Swift 3 | macOs | Write NSImage(s) to a movie file. Modified from http://stackoverflow.com/a/36297656/1275125

MovieWriter.swift

import AppKit
import AVFoundation

class MovieWriter: NSObject {
    func writeImagesAsMovie(_ allImages: [NSImage], videoPath: String, videoSize: CGSize, videoFPS: Int32) {
        // Create AVAssetWriter to write video
        guard let assetWriter = createAssetWriter(videoPath, size: videoSize) else {
            print("Error converting images to video: AVAssetWriter not created")
            return
        }
        
        // If here, AVAssetWriter exists so create AVAssetWriterInputPixelBufferAdaptor
        let writerInput = assetWriter.inputs.filter{ $0.mediaType == AVMediaTypeVideo }.first!
        let sourceBufferAttributes : [String : AnyObject] = [
            kCVPixelBufferPixelFormatTypeKey as String : Int(kCVPixelFormatType_32ARGB) as AnyObject,
            kCVPixelBufferWidthKey as String : videoSize.width as AnyObject,
            kCVPixelBufferHeightKey as String : videoSize.height as AnyObject,
            ]
        let pixelBufferAdaptor = AVAssetWriterInputPixelBufferAdaptor(assetWriterInput: writerInput, sourcePixelBufferAttributes: sourceBufferAttributes)
        
        // Start writing session
        assetWriter.startWriting()
        assetWriter.startSession(atSourceTime: kCMTimeZero)
        if (pixelBufferAdaptor.pixelBufferPool == nil) {
            print("Error converting images to video: pixelBufferPool nil after starting session")
            return
        }
        
        // -- Create queue for <requestMediaDataWhenReadyOnQueue>
        let mediaQueue = DispatchQueue(label: "mediaInputQueue", attributes: [])
        
        // -- Set video parameters
        let frameDuration = CMTimeMake(1, videoFPS)
        var frameCount = 0
        
        // -- Add images to video
        let numImages = allImages.count
        writerInput.requestMediaDataWhenReady(on: mediaQueue, using: { () -> Void in
            // Append unadded images to video but only while input ready
            while (writerInput.isReadyForMoreMediaData && frameCount < numImages) {
                let lastFrameTime = CMTimeMake(Int64(frameCount), videoFPS)
                let presentationTime = frameCount == 0 ? lastFrameTime : CMTimeAdd(lastFrameTime, frameDuration)
                
                if !self.appendPixelBufferForImageAtURL(allImages[frameCount], pixelBufferAdaptor: pixelBufferAdaptor, presentationTime: presentationTime) {
                    print("Error converting images to video: AVAssetWriterInputPixelBufferAdapter failed to append pixel buffer")
                    return
                }
                
                frameCount += 1
            }
            
            // No more images to add? End video.
            if (frameCount >= numImages) {
                writerInput.markAsFinished()
                assetWriter.finishWriting {
                    if (assetWriter.error != nil) {
                        print("Error converting images to video: \(assetWriter.error)")
                    } else {
                        print("Converted images to movie @ \(videoPath)")
                    }
                }
            }
        })
    }
    
    
    func createAssetWriter(_ path: String, size: CGSize) -> AVAssetWriter? {
        // Convert <path> to NSURL object
        let pathURL = URL(fileURLWithPath: path)
        
        // Return new asset writer or nil
        do {
            // Create asset writer
            let newWriter = try AVAssetWriter(outputURL: pathURL, fileType: AVFileTypeMPEG4)
            
            // Define settings for video input
            let videoSettings: [String : AnyObject] = [
                AVVideoCodecKey  : AVVideoCodecH264 as AnyObject,
                AVVideoWidthKey  : size.width as AnyObject,
                AVVideoHeightKey : size.height as AnyObject,
                ]
            
            // Add video input to writer
            let assetWriterVideoInput = AVAssetWriterInput(mediaType: AVMediaTypeVideo, outputSettings: videoSettings)
            newWriter.add(assetWriterVideoInput)
            
            // Return writer
            print("Created asset writer for \(size.width)x\(size.height) video")
            return newWriter
        } catch {
            print("Error creating asset writer: \(error)")
            return nil
        }
    }
    
    
    func appendPixelBufferForImageAtURL(_ image: NSImage, pixelBufferAdaptor: AVAssetWriterInputPixelBufferAdaptor, presentationTime: CMTime) -> Bool {
        var appendSucceeded = false
        
        autoreleasepool {
            if  let pixelBufferPool = pixelBufferAdaptor.pixelBufferPool {
                let pixelBufferPointer = UnsafeMutablePointer<CVPixelBuffer?>.allocate(capacity:1)
                let status: CVReturn = CVPixelBufferPoolCreatePixelBuffer(
                    kCFAllocatorDefault,
                    pixelBufferPool,
                    pixelBufferPointer
                )
                
                if let pixelBuffer = pixelBufferPointer.pointee , status == 0 {
                    fillPixelBufferFromImage(image, pixelBuffer: pixelBuffer)
                    appendSucceeded = pixelBufferAdaptor.append(pixelBuffer, withPresentationTime: presentationTime)
                    pixelBufferPointer.deinitialize()
                } else {
                    NSLog("Error: Failed to allocate pixel buffer from pool")
                }
                
                pixelBufferPointer.deallocate(capacity: 1)
            }
        }
        
        return appendSucceeded
    }
    
    
    func fillPixelBufferFromImage(_ image: NSImage, pixelBuffer: CVPixelBuffer) {
        CVPixelBufferLockBaseAddress(pixelBuffer, CVPixelBufferLockFlags(rawValue: CVOptionFlags(0)))
        
        let pixelData = CVPixelBufferGetBaseAddress(pixelBuffer)
        let rgbColorSpace = CGColorSpaceCreateDeviceRGB()
        
        // Create CGBitmapContext
        let context = CGContext(
            data: pixelData,
            width: Int(image.size.width),
            height: Int(image.size.height),
            bitsPerComponent: 8,
            bytesPerRow: CVPixelBufferGetBytesPerRow(pixelBuffer),
            space: rgbColorSpace,
            bitmapInfo: CGImageAlphaInfo.premultipliedFirst.rawValue
        )!
        
        // Draw image into context
        let drawCGRect = CGRect(x:0, y:0, width:image.size.width, height:image.size.height)
        var drawRect = NSRectFromCGRect(drawCGRect);
        let cgImage = image.cgImage(forProposedRect: &drawRect, context: nil, hints: nil)!
        context.draw(cgImage, in: CGRect(x: 0.0,y: 0.0,width: image.size.width,height: image.size.height))

        CVPixelBufferUnlockBaseAddress(pixelBuffer, CVPixelBufferLockFlags(rawValue: CVOptionFlags(0)))
    }
}

i got this error : pixelBufferPool nil after starting session