Fix silent failure causing audio-only exports (Issue #38)

  Previously, when video output configuration was incompatible with the
  asset, setupVideoOutput would silently return and the export would
  continue with only audio tracks. This caused intermittent reports of
  videos exporting without video content.

  Changes:
  - Add validation to detect when video setup fails for assets with video tracks
  - Fail fast with clear error: "Video output configuration is not compatible"
  - Track setup errors and surface them through completion handler
  - Document issue and common causes in README troubleshooting section
  - Remove unused Photos import

  Fixes #38

Author: piemonte

Sources/NextLevelSessionExporter.swift

@@ -167,6 +167,9 @@ open class NextLevelSessionExporter: NSObject, @unchecked Sendable {
     fileprivate var _duration: TimeInterval = 0
     fileprivate var _lastSamplePresentationTime: CMTime = .invalid
 
+    fileprivate var _videoSetupFailed: Bool = false
+    fileprivate var _videoSetupError: Error?
+
     // MARK: - object lifecycle
 
     /// Initializes a session with an asset to export.
@@ -253,6 +256,8 @@ extension NextLevelSessionExporter {
         }
 
         self._progress = 0
+        self._videoSetupFailed = false
+        self._videoSetupError = nil
 
         do {
             self._reader = try AVAssetReader(asset: asset)
@@ -306,6 +311,17 @@ extension NextLevelSessionExporter {
         self.setupAudioOutput(withAsset: asset)
         self.setupAudioInput()
 
+        // Validate that video setup succeeded if video tracks exist (Issue #38)
+        let videoTracks = asset.tracks(withMediaType: AVMediaType.video)
+        if videoTracks.count > 0 && self._videoSetupFailed {
+            print("NextLevelSessionExporter, video tracks exist but video setup failed - aborting export")
+            DispatchQueue.main.async {
+                let error = self._videoSetupError ?? NextLevelSessionExporterError.setupFailure("Video output could not be configured")
+                self._completionHandler?(.failure(error))
+            }
+            return
+        }
+
         // export
 
         self._writer?.startWriting()
@@ -314,7 +330,7 @@ extension NextLevelSessionExporter {
 
         let dispatchGroup = DispatchGroup()
 
-        let videoTracks = asset.tracks(withMediaType: AVMediaType.video)
+        // Reuse videoTracks from validation above (Issue #38)
         if let videoInput = self._videoInput,
            let videoOutput = self._videoOutput,
            videoTracks.count > 0 {
@@ -395,7 +411,10 @@ extension NextLevelSessionExporter {
             self._videoInput = AVAssetWriterInput(mediaType: AVMediaType.video, outputSettings: self.videoOutputConfiguration)
             self._videoInput?.expectsMediaDataInRealTime = self.expectsMediaDataInRealTime
         } else {
-            print("Unsupported output configuration")
+            // Mark video setup as failed - this will cause the export to fail with a clear error
+            self._videoSetupFailed = true
+            self._videoSetupError = NextLevelSessionExporterError.invalidConfiguration("Video output configuration is not compatible with this asset. The writer cannot apply the specified video settings.")
+            print("NextLevelSessionExporter, video output configuration is not supported by the writer")
             return
         }
 
@@ -740,6 +759,9 @@ extension NextLevelSessionExporter {
         self._progressHandler = nil
         self._renderHandler = nil
         self._completionHandler = nil
+
+        self._videoSetupFailed = false
+        self._videoSetupError = nil
     }
 
 }