Unable to export AVMutableComposition using AVAssetExportSession only on iOS7 swift - swift

Question

I'm trying to export an AVMutableComposition using AVAssetExportSession to a mp4 file in NSTemporaryDirectory(). It works great on iOS8 but on iOS7 I got a -1100 error on exportAsynchronouslyWithCompletionHandler
The requested URL was not found on this server.
My composition contains 2 AVAssets, both getting sources from the web, one for the video, one for the audio.
At first, I though that this was a folder problem but I can create / delete a file using NSFileManager without trouble.
I don't really understand why it should work on iOS8 but not on iOS7 and I didn't spot any particular "introduced=8.0"
Here is my code:
func compose() {
var composition = AVMutableComposition()
var videoTrack = composition.addMutableTrackWithMediaType(AVMediaTypeVideo, preferredTrackID: Int32(kCMPersistentTrackID_Invalid))
var audioTrack = composition.addMutableTrackWithMediaType(AVMediaTypeAudio, preferredTrackID: Int32(kCMPersistentTrackID_Invalid))
var videoAsset = AVAsset.assetWithURL(videoUrl) as! AVAsset //Not a local URL
var audioAsset = AVAsset.assetWithURL(audioUrl) as! AVAsset //Not a local URL
var videoTimeRange = CMTimeRangeMake(kCMTimeZero, videoAsset.duration)
var videoError: NSError?
if let tracks = videoAsset.tracksWithMediaType(mediaType) as? [AVMediaTypeVideo] where tracks.isEmpty == false {
videoTrack.insertTimeRange(videoTimeRange, ofTrack: tracks[0], atTime: duration, error: &videoError)
}
var audioTimeRange = CMTimeRangeMake(kCMTimeZero, audioAsset.duration)
var audioError: NSError?
if let tracks = audioAsset.tracksWithMediaType(mediaType) as? [AVMediaTypeAudio] where tracks.isEmpty == false {
audioTrack.insertTimeRange(audioTimeRange, ofTrack: tracks[0], atTime: duration, error: &audioError)
}
self.export(composition)
}
func export(composition: AVMutableComposition) {
self.exporter = AVAssetExportSession(asset: composition, presetName: AVAssetExportPresetMediumQuality)
let filename = "composition.mp4"
let outputPath = NSTemporaryDirectory().stringByAppendingPathComponent(filename)
//Check if file already exists and delete it if needed
if let fileUrl = NSURL(fileURLWithPath: outputPath) {
let manager = NSFileManager.defaultManager()
if manager.fileExistsAtPath(outputPath) {
var error: NSError? = nil
if manager.removeItemAtPath(outputPath, error: &error) == true {
println("Removed")
}
}
//File should be 30 seconds max
let maxDuration = CMTimeMakeWithSeconds(30.0, composition.duration.timescale)
let exportInterval = CMTimeRangeMake(kCMTimeZero, maxDuration)
self.exporter.outputFileType = AVFileTypeMPEG4
self.exporter.outputURL = fileUrl
self.exporter.timeRange = exportInterval
self.exporter.exportAsynchronouslyWithCompletionHandler({ () -> Void in
dispatch_async(dispatch_get_main_queue(), {
if self.exporter.status == AVAssetExportSessionStatus.Completed {
println("Success")
}
else {
println(self.exporter.error?.localizedDescription)
//The requested URL was not found on this server.
}
})
})
}
I'm getting stuck on this problem for a while now, so any advice would be much appreciated !
Thanks in advance,
Cordially, Louis.
Edit 7th July:
I found out that it is not possible to do so since AVMutableComposition does not support web URLs in iOS7.
For now, I haven't been able to find a workaround and would really appreciate any advice.