how could two different View Controllers Communicate with each other - ios

Question

I have an application with tab bar and 3 different View Controller. one of these imanage a UItableView that I designed through Interface Builder (storyboard) and I set the it's view controller class in the Inspector -> inspector identity -> and I set class field there, hence, I have no control when this view controller get instantiated, as it's done through storyboard when user click on tab bar. notice, i'm new to objective C and iOS programming.
the issue that I'm facing, i'm also using remote notification. hence, when I receive a remote notification message in "didReceiveRemoteNotification" in the AppDelgate class. I need to update UI interface (above ViewController), but the issue I don't have a reference (pointer) to this ViewController from my AppDelgate class ( or do I?). the problem this ViewController instantiated by storyboard nor programmatically, otherwise I could have kept a reference to it.
I did some reading and I understand I could do this communication via NSNotification, but I think this will be an overkill for a problem that maybe arise just because I'm new to this and I don't have full understanding of iOS development.
Thanks,

Answer 1

NSNotifications are easy to use and are probably the right solution.
In the app delegate that needs to send the message, just put:
[[NSNotificationCenter defaultCenter] postNotificationName:#"MyNotification" object:someObjectYouWantToPassCouldBeAppDelegateOrRemoteNotificationObjectOrAnything];
In the view controller that is receiving the message, put:
-(void)viewDidLoad
{
[super viewDidLoad];
//you can add as many of these as you like to handle different notifications
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(handleNotification:) name:#"MyNotification" object:nil];
}
-(void)viewDidUnload
{
//make sure you remove every observer you've added here
[[NSNotificationCenter defaultCenter] removeObserver:self name:#"MyNotification" object:nil];
[super viewDidUnload];
}
-(void)dealloc
{
//clean up in case viewDidUnload wasn't called (it sometimes isn't)
[[NSNotificationCenter defaultCenter] removeObserver:self];
[super dealloc];
}
//use a different handler method for each notification
//the method name should match the selector in your observe call in viewDidLoad
-(void)handleNotification:(NSNotification *)notification
{
WhateverClassOfObjectYouWerePassing *object = notification.object;
//now you have a reference to the object that was passed from your app delegate
}
For different methods you want to call, just and a new notification name and a new handler method.

Answer 2

Your app Delegate will have a window property which points to the apps window.
Window Property has a -rootViewController property/method.
For your Tab Based Application it would return you the TabViewController.
Each TabViewController have a method -(NSArray *)viewControllers which returns the ViewControllers inside the Tab. These are arranged in the order.
To Access your applications AppDelegate use [[UIApplication sharedApplication]delegate]
Once you have these viewcontrollers you would know which all viewController these are since you have added it in the XIB files. and can perform your methods

Answer 3

1. Communicate two ViewControllers
If you want to communicate two ViewControllers, you should use #protocol as Apple recommended:
ViewController1.h
#interface ViewController1 : UIViewController<ViewController2Delegate, ViewController2DataSource>
#end
ViewController1.m
- (IBAction)goToViewController2:(id)sender{
if(viewController2 == nil) {
ViewController2 *viewController = [[ViewController2 alloc]
initWithNibName:#"View2" bundle:[NSBundle mainBundle]];
viewController2 = viewController;
}
//...
viewController2.delegate = self;
viewController2.dataSource = self;
[self.navigationController pushViewController:viewController2 animated:YES];
}
- (NSString)viewController:(ViewController2 *)controller itemForSomethingAtIndex:(NSInteger)index{
//Send to viewController2 what it needs
return [items objectAtIndex: index];
}
- (void)viewController:(ViewController2 *)controller didFinishEnteringItem:(NSString *)item{
//Handle the result from the viewController2
NSLog(#"result: %#", item);
}
ViewController2.h
#import <UIKit/UIKit.h>
// Define your delegate methods to return items to the delegate of this viewController
#protocol ViewController2Delegate <NSObject>
- (void)viewController:(ViewController2 *)controller didFinishEnteringItem:(NSString *)item;
#end
// Define your dataSource methods to send items from the dataSource to this viewController
#protocol ViewController2DataSource <NSObject>
- (NSString)viewController:(ViewController2 *)controller itemForSomethingAtIndex:(NSInteger)index;
#end
#interface ViewController2 : UIViewController
#property (nonatomic) id <ViewController2Delegate> delegate;
#property (nonatomic) id <ViewController2DataSource> dataSource;
#end
ViewController2.m
#import "ViewController2.h"
#interface ViewController2 ()
#end
#implementation ViewController2
#synthesize //...;
- (void)someMethod {
//Get someThing from controller1
NSString *item = [dataSource viewController: self itemForSomethingAtIndex:0];
//Return someThing to controller1
[delegate viewController: self didFinishEnteringItem: item];
}
2. Communicate backgroundTask with viewController
If you want to communicate a background task or handle a push notification, use #NickLockwood's answer. But this dont gonna work if the viewController its not loaded. In this case you should handle that in the AppDelegate:
//Get the appDelegate instance
AppDelegate *appDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];
//And call your custom method to show what it needs
[appDelegate customMethod];
Your custom method should call controllers consecutevly like:
AppDelegate > RootController > ViewController1 > ViewController2 > myMethod
//do something if viewController2 is visible to the user or push it before do something.
//if you use navigation controller, then you need to ask for the position and className