问题
I already have read Read and write data from text file
I need to append the data (a string) to the end of my text file.
One obvious way to do it is to read the file from disk and append the string to the end of it and write it back, but it is not efficient, especially if you are dealing with large files and doing in often.
So the question is \"How to append string to the end of a text file, without reading the file and writing the whole thing back\"?
so far I have:
let dir:NSURL = NSFileManager.defaultManager().URLsForDirectory(NSSearchPathDirectory.CachesDirectory, inDomains: NSSearchPathDomainMask.UserDomainMask).last as NSURL
let fileurl = dir.URLByAppendingPathComponent(\"log.txt\")
var err:NSError?
// until we find a way to append stuff to files
if let current_content_of_file = NSString(contentsOfURL: fileurl, encoding: NSUTF8StringEncoding, error: &err) {
\"\\(current_content_of_file)\\n\\(NSDate()) -> \\(object)\".writeToURL(fileurl, atomically: true, encoding: NSUTF8StringEncoding, error: &err)
}else {
\"\\(NSDate()) -> \\(object)\".writeToURL(fileurl, atomically: true, encoding: NSUTF8StringEncoding, error: &err)
}
if err != nil{
println(\"CANNOT LOG: \\(err)\")
}
回答1:
You should use NSFileHandle, it can seek to the end of the file
let dir:NSURL = NSFileManager.defaultManager().URLsForDirectory(NSSearchPathDirectory.CachesDirectory, inDomains: NSSearchPathDomainMask.UserDomainMask).last as NSURL
let fileurl = dir.URLByAppendingPathComponent("log.txt")
let string = "\(NSDate())\n"
let data = string.dataUsingEncoding(NSUTF8StringEncoding, allowLossyConversion: false)!
if NSFileManager.defaultManager().fileExistsAtPath(fileurl.path!) {
var err:NSError?
if let fileHandle = NSFileHandle(forWritingToURL: fileurl, error: &err) {
fileHandle.seekToEndOfFile()
fileHandle.writeData(data)
fileHandle.closeFile()
}
else {
println("Can't open fileHandle \(err)")
}
}
else {
var err:NSError?
if !data.writeToURL(fileurl, options: .DataWritingAtomic, error: &err) {
println("Can't write \(err)")
}
}
回答2:
Here's an update for PointZeroTwo's answer in Swift 3.0, with one quick note - in the playground testing using a simple filepath works, but in my actual app I needed to build the URL using .documentDirectory (or which ever directory you chose to use for reading and writing - make sure it's consistent throughout your app):
extension String {
func appendLineToURL(fileURL: URL) throws {
try (self + "\n").appendToURL(fileURL: fileURL)
}
func appendToURL(fileURL: URL) throws {
let data = self.data(using: String.Encoding.utf8)!
try data.append(fileURL: fileURL)
}
}
extension Data {
func append(fileURL: URL) throws {
if let fileHandle = FileHandle(forWritingAtPath: fileURL.path) {
defer {
fileHandle.closeFile()
}
fileHandle.seekToEndOfFile()
fileHandle.write(self)
}
else {
try write(to: fileURL, options: .atomic)
}
}
}
//test
do {
let dir: URL = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).last! as URL
let url = dir.appendingPathComponent("logFile.txt")
try "Test \(Date())".appendLineToURL(fileURL: url as URL)
let result = try String(contentsOf: url as URL, encoding: String.Encoding.utf8)
}
catch {
print("Could not write to file")
}
Thanks PointZeroTwo.
回答3:
Here's a version for Swift 2, using extension methods on String and NSData.
