直接上代码:       

NSData *data = [[NSString stringWithFormat:@"%@\n", self.textField.text] dataUsingEncoding:NSUTF8StringEncoding];
        
        NSString *document = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) firstObject];
        NSString *filePath = [document stringByAppendingPathComponent:@"file.txt"];
        NSLog(@"file path %@", filePath);
        
        NSFileHandle *fileHandle = [NSFileHandle fileHandleForWritingAtPath:filePath];
        if (!fileHandle) {
            NSLog(@"no file handle!");
            [self.textField.text writeToFile:filePath atomically:YES encoding:NSUTF8StringEncoding error:nil];
        }
        
        [fileHandle seekToEndOfFile];
        [fileHandle writeData:data];
        [fileHandle closeFile];

文件写入