Convert PHAsset to UIImage with Swift

getAssetThumbnail.swift

// it returns a square thumbnail.

func getAssetThumbnail(asset: PHAsset, size: CGFloat) -> UIImage {
        let retinaScale = UIScreen.mainScreen().scale
        let retinaSquare = CGSizeMake(size * retinaScale, size * retinaScale)
        let cropSizeLength = min(asset.pixelWidth, asset.pixelHeight)
        let square = CGRectMake(0, 0, CGFloat(cropSizeLength), CGFloat(cropSizeLength))
        let cropRect = CGRectApplyAffineTransform(square, CGAffineTransformMakeScale(1.0/CGFloat(asset.pixelWidth), 1.0/CGFloat(asset.pixelHeight)))
        
        let manager = PHImageManager.defaultManager()
        let options = PHImageRequestOptions()
        var thumbnail = UIImage()
        
        options.synchronous = true
        options.deliveryMode = .HighQualityFormat
        options.resizeMode = .Exact
        options.normalizedCropRect = cropRect
        
        manager.requestImageForAsset(asset, targetSize: retinaSquare, contentMode: .AspectFit, options: options, resultHandler: {(result, info)->Void in
            thumbnail = result!
        })
        return thumbnail
    }

Swift 5

func getAssetThumbnail(asset: PHAsset, size: CGFloat) -> UIImage {
        let retinaScale = UIScreen.main.scale
        let retinaSquare = CGSize(width: size * retinaScale, height: size * retinaScale)
        let cropSizeLength = min(asset.pixelWidth, asset.pixelHeight)
        let square = CGRect(x: 0, y: 0, width: CGFloat(cropSizeLength), height: CGFloat(cropSizeLength))
        let cropRect = square.applying(CGAffineTransform(scaleX: 1.0/CGFloat(asset.pixelWidth), y: 1.0/CGFloat(asset.pixelHeight)))
        
        let manager = PHImageManager.default()
        let options = PHImageRequestOptions()
        var thumbnail = UIImage()
        
        options.isSynchronous = true
        options.deliveryMode = .highQualityFormat
        options.resizeMode = .exact
        
        options.normalizedCropRect = cropRect
        
        manager.requestImage(for: asset, targetSize: retinaSquare, contentMode: .aspectFit, options: options, resultHandler: {(result, info)->Void in
            thumbnail = result!
        })
        return thumbnail
    }