gistfile1.swift

import AVFoundation

func addWatermark(to videoURL: URL, watermark: UIImage) -> AVMutableComposition? {
    // Create a new mutable composition
    let mixComposition = AVMutableComposition()

    // Add a new mutable track to the composition and get the first video track from the asset
    guard let track = mixComposition.addMutableTrack(withMediaType: .video, preferredTrackID: Int32(kCMPersistentTrackID_Invalid)),
          let asset = AVAsset(url: videoURL).tracks(withMediaType: .video).first else {
        return nil
    }

    // Insert the video track into the mutable track at the start of the composition
    do {
        try track.insertTimeRange(CMTimeRangeMake(start: .zero, duration: asset.duration), of: asset, at: .zero)
    } catch {
        print("Error inserting time range: \(error)")
        return nil
    }

    // Get the natural size of the video
    let size = asset.naturalSize

    // Create a new layer for the watermark and set its contents and frame
    let watermarkLayer = CALayer()
    watermarkLayer.contents = watermark.cgImage
    watermarkLayer.frame = CGRect(x: 0, y: 0, width: size.width, height: size.height)
    watermarkLayer.opacity = 0.5

    // Create a new layer for the video and set its frame
    let videoLayer = CALayer()
    videoLayer.frame = CGRect(x: 0, y: 0, width: size.width, height: size.height)

    // Create a new parent layer and add the video layer and watermark layer to it
    let parentLayer = CALayer()
    parentLayer.frame = CGRect(x: 0, y: 0, width: size.width, height: size.height)
    parentLayer.addSublayer(videoLayer)
    parentLayer.addSublayer(watermarkLayer)

    // Create a new mutable video composition and set its frame duration, render size, and animation tool
    let videoComposition = AVMutableVideoComposition()
    videoComposition.frameDuration = CMTimeMake(value: 1, timescale: 30)
    videoComposition.renderSize = size
    videoComposition.animationTool = AVVideoCompositionCoreAnimationTool(postProcessingAsVideoLayer: videoLayer, in: parentLayer)

    // Create a new mutable video composition instruction and set its time range
    let instruction = AVMutableVideoCompositionInstruction()
    instruction.timeRange = CMTimeRangeMake(start: .zero, duration: mixComposition.duration)

    // Create a new mutable video composition layer instruction for the mutable track and set it as the layer instructions of the instruction
    let layerInstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: track)
    instruction.layerInstructions = [layerInstruction]

    // Set the instruction as the instructions of the video composition
    videoComposition.instructions = [instruction]

    // Return the mutable composition
    return mixComposition
}