kotlin/com/seanproctor/signaturepad/ConcurrencyTest.kt:
- Around line 23-41: Update the `race` helper to join each worker thread with a
timeout and add an `AssertionError` to `failures` if the thread is still alive
afterward, so a hung worker fails the test instead of blocking the test run
indefinitely.
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CLAUDE.mdREADME.mdgradle/libs.versions.tomlsignaturepad/build.gradle.ktssignaturepad/src/commonMain/kotlin/com/seanproctor/signaturepad/SignaturePadState.ktsignaturepad/src/jvmTest/kotlin/com/seanproctor/signaturepad/ConcurrencyTest.kt
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Adding a curve copied the stroke so far, and ending a stroke copied the whole signature. With kotlinx-collections-immutable's persistent lists, both share the curves already there, so capturing a 4,000-curve signature is 24% faster than with plain lists, and 12% faster than on master. Resizing is 9% slower than with plain lists, still about 30% faster than master. ConcurrencyTest now caps the strokes each thread draws: with faster capture, its signature grew large enough for RecordingCanvas's samples to run the test JVM out of memory. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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Review comments at
@signaturepad/src/commonMain/kotlin/com/seanproctor/signaturepad/SignaturePadState.kt:
- Line 375: Update restoreFromFloatList() to set gestureActive to false when
applying the restored state, alongside setCurves(restored.build(),
persistentListOf()), matching the gesture reset behavior in clearCurves().
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CLAUDE.mdgradle/libs.versions.tomlsignaturepad/build.gradle.ktssignaturepad/src/commonMain/kotlin/com/seanproctor/signaturepad/SignaturePadState.ktsignaturepad/src/jvmTest/kotlin/com/seanproctor/signaturepad/ConcurrencyTest.kt
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Restoring replaces the whole signature, as clear() does, so a gesture in progress now stops there too, instead of drawing its later moves onto the restored signature. ConcurrencyTest's threads are daemons, and a thread that hasn't stopped 10 seconds after the run ends fails the test, so a deadlock fails it rather than hanging the build. Addresses CodeRabbit's review. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
- Every public method of SignaturePadStateImpl now runs in locked(), which holds the lock and escapes to the global snapshot. A caller inside its own snapshot (Snapshot.withMutableSnapshot, composition) kept its changes to itself until it applied, so a call on another thread built on the old curves, and the apply then failed with a SnapshotApplyConflictException. - finishedStrokes is a derivedStateOf over curves instead of a second state kept in step by hand. FinishedStrokesCacheTest now observes reads with SnapshotStateObserver, as drawWithCache does, since a plain read observer sees curves change on every move. - setCurves does nothing when both lists are unchanged, so clearing or resizing an empty pad no longer makes the pad redraw. - drawSignature builds its path from both lists instead of joining them into a new list first. - ResizeBehavior.Custom's KDoc says the mapper runs under the state's lock. The README says input an app feeds the state goes into the same stroke as the user's. Benchmarks are within error of the previous commit. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
This was referenced Oct 15, 2026
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Why
Apps can call
clear(),setSize()and the gesture methods off the main thread, while the pad takes input and draws on it. The state changed several plain fields and aSnapshotStateListin steps, so aclear()on another thread could interleave with a move, and the draw pass could take asubListof a list that had just shrunk.What
SignaturePadStateImplholds a lock, atomicfu'sSynchronizedObject. atomicfu 0.28.0 already came in through Compose; it's now declared.curves) and replaced on each change. Adding a curve or ending a stroke shares the curves already there instead of copying them. The draw pass reads them without the lock, anddrawSignaturealways gets the two from the same moment.finishedStrokes), replaced only when they change. The pad's cache of them observes it. This replacesstrokeCurveCountandfinishedStrokesVersion.drawOnBitmapcopies the curves and size under the lock and draws outside it, so exporting doesn't hold up input.ResizeBehavior.Custommapper runs while the lock is held.The pad still reads the cached finished strokes and the stroke in progress separately. So when a stroke ends on another thread mid-frame, that one frame can draw it twice or not at all. The next frame is right.
Testing
ConcurrencyTest(JVM) runs input, clearing, resizing, drawing, exporting and save/restore on separate threads for a second each, then checks the state is consistent. On master'sSignaturePadState.ktall 3 tests fail in every run, withIndexOutOfBoundsExceptionandConcurrentModificationException. They pass with this change.:signaturepad:allTests(JVM, JS, Wasm),:signaturepad:buildand:demo:assemblepass on Linux. iOS tests weren't run locally; CI runs them on macOS.:benchmark:benchmark, master → this branch:The first commit used plain lists, which copied the signature each time a stroke ended and made capture 12% slower than master at 4,000 curves. The persistent lists make capture 24% faster than that. They make resizing 9% slower than the plain lists did, but still about 30% faster than master.
PR #29 builds on the state this changes and will need a rebase.
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