iOS uses NSURLConnection to achieve breakpoint continuous transmission download

fileprivate var totalSize: Int64 = 0   // Total sizefileprivate var currentSize: Int64 = 0 // Current sizefileprivate var fileName: String?      // file namefileprivate var fullPath: String?      // File Lu Jinfileprivate var handle: FileHandle?    // Handlefileprivate var connection: NSURLConnection?

// Download the filefunc urlConnectionDownload(_ url: String) -> NSURLConnection? {
        var request = URLRequest(url: URL(string: url)!)
        // Set request header information        /*
 bytes=0-1000 indicates that data of 0-1000 is downloaded
 bytes=0-   Indicates that download starts from 0 until the download is completed
 bytes=100-   Indicates that download starts from 100 until the download is completed
 */
        ("bytes=\(currentSize)", forHTTPHeaderField: "Range")
        // Send asynchronous request        connection = NSURLConnection(request: request, delegate: self)
        return connection
    }
    // Cancel download the file    func urlConnectionCacel() {
        connection?.cancel()
    }

// When receiving the response header information, it will be called (the first method called), and it will only be called once    func connection(_ connection: NSURLConnection, didReceive response: URLResponse) {
        print("didReceive response")
        // Determine whether it has been downloaded        if currentSize > 0 {
            // If you have already downloaded it, you don't need to accept the response again            return
        }
        // Total file size        totalSize = 
        // The file name obtained        fileName = 
        //Write files into the sandbox while receiving data        // 1. Get the full path of the file        if let cache = NSSearchPathForDirectoriesInDomains(.cachesDirectory, .userDomainMask, true).last {
            let nsCache = cache as NSString
            fullPath = (fileName!)
            // Create an empty file            (atPath: fullPath!, contents: nil, attributes: nil)
            // Create a handle            handle = FileHandle(forWritingAtPath: fullPath!)
        }
    }
    
    func connection(_ connection: NSURLConnection, didReceive data: Data) {
        print("didReceive data")
        // Move the file handle to the end of the file        handle?.seekToEndOfFile()
        // Write data using file handle        handle?.write(data)
        currentSize += 
        print(currentSize / totalSize)
    }
    
    func connectionDidFinishLoading(_ connection: NSURLConnection) {
        print("didFinish loading")
        print(fullPath!)
        handle?.closeFile()
        handle = nil
    }