This post gives a small idea about the push notification of iOS and Android.

Life Cycle of Push notification looks as follows

Push Notification overview of APN and GCM Server as follows

PUSH NOTIFICATION
IPhone / IPad	Android
Login into developer console	Login into Google API console,
1. Creating the SSL certificate 1.1. Generating a Certificate Request Launch the Keychain Access application on your Mac. Select the menu item Keychain Access > Certificate Assistant > Request a Certificate From a Certificate Authority… Enter you name and email address (leave CA Email Address blank). Select "Saved to disk" to download the .certSigningRequest file to your desktop. 1.2 Creating and Configuring an AppID Navigate to the Apple Developer Member Center website, and enter the iOS Provisioning Portal. Select App IDs from the menu on the left. Select the "New App ID" button and create a new App ID. Make sure you do not enter the wildcard character ("") in the "Bundle Identifier" field. Select "Configure" beside your newly created App ID. Check "Enable for Apple Push Notification service". Then, Click "Configure" beside the "Development Push SSL Certificate". The "Apple Push Notification service SSL Certificate Assistant" wizard should be displayed. Select "Continue", then select "Choose File" and locate the .certSigningRequest file you created in the previous step. Select "Generate" and download the generated SSL certificate. Double click on the downloaded SSL certificate to install it in your Keychain. In your keychain, under "My Certificates", find the certificate you just added. It should be called "Apple Development IOS Push Services: xxx". Right-click on it, select "Export Apple…", and save it as a .p12 file. Do not enter an export password when you prompted! 2. Creating the Provisioning Profile* Select the "Provisioning" tab in the iOS Provisioning Portal. Create a new profile by selecting "New Profile". Fill in the information. Make sure you select your developer certificate, the App ID you just created, and the devices you'll be testing with. Download the generated Provisioning Profile by clicking the "Download" button under the "Actions" column. Install the profile by double-clicking on the downloaded file. This should open the iPhone Configuration Utility App. 3. Configuring the Parse App If you want your users to be able to send push notifications, you will need to toggle the "Client push enabled?" option to "on" under the "Push Notification Settings" header. This is useful for applications like chat clients, where users need to send push messages. For this tutorial, select toggle the option to "on".	If you haven't created an API project yet, this page will prompt you to do so: Click Create project. Your browser URL will change to something like: https://code.google.com/apis/console/#project:4815162342 Note: #project: (4815162342 in this example). This is your project ID, To enable the GCM service: In the main Google APIs Console page, select Services. Turn the Google Cloud Messaging toggle to ON.
Use bundle identifier in the project	Use the project ID (4815162342) in the project
P12 key is generated for the above bundle identifier, Use the p12 key and respective pwd to send push notification from your server to apple server	Create new Server key, which looks like AIzaxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx and used to send push notification from your server to GCM server

Client Side Code
// Register for push notifications [application registerForRemoteNotificationTypes: UIRemoteNotificationTypeBadge \| UIRemoteNotificationTypeAlert \| UIRemoteNotificationTypeSound];	// Register for push notifications Manifest.xml
Handling Remote Notifications didReceiveRemoteNotification: didRegisterForRemoteNotificationsWithDeviceToken: didFailToRegisterForRemoteNotificationsWithError: For more details for push notification visit apple documentation ApplePushService	public class GCMIntentService extends GCMBaseIntentService { public GCMIntentService() { super("PRODUCT_ID"); } // Here PRODUCT_ID is 4815162342 @Override protected void onError(Context arg0, String arg1) { } @Override protected void onMessage(Context arg0, Intent arg1) { } @Override protected void onRegistered(Context arg0, String arg1) { } @Override protected void onUnregistered(Context arg0, String arg1) { } } for more details visit Android Guide GCM
didRegisterForRemoteNotificationsWithDeviceToken: Tells the delegate that the application successfully registered with Apple Push Service (APS). Device Token is generated Length of the device token is 64 bit. Send Device Token to server, so it can use it to send push notification	onRegistered(Context context, String regId): Called after a registration intent is received, passes the registration ID assigned by GCM to that device/application pair as parameter. Typically, you should send the regid to your server so it can use it to send messages to this device.
Java Server Side Code
Add jar file : JavaPNS_2.2.jar	Add Jar file : gcm-server.jar
APNTest iOS Code Here	APNTest Android Code Here

Thanks for reading :).
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There are several ways to pass the data between the view controllers.

1. Delegates & Protocols

2. NSUserDefaults

3. Database

4. Singleton Object

Here I am going to explain using Delegates & Protocols.
First we will get an idea about the protocols and delegates.

Protocols

Protocols declare methods that can be implemented by any class. Protocols are useful in at least three situations:

· To declare methods that others are expected to implement

· To declare the interface to an object while concealing its class

· To capture similarities among classes that are not hierarchically related

Declaring a Protocol

You declare formal protocols with the @protocol directive:

@protocol ProtocolName

method declarations

@end

Optional Protocol Methods

Protocol methods can be marked as optional using the @optional keyword. Corresponding to the @optional modal keyword, there is a @required keyword to formally denote the semantics of the default behavior. You can use @optional and @required to partition your protocol into sections as you see fit. If you do not specify any keyword, the default is @required.

@protocol MyProtocol

- (void)requiredMethod;

@optional

- (void)anOptionalMethod;

- (void)anotherOptionalMethod;

@required

- (void)anotherRequiredMethod;

@end

Delegates

iOS uses Objective-C delegates to implement the delegation pattern, in which one object passes work off to another. The object doing the work is the delegate of the first object. An object tells its delegate to do work by sending it messages after certain things happen.
Delegates are basically a way of extending the behavior of a class without needing to create subclasses. Applications in iOS often use delegates when one class calls back to another after an important action occurs.

For Java Developers:

Protocol is similar to Interface

Protocol	Interface
@protocol ProtocolName -(void)methodName(NSString *):value; @end	public interface ProtocolName { public void methodName(String value); }
Declare protocol in .h file and import the .h file, wherever protocol has to be called	implements the interface and have a logic in a class