A method to calculate the accumulated size of a directory on the volume in bytes.

NRFileManager.swift

//
//  Created by Nikolai Ruhe on 2016-02-10.
//  Copyright (c) 2016 Nikolai Ruhe. All rights reserved.
// updated for Swift 3, Tyler Arnold 2017-04-25
//

import Foundation 
extension FileManager {
    
    func allocatedSizeOfDirectoryAtURL(directoryURL: URL) throws -> UInt64 {
        var accumulatedSize = UInt64(0)
        let prefetchedProperties = [URLResourceKey.isRegularFileKey, URLResourceKey.fileAllocatedSizeKey, URLResourceKey.totalFileAllocatedSizeKey]
        var errorDidOccur: Error?
        let errorHandler: (URL, Error) -> Bool = { _, error in
            errorDidOccur = error
            return false
        }
        
        if let e = self.enumerator(at: directoryURL,
                                   includingPropertiesForKeys: prefetchedProperties,
                                   options: FileManager.DirectoryEnumerationOptions(),
                                   errorHandler: errorHandler) {
            
            let keys: Set<URLResourceKey> = [URLResourceKey.isRegularFileKey, URLResourceKey.totalFileAllocatedSizeKey, URLResourceKey.fileAllocatedSizeKey]
            for item in e {
                if let contentItemURL = item as? URL {
                    
                    if let error = errorDidOccur { throw error }
                    let resourceValues = try contentItemURL.resourceValues(forKeys: keys)
                    
                    guard let isRegularFile = resourceValues.isRegularFile else {
                        preconditionFailure()
                    }
                    guard isRegularFile == true else {
                        continue
                    }
                    var fileSize = resourceValues.fileSize
                    fileSize = fileSize ?? resourceValues.totalFileAllocatedSize
                    guard let size = fileSize else {
                        preconditionFailure("huh? NSURLFileAllocatedSizeKey should always return a value")
                    }
                    // We're good, add up the value.
                    accumulatedSize += UInt64(size)
                }
            }
            
        }
        if let error = errorDidOccur { throw error }
        return accumulatedSize
    }
}