Allows videos to rotate in landscape when displayed in a portrait only iOS app

AppDelegate.m

#import "AppDelegate.h"
#import <MediaPlayer/MediaPlayer.h>

@implementation AppDelegate
{
    BOOL _isFullScreen;
}

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    // We register ourselves to be notified when the movie player enters or exits full screen
    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(willEnterFullScreen:)
                                                 name:MPMoviePlayerWillEnterFullscreenNotification
                                               object:nil];
    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(willExitFullScreen:)
                                                 name:MPMoviePlayerWillExitFullscreenNotification
                                               object:nil];
    
    return YES;
}

#pragma mark - Allowing the movie players to rotate in fullscreen

- (void)willEnterFullScreen:(NSNotification *)notification
{
    _isFullScreen = YES;
}

- (void)willExitFullScreen:(NSNotification *)notification
{
    _isFullScreen = NO;
}

- (NSUInteger)application:(UIApplication *)application supportedInterfaceOrientationsForWindow:(UIWindow *)window
{
    if (_isFullScreen) {
        return UIInterfaceOrientationMaskPortrait | UIInterfaceOrientationMaskLandscapeLeft | UIInterfaceOrientationMaskLandscapeRight;
    } else {
        return UIInterfaceOrientationMaskPortrait;
    }
}

@Serlight In iOS 10.0 onwards I will prefer to use AVPlayerViewController from AVKit. It has the auto rotate support inbuilt even if you have the app locked in portrait mode. I know I am very late to reply but it might be helpful for other developers.

@ShivamKJJW, good call. No need to add landscape support to a portrait only app just for AVPlayerViewController.