Q:

Checking iOS device Orientation

I need to get orientation of iOS device for each time iOS device changes orientation remember not on launch of app or at loading time, for every single time device change orientation i need to register it . 

A:

As per apple UIKit Framework Reference following method called every time when device orientation changed.
For iOS8 and later
Notifies the container that the size of its view is about to change. (required)
- (void)viewWillTransitionToSize:(CGSize)size
       withTransitionCoordinator:(id<UIViewControllerTransitionCoordinator>)coordinator

Parameters

size - The new size for the container’s view.
coordinator    - The transition coordinator object managing the size
  change. You can use this object to animate your changes or get
  information about the transition that is in progress.

Discussion

UIKit calls this method before changing the size of a presented view
  controller’s view. You can override this method in your own objects
  and use it to perform additional tasks related to the size change. For
  example, a container view controller might use this method to override
  the traits of its embedded child view controllers. Use the provided
  coordinator object to animate any changes you make.
If you override this method in your custom view controllers, always
  call super at some point in your implementation so that UIKit can
  forward the size change message appropriately. View controllers
  forward the size change message to their views and child view
  controllers. Presentation controllers forward the size change to their
  presented view controller.

For iOS2 upto iOS8
Sent to the view controller after the user interface rotates.
Deprecation Statement

Use viewWillTransitionToSize:withTransitionCoordinator: to make
  interface-based adjustments.

Declaration
- (void)didRotateFromInterfaceOrientation:(UIInterfaceOrientation)fromInterfaceOrientation

Parameters

fromInterfaceOrientation - The old orientation of the user interface.
  For possible values, see UIInterfaceOrientation.

Discussion

Subclasses may override this method to perform additional actions
  immediately after the rotation. For example, you might use this method
  to reenable view interactions, start media playback again, or turn on
  expensive drawing or live updates. By the time this method is called,
  the interfaceOrientation property is already set to the new
  orientation. Your implementation of this method must call super at
  some point during its execution.
This method is called regardless of whether your code performs
  one-step or two-step rotations.

Source - UIKit Framework Reference (hope this helpful to you)

