feat: Implement audio pre-rendering for custom tracks

CHANGELOG.md

@@ -1,3 +1,6 @@
+## 1.16.2
+- **FIX**(android, iOS, macOS): Eliminate audible clicks/gaps at custom audio loop boundaries and clip transitions. Each custom audio track is now pre-rendered to a single gap-less PCM WAV file before being inserted into the composition, avoiding AAC encoder frame realignment (Android Media3) and codec priming/padding artifacts (AVFoundation `insertTimeRange` per loop iteration on iOS/macOS). Volume control remains in the mixer layer for post-hoc adjustments without re-decoding.
+
 ## 1.16.1
 - **FIX**(iOS, macOS): Fix crash `No source tracks available for compositing` when rendering H.264 MP4 files whose container duration slightly exceeds the video track's actual frame duration. The compositor time range is now clamped to the video track's real `timeRange` before inserting into the composition, preventing AVFoundation from requesting frames that have no pixel buffer. A black-frame fallback was also added as a safety net.
 

android/src/main/kotlin/ch/waio/pro_video_editor/src/features/render/RenderVideo.kt

@@ -310,15 +310,35 @@ class RenderVideo(private val context: Context) {
         // Create composition (now fast - no manual audio mixing needed, Media3 handles it natively)
         Thread {
             try {
-                val composition = applyComposition(
+                val compositionResult = applyComposition(
                     context = context,
                     config = config,
                     videoEffects = videoEffects,
                     audioEffects = audioEffects
                 )
 
                 mainHandler.post {
-                    if (composition != null) {
+                    if (compositionResult != null) {
+                        val composition = compositionResult.composition
+                        val audioTempFiles = compositionResult.temporaryFiles
+
+                        transformer.addListener(object : Transformer.Listener {
+                            override fun onCompleted(
+                                composition: Composition,
+                                result: ExportResult
+                            ) {
+                                cleanupAudioTempFiles(audioTempFiles)
+                            }
+
+                            override fun onError(
+                                composition: Composition,
+                                result: ExportResult,
+                                exception: ExportException
+                            ) {
+                                cleanupAudioTempFiles(audioTempFiles)
+                            }
+                        })
+
                         transformer.start(composition, outputFile.absolutePath)
 
                         // Start progress tracking loop
@@ -349,4 +369,27 @@ class RenderVideo(private val context: Context) {
             }
         }.start()
     }
+
+    /**
+     * Deletes pre-rendered audio temp files (typically WAVs from
+     * AudioPreRenderer) created while building the composition.
+     */
+    private fun cleanupAudioTempFiles(files: List<File>) {
+        for (file in files) {
+            try {
+                if (file.exists()) {
+                    val deleted = file.delete()
+                    Log.d(
+                        RENDER_TAG,
+                        "Cleanup pre-rendered audio file: ${file.name}, deleted=$deleted"
+                    )
+                }
+            } catch (e: Exception) {
+                Log.w(
+                    RENDER_TAG,
+                    "Failed to delete pre-rendered audio file ${file.name}: ${e.message}"
+                )
+            }
+        }
+    }
 }

android/src/main/kotlin/ch/waio/pro_video_editor/src/features/render/helpers/ApplyComposition.kt

@@ -6,6 +6,18 @@ import androidx.media3.common.audio.AudioProcessor
 import androidx.media3.common.util.UnstableApi
 import androidx.media3.transformer.Composition
 import ch.waio.pro_video_editor.src.features.render.models.RenderConfig
+import java.io.File
+
+/**
+ * Result of building a composition: the composition itself plus a list of
+ * temporary files that were created during the build (e.g. pre-rendered
+ * audio WAVs). The caller MUST delete these temporary files after the
+ * Transformer export finishes (success or failure).
+ */
+data class CompositionResult(
+    val composition: Composition,
+    val temporaryFiles: List<File>
+)
 
 /**
  * Creates a Media3 Composition from render configuration.
@@ -18,17 +30,19 @@ import ch.waio.pro_video_editor.src.features.render.models.RenderConfig
  * @param config The render configuration containing all composition parameters
  * @param videoEffects List of video effects to apply (from EffectsProcessor)
  * @param audioEffects List of audio effects to apply (from EffectsProcessor)
- * @return Composition ready for transformer, or null if no video clips provided
+ * @return [CompositionResult] with the composition and any temporary files
+ *   created during the build, or null if no video clips were provided.
  */
 @UnstableApi
 fun applyComposition(
     context: Context,
     config: RenderConfig,
     videoEffects: List<Effect>,
     audioEffects: List<AudioProcessor>
-): Composition? {
-    return CompositionBuilder(context, config)
+): CompositionResult? {
+    val builder = CompositionBuilder(context, config)
         .setVideoEffects(videoEffects)
         .setAudioEffects(audioEffects)
-        .build()
+    val composition = builder.build() ?: return null
+    return CompositionResult(composition, builder.temporaryFiles.toList())
 }