Shoot from the volume keys, while the camera is up

modules/recipescam-volume/android/build.gradle

@@ -0,0 +1,15 @@
+plugins {
+  id 'com.android.library'
+  id 'expo-module-gradle-plugin'
+}
+
+group = 'com.locphamtran.recipescamera'
+version = '0.1.0'
+
+android {
+  namespace "com.locphamtran.recipescamera.volume"
+  defaultConfig {
+    versionCode 1
+    versionName "0.1.0"
+  }
+}

modules/recipescam-volume/android/src/main/java/com/locphamtran/recipescamera/volume/VolumeShutterModule.kt

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+package com.locphamtran.recipescamera.volume
+
+import android.view.KeyEvent
+import expo.modules.kotlin.modules.Module
+import expo.modules.kotlin.modules.ModuleDefinition
+
+/**
+ * Volume keys as a shutter release.
+ *
+ * Android exposes no key stream to JS, and the volume keys never reach a view -
+ * the window (PhoneWindow.OnKeyListener) turns them into a stream-volume change
+ * before any view sees them. The one place that runs first is the activity's
+ * dispatchKeyEvent, so MainActivity hands every key here and only passes on
+ * what comes back false.
+ *
+ * Enabled from JS only while the camera screen is up: with the flag off the keys
+ * go straight back to the window, so volume still works in the library.
+ */
+class VolumeShutterModule : Module() {
+  override fun definition() = ModuleDefinition {
+    Name("RecipesCamVolumeShutter")
+
+    Events("onShutter")
+
+    // The activity is the one dispatching, but it does not own the module -
+    // publish the single live instance so it can be reached from there.
+    OnCreate { instance = this@VolumeShutterModule }
+    OnDestroy { if (instance === this@VolumeShutterModule) instance = null }
+
+    Function("setEnabled") { enabled: Boolean -> VolumeShutterModule.enabled = enabled }
+  }
+
+  companion object {
+    @Volatile private var instance: VolumeShutterModule? = null
+
+    @Volatile private var enabled = false
+
+    /**
+     * Every key event of the activity goes through here first. Returns true when
+     * the key is ours and has been consumed - that consumption is what keeps the
+     * press from also changing the media volume, the way a stock camera app
+     * behaves.
+     *
+     * Only ACTION_DOWN with repeatCount 0 fires: the UP and the auto-repeat of a
+     * held button are still swallowed (so holding it does not leak a volume
+     * change on release) but produce no second frame.
+     */
+    fun onKeyEvent(event: KeyEvent): Boolean {
+      if (!enabled) return false
+      if (event.keyCode != KeyEvent.KEYCODE_VOLUME_UP && event.keyCode != KeyEvent.KEYCODE_VOLUME_DOWN) {
+        return false
+      }
+      if (event.action == KeyEvent.ACTION_DOWN && event.repeatCount == 0) {
+        instance?.sendEvent("onShutter")
+      }
+      return true
+    }
+  }
+}