Swift Playground code showing how to convert a video from H264 (in my case) to HEVC/H265 using AVFoundation

Export-HEVC.swift

import AVFoundation

// Get test video from bundle
let url = Bundle.main.url(forResource: "test", withExtension: "mp4")!
let anAsset = AVURLAsset(url: url)

// get properties of video track
let track = anAsset.tracks(withMediaType: AVMediaType.video).first!
let size = track.naturalSize.applying(track.preferredTransform)
let frameSize = CGSize(width: abs(size.width), height: abs(size.height))
let frameRate = track.nominalFrameRate
let bps = track.estimatedDataRate

let duration = anAsset.duration.seconds

let fileSize = Double(bps) * duration / 8

// We want half the size...
let desiredHEVCFileSize = Int64(fileSize / 2)

// Location of exported HEVC file
let outputUrl = FileManager.default.urls(for: .moviesDirectory, in: .userDomainMask).first!.appendingPathComponent("RocketSim/output-\(Int(Date().timeIntervalSinceReferenceDate)).mp4")

//print("All export presets: ", AVAssetExportSession.exportPresets(compatibleWith: anAsset))
var preset = AVAssetExportPresetHEVCHighestQuality
let outFileType = AVFileType.m4v

AVAssetExportSession.determineCompatibility(ofExportPreset: preset, with: anAsset, outputFileType: outFileType, completionHandler: { (isCompatible) in
    if !isCompatible {
        print("incompatible")
        return
    }

    guard let export = AVAssetExportSession(asset: anAsset, presetName: preset) else {
        print("Unable to create AVAssetExportSession...")
        return
    }

    export.shouldOptimizeForNetworkUse = true
    export.canPerformMultiplePassesOverSourceMediaData = true
    export.fileLengthLimit = desiredHEVCFileSize
    export.outputFileType = outFileType
    export.outputURL = outputUrl

    print("Exporting...")
    export.exportAsynchronously { () -> Void in
       // Handle export results.
        print("Done exporting to \(outputUrl.path)")
    }
}
)