How to save data in other class not in Appdelegate?

Objective-C Programming (2nd Edition) 31. Your First iOS Application
1

If I can save data in other class, such as my view controller?

Here is my code

#import "MyviewController.h"
NSString *docPath(){...};
...
@implementation MyViewController

-(void)viewDidLoad{
   NSArray *plist = [NSArray arrayWithContentsOfFile:docPath()];
   if(plist){
        todoItems = [plist mutableCopy];
    }else{    
    todoItems = [[NSMutableArray alloc] init];
}

//here I want to save data in this controller, which is turned out to be wrong.

- (void)applicationDidEnterBackground:(UIApplication *)application{
    [todoItems writeToFile:docPath() atomically:YES];
}

So, How to save data in other class not in Appdelegate?