swift: how to delete part of audio? - swift

Question

I'm creating a simple audio editing tool to trim and delete from an audio.
I implemented the trim function and it is working fine. However I searched and tried to implement the delete function and here is my code:
func deleteExportAsset(_ asset: AVAsset, fileName: String, completeAudioTime: CGFloat) -> URL {
print("\(#function)")
let documentsDirectory = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)[0]
let trimmedSoundFileURL = documentsDirectory.appendingPathComponent(fileName)
print("saving to \(trimmedSoundFileURL.absoluteString)")
if FileManager.default.fileExists(atPath: trimmedSoundFileURL.absoluteString) {
print("sound exists, removing \(trimmedSoundFileURL.absoluteString)")
do {
if try trimmedSoundFileURL.checkResourceIsReachable() {
print("is reachable")
}
try FileManager.default.removeItem(atPath: trimmedSoundFileURL.absoluteString)
} catch {
print("could not remove \(trimmedSoundFileURL)")
print(error.localizedDescription)
}
}
print("creating export session for \(asset)")
if let exporter = AVAssetExportSession(asset: asset, presetName: AVAssetExportPresetAppleM4A) {
exporter.outputFileType = AVFileType.m4a
exporter.outputURL = trimmedSoundFileURL
let timeRange1 = CMTimeRangeFromTimeToTime(CMTime(seconds: 0, preferredTimescale: 100), CMTime(seconds: endTimeOfRange1, preferredTimescale: 100))
let timeRange2 = CMTimeRangeFromTimeToTime(CMTime(seconds: startTimeOfRange2)), preferredTimescale: 100), CMTime(seconds: Double(completeAudioTime), preferredTimescale: 100))
exporter.timeRange = CMTimeRangeGetUnion(timeRange1, timeRange2)
// do it
exporter.exportAsynchronously(completionHandler: {
print("export complete \(exporter.status)")
switch exporter.status {
case AVAssetExportSessionStatus.failed:
if let e = exporter.error {
print("export failed \(e)")
}
case AVAssetExportSessionStatus.cancelled:
print("export cancelled \(String(describing: exporter.error))")
default:
print("export complete")
}
})
} else {
print("cannot create AVAssetExportSession for asset \(asset)")
}
return trimmedSoundFileURL
}
what I'm doing here is creating 2 Ranges. Range1 from 0 ->time1 and Range2 from time2->endOfAudio. (I want to delete from time1 -> time2)
then I'm creating the union between 2 ranges.
however, nothing is happening to the audio. It is saved exactly like it was before this function.

Answer 1

CMTimeRangeGetUnion returns another CMTimeRange, which is just a (start-)time and a duration. So there is nothing than can hold the two time ranges required to do what you are expecting. In extension, AVAssetExportSession has no API that takes a list of time ranges to export.
But there is a way to accomplish it. The idea is to create an editable copy of the asset, delete the time range, and then export the editable copy. AVMutableComposition does this:
// assuming 'asset', 'endTimeOfRange1' and 'startTimeOfRange2' from the question:
// create empty mutable composition
let composition: AVMutableComposition = AVMutableComposition()
// copy all of original asset into the mutable composition, effectively creating an editable copy
try composition.insertTimeRange( CMTimeRangeMake( kCMTimeZero, asset.duration), of: asset, at: kCMTimeZero)
// now edit as required, e.g. delete a time range
let startTime = CMTime(seconds: endTimeOfRange1, preferredTimescale: 100)
let endTime = CMTime(seconds: startTimeOfRange2, preferredTimescale: 100)
composition.removeTimeRange( CMTimeRangeFromTimeToTime( startTime, endTime))
// since AVMutableComposition is an AVAsset subclass, it can be exported with AVAssetExportSession (or played with an AVPlayer(Item))
if let exporter = AVAssetExportSession(asset: composition, presetName: AVAssetExportPresetAppleM4A)
{
// configure session and exportAsynchronously as above.
// You don't have to set the timeRange of the exportSession
}
Note that copying from the asset to the composition only modifies some in-memory structures defining which samples go where on the time line, but doesn't actually moves any media samples around. This is not done until exporting; as result, editing is (relatively) fast, and you have to keep the source file around at least until export is finished.