Appending to the end of a file with NSMutableString

Question

I have a log file that I'm trying to append data to the end of. I have an NSMutableString textToWrite variable, and I am doing the following:

[textToWrite writeToFile:filepath atomically:YES 
                                    encoding: NSUnicodeStringEncoding error:&err];

However, when I do this all the text inside the file is replaced with the text in textToWrite. How can I instead append to the end of the file? (Or even better, how can I append to the end of the file on a new line?)

Answer 1

Initially I thought that using the FileHandler method in the accepted answer that I was going to get a bunch of hex data values written to my file, but I got readable text which is all I need. So based off the accepted answer, this is what I came up with:

-(void) writeToLogFile:(NSString*)content{
    content = [NSString stringWithFormat:@"%@\n",content];

    //get the documents directory:
    NSString *documentsDirectory = [NSHomeDirectory() stringByAppendingPathComponent:@"Documents"];
    NSString *fileName = [documentsDirectory stringByAppendingPathComponent:@"hydraLog.txt"];

    NSFileHandle *fileHandle = [NSFileHandle fileHandleForWritingAtPath:fileName];
    if (fileHandle){
        [fileHandle seekToEndOfFile];
        [fileHandle writeData:[content dataUsingEncoding:NSUTF8StringEncoding]];
        [fileHandle closeFile];
    }
    else{
        [content writeToFile:fileName
                  atomically:NO
                    encoding:NSUTF8StringEncoding
                       error:nil];
    }
}

This way if the file doesn't yet exist, you create it. If it already exists then you only append to it. Also, if you go into the plist and add a key under the information property list UIFileSharingEnabled and set the value to true then the user can sync with their computer and see the log file through iTunes.

Answer 2

And here is a (slightly adopted) Swift version of Chase Roberts' solution:

static func writeToFile(content: String, fileName: String = "log.txt") {
    let contentWithNewLine = content+"\n"
    let filePath = NSHomeDirectory() + "/Documents/" + fileName
    let fileHandle = NSFileHandle(forWritingAtPath: filePath)
    if (fileHandle != nil) {
        fileHandle?.seekToEndOfFile()
        fileHandle?.writeData(contentWithNewLine.dataUsingEncoding(NSUTF8StringEncoding)!)
    }
    else {
        do {
            try contentWithNewLine.writeToFile(filePath, atomically: true, encoding: NSUTF8StringEncoding)
        } catch {
            print("Error while creating \(filePath)")
        }
    }
}