更新： Xcode 11.4.1•Swift 5.2

extension URL {
    /// check if the URL is a directory and if it is reachable 
    func isDirectoryAndReachable() throws -> Bool {
        guard try resourceValues(forKeys: [.isDirectoryKey]).isDirectory == true else {
            return false
        }
        return try checkResourceIsReachable()
    }

    /// returns total allocated size of a the directory including its subFolders or not
    func directoryTotalAllocatedSize(includingSubfolders: Bool = false) throws -> Int? {
        guard try isDirectoryAndReachable() else { return nil }
        if includingSubfolders {
            guard
                let urls = FileManager.default.enumerator(at: self, includingPropertiesForKeys: nil)?.allObjects as? [URL] else { return nil }
            return try urls.lazy.reduce(0) {
                    (try $1.resourceValues(forKeys: [.totalFileAllocatedSizeKey]).totalFileAllocatedSize ?? 0) + $0
            }
        }
        return try FileManager.default.contentsOfDirectory(at: self, includingPropertiesForKeys: nil).lazy.reduce(0) {
                 (try $1.resourceValues(forKeys: [.totalFileAllocatedSizeKey])
                    .totalFileAllocatedSize ?? 0) + $0
        }
    }

    /// returns the directory total size on disk
    func sizeOnDisk() throws -> String? {
        guard let size = try directoryTotalAllocatedSize(includingSubfolders: true) else { return nil }
        URL.byteCountFormatter.countStyle = .file
        guard let byteCount = URL.byteCountFormatter.string(for: size) else { return nil}
        return byteCount + " on disk"
    }
    private static let byteCountFormatter = ByteCountFormatter()
}

用法：

do {
    let documentDirectory = try FileManager.default.url(for: .documentDirectory, in: .userDomainMask, appropriateFor: nil, create: false)
    if let sizeOnDisk = try documentDirectory.sizeOnDisk() {
        print("Size:", sizeOnDisk) // Size: 3.15 GB on disk
    }
} catch {
    print(error)
}