Play a shutter click when a photo is taken

A camera you are not looking at has to say it heard the tap. A synthesised
shutter_click.wav (110 ms, two filtered noise transients for the shutter
open and close) is bundled and played through SoundPool on each accepted
shot.

The click sits behind the capture guards, so a tap the camera refuses -
still busy, or not ready - stays silent and the click always means
"taken". SoundPool loads asynchronously, so a tap that beats the sample
into memory plays a short system tone instead of nothing. The sample is
released in onDestroy.

Verified on device: with system touch sounds disabled, dumpsys audio shows
player piid:1263 (package:com.bm.pagescribe, type:android.media.SoundPool)
emitting event:started on the tap, while AudioFlinger's mixer leaves
standby at the same moment.

README.md

@@ -42,7 +42,10 @@ it can go to Drive, email, or the Files app.
 
 - **Shelf** (`MainActivity`) — one row per book, plus a `+` to create one.
 - **Capture** (`CaptureActivity`) — CameraX preview, flash toggle, shutter, and
-  a jump to the book.
+  a jump to the book. Every accepted shot is confirmed with a short shutter
+  click (a bundled sample played through `SoundPool`), so a batch of spreads can
+  be shot by feel. A tap that the camera refuses — still busy, not ready —
+  stays silent, so the click always means "taken".
 - **Book** (`BookActivity`) — the pages read so far, the log, **Export text**,
   **Capture**, and **Delete book**.
 

app/src/main/java/com/bm/pagescribe/CaptureActivity.kt

@@ -26,6 +26,7 @@ import androidx.core.content.ContextCompat
 import com.bm.pagescribe.book.Book
 import com.bm.pagescribe.book.BookRepository
 import com.bm.pagescribe.ocr.WorkScheduler
+import com.bm.pagescribe.util.ClickSound
 import com.bm.pagescribe.util.EdgeToEdge
 import com.google.android.material.appbar.MaterialToolbar
 import com.google.android.material.button.MaterialButton
@@ -44,6 +45,7 @@ class CaptureActivity : AppCompatActivity() {
     private lateinit var shutter: ImageButton
     private lateinit var flashButton: MaterialButton
     private lateinit var permissionPanel: View
+    private var clickSound: ClickSound? = null
 
     private var imageCapture: ImageCapture? = null
     private var book: Book? = null
@@ -95,6 +97,7 @@ class CaptureActivity : AppCompatActivity() {
             return
         }
         supportActionBar?.title = book?.title
+        clickSound = ClickSound(this)
 
         shutter.setOnClickListener { shoot() }
         flashButton.setOnClickListener { toggleFlash() }
@@ -126,6 +129,12 @@ class CaptureActivity : AppCompatActivity() {
         handler.removeCallbacks(poll)
     }
 
+    override fun onDestroy() {
+        clickSound?.release()
+        clickSound = null
+        super.onDestroy()
+    }
+
     private fun hasCameraPermission(): Boolean =
         ContextCompat.checkSelfPermission(this, Manifest.permission.CAMERA) ==
             PackageManager.PERMISSION_GRANTED
@@ -174,6 +183,9 @@ class CaptureActivity : AppCompatActivity() {
 
         val target = File(current.queueDir, "page-${System.currentTimeMillis()}.jpg")
         val options = ImageCapture.OutputFileOptions.Builder(target).build()
+        // Confirmation for the tap: only once the shot is really under way, so a
+        // click never plays for a tap that was refused.
+        clickSound?.play()
         capturing = true
         shutter.isEnabled = false
 

app/src/main/java/com/bm/pagescribe/util/ClickSound.kt

@@ -0,0 +1,72 @@
+package com.bm.pagescribe.util
+
+import android.content.Context
+import android.media.AudioAttributes
+import android.media.AudioManager
+import android.media.SoundPool
+import android.media.ToneGenerator
+import com.bm.pagescribe.R
+
+/**
+ * The short click the Camera button makes when a shot is taken.
+ *
+ * The sound is a bundled shutter sample played through [SoundPool], which is
+ * built for short UI sounds and keeps the latency low enough that the click
+ * lands with the tap.  [SoundPool.load] is asynchronous, so a tap that beats
+ * the sample into memory falls back to a system tone rather than going silent.
+ *
+ * Call [release] when the owning activity is destroyed.
+ */
+class ClickSound(context: Context) {
+
+    private val appContext = context.applicationContext
+    private val pool: SoundPool = SoundPool.Builder()
+        .setMaxStreams(1)
+        .setAudioAttributes(
+            AudioAttributes.Builder()
+                .setUsage(AudioAttributes.USAGE_ASSISTANCE_SONIFICATION)
+                .setContentType(AudioAttributes.CONTENT_TYPE_SONIFICATION)
+                .build()
+        )
+        .build()
+
+    private var sampleId = 0
+    private var sampleReady = false
+    private var tone: ToneGenerator? = null
+    private var released = false
+
+    init {
+        pool.setOnLoadCompleteListener { _, _, status -> sampleReady = status == 0 }
+        sampleId = pool.load(appContext, R.raw.shutter_click, 1)
+    }
+
+    /** Play the click. Safe to call on every tap. */
+    fun play() {
+        if (released) return
+        if (sampleReady && sampleId != 0) {
+            pool.play(sampleId, VOLUME, VOLUME, 1, 0, 1f)
+            return
+        }
+        // The sample has not finished loading yet; a system tone still confirms
+        // the tap. (ToneGenerator can fail on some devices, hence runCatching.)
+        runCatching {
+            val generator = tone ?: ToneGenerator(AudioManager.STREAM_SYSTEM, TONE_VOLUME)
+                .also { tone = it }
+            generator.startTone(ToneGenerator.TONE_PROP_ACK, TONE_MS)
+        }
+    }
+
+    fun release() {
+        if (released) return
+        released = true
+        pool.release()
+        runCatching { tone?.release() }
+        tone = null
+    }
+
+    private companion object {
+        const val VOLUME = 1f
+        const val TONE_VOLUME = 60
+        const val TONE_MS = 80
+    }
+}