//: Playground - noun: a place where people can play
import UIKit
extension String {
func appendLineToURL(fileURL: NSURL) throws {
try self.stringByAppendingString("\n").appendToURL(fileURL)
}
func appendToURL(fileURL: NSURL) throws {
let data = self.dataUsingEncoding(NSUTF8StringEncoding)!
try data.appendToURL(fileURL)
}
}
extension NSData {
func appendToURL(fileURL: NSURL) throws {
if let fileHandle = try? NSFileHandle(forWritingToURL: fileURL) {
defer {
fileHandle.closeFile()
}
fileHandle.seekToEndOfFile()
fileHandle.writeData(self)
}
else {
try writeToURL(fileURL, options: .DataWritingAtomic)
}
}
}
// Test
do {
let url = NSURL(fileURLWithPath: "test.log")
try "Test \(NSDate())".appendLineToURL(url)
let result = try String(contentsOfURL: url)
}
catch {
print("Could not write to file")
}
回答4:
Here is a way to update a file in a much more efficient way.
let monkeyLine = "\nAdding a 🐵 to the end of the file via FileHandle"
if let fileUpdater = try? FileHandle(forUpdating: newFileUrl) {
// function which when called will cause all updates to start from end of the file
fileUpdater.seekToEndOfFile()
// which lets the caller move editing to any position within the file by supplying an offset
fileUpdater.write(monkeyLine.data(using: .utf8)!)
//Once we convert our new content to data and write it, we close the file and that’s it!
fileUpdater.closeFile()
}
回答5:
In order to stay in the spirit PointZero Two. Here an update of his code for Swift 4.1
extension String {
func appendLine(to url: URL) throws {
try self.appending("\n").append(to: url)
}
func append(to url: URL) throws {
let data = self.data(using: String.Encoding.utf8)
try data?.append(to: url)
}
}
extension Data {
func append(to url: URL) throws {
if let fileHandle = try? FileHandle(forWritingTo: url) {
defer {
fileHandle.closeFile()
}
fileHandle.seekToEndOfFile()
fileHandle.write(self)
} else {
try write(to: url)
}
}
}
回答6:
Update: I wrote a blog post on this, which you can find here!
Keeping things Swifty, here is an example using a FileWriter protocol with default implementation (Swift 4.1 at the time of this writing):
- To use this, have your entity (class, struct, enum) conform to this protocol and call the write function (fyi, it throws!).
- Writes to the document directory.
- Will append to the text file if the file exists.
- Will create a new file if the text file doesn't exist.
Note: this is only for text. You could do something similar to write/append
Data.import Foundation enum FileWriteError: Error { case directoryDoesntExist case convertToDataIssue } protocol FileWriter { var fileName: String { get } func write(_ text: String) throws } extension FileWriter { var fileName: String { return "File.txt" } func write(_ text: String) throws { guard let dir = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first else { throw FileWriteError.directoryDoesntExist } let encoding = String.Encoding.utf8 guard let data = text.data(using: encoding) else { throw FileWriteError.convertToDataIssue } let fileUrl = dir.appendingPathComponent(fileName) if let fileHandle = FileHandle(forWritingAtPath: fileUrl.path) { fileHandle.seekToEndOfFile() fileHandle.write(data) } else { try text.write(to: fileUrl, atomically: false, encoding: encoding) } } }
回答7:
A variation over some of the posted answers, with following characteristics:
- based on Swift 5
- accessible as a static function
- appends new entries to the end of the file, if it exists
- creates the file, if it doesn't exist
- no cast to NS objects (more Swiftly)
fails silently if the text cannot be encoded or the path does not exist
class Logger { static var logFile: URL? { guard let documentsDirectory = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first else { return nil } let formatter = DateFormatter() formatter.dateFormat = "dd-MM-yyyy" let dateString = formatter.string(from: Date()) let fileName = "\(dateString).log" return documentsDirectory.appendingPathComponent(fileName) } static func log(_ message: String) { guard let logFile = logFile else { return } let formatter = DateFormatter() formatter.dateFormat = "HH:mm:ss" let timestamp = formatter.string(from: Date()) guard let data = (timestamp + ": " + message + "\n").data(using: String.Encoding.utf8) else { return } if FileManager.default.fileExists(atPath: logFile.path) { if let fileHandle = try? FileHandle(forWritingTo: logFile) { fileHandle.seekToEndOfFile() fileHandle.write(data) fileHandle.closeFile() } } else { try? data.write(to: logFile, options: .atomicWrite) } } }
来源：https://stackoverflow.com/questions/27327067/append-text-or-data-to-text-file-in-swift