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Classes/AVPlayerDemoPlaybackViewController.m

/*

     File: AVPlayerDemoPlaybackViewController.m

 Abstract: UIViewController managing a playback view, thumbnail view, and associated playback UI.

  Version: 1.3

 Disclaimer: IMPORTANT:  This Apple software is supplied to you by Apple

 Inc. ("Apple") in consideration of your agreement to the following

 terms, and your use, installation, modification or redistribution of

 this Apple software constitutes acceptance of these terms.  If you do

 not agree with these terms, please do not use, install, modify or

 redistribute this Apple software.

 In consideration of your agreement to abide by the following terms, and

 subject to these terms, Apple grants you a personal, non-exclusive

 license, under Apple's copyrights in this original Apple software (the

 "Apple Software"), to use, reproduce, modify and redistribute the Apple

 Software, with or without modifications, in source and/or binary forms;

 provided that if you redistribute the Apple Software in its entirety and

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 text and disclaimers in all such redistributions of the Apple Software.

 Neither the name, trademarks, service marks or logos of Apple Inc. may

 be used to endorse or promote products derived from the Apple Software

 without specific prior written permission from Apple.  Except as

 expressly stated in this notice, no other rights or licenses, express or

 implied, are granted by Apple herein, including but not limited to any

 patent rights that may be infringed by your derivative works or by other

 works in which the Apple Software may be incorporated.

 The Apple Software is provided by Apple on an "AS IS" basis.  APPLE

 MAKES NO WARRANTIES, EXPRESS OR IMPLIED, INCLUDING WITHOUT LIMITATION

 THE IMPLIED WARRANTIES OF NON-INFRINGEMENT, MERCHANTABILITY AND FITNESS

 FOR A PARTICULAR PURPOSE, REGARDING THE APPLE SOFTWARE OR ITS USE AND

 OPERATION ALONE OR IN COMBINATION WITH YOUR PRODUCTS.

 IN NO EVENT SHALL APPLE BE LIABLE FOR ANY SPECIAL, INDIRECT, INCIDENTAL

 OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF

 SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS

 INTERRUPTION) ARISING IN ANY WAY OUT OF THE USE, REPRODUCTION,

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 AND WHETHER UNDER THEORY OF CONTRACT, TORT (INCLUDING NEGLIGENCE),

 STRICT LIABILITY OR OTHERWISE, EVEN IF APPLE HAS BEEN ADVISED OF THE

 POSSIBILITY OF SUCH DAMAGE.

 Copyright (C) 2014 Apple Inc. All Rights Reserved.

*/

#import "AVPlayerDemoPlaybackViewController.h"

#import "AVPlayerDemoPlaybackView.h"

#import "AVPlayerDemoMetadataViewController.h"

@interface AVPlayerDemoPlaybackViewController ()

- (void)play:(id)sender;

- (void)pause:(id)sender;

- (void)showMetadata:(id)sender;

- (void)initScrubberTimer;

- (void)showPlayButton;

- (void)showStopButton;

- (void)syncScrubber;

- (IBAction)beginScrubbing:(id)sender;

- (IBAction)scrub:(id)sender;

- (IBAction)endScrubbing:(id)sender;

- (BOOL)isScrubbing;

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil;

- (id)init;

- (void)dealloc;

- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation;

- (void)viewDidLoad;

- (void)viewWillDisappear:(BOOL)animated;

- (void)handleSwipe:(UISwipeGestureRecognizer*)gestureRecognizer;

- (void)syncPlayPauseButtons;

- (void)setURL:(NSURL*)URL;

- (NSURL*)URL;

@end

@interface AVPlayerDemoPlaybackViewController (Player)

- (void)removePlayerTimeObserver;

- (CMTime)playerItemDuration;

- (BOOL)isPlaying;

- (void)playerItemDidReachEnd:(NSNotification *)notification ;

- (void)observeValueForKeyPath:(NSString*) path ofObject:(id)object change:(NSDictionary*)change context:(void*)context;

- (void)prepareToPlayAsset:(AVURLAsset *)asset withKeys:(NSArray *)requestedKeys;

@end

static void *AVPlayerDemoPlaybackViewControllerRateObservationContext = &AVPlayerDemoPlaybackViewControllerRateObservationContext;

static void *AVPlayerDemoPlaybackViewControllerStatusObservationContext = &AVPlayerDemoPlaybackViewControllerStatusObservationContext;

static void *AVPlayerDemoPlaybackViewControllerCurrentItemObservationContext = &AVPlayerDemoPlaybackViewControllerCurrentItemObservationContext;

#pragma mark -

@implementation AVPlayerDemoPlaybackViewController

@synthesize mPlayer, mPlayerItem, mPlaybackView, mToolbar, mPlayButton, mStopButton, mScrubber;

#pragma mark Asset URL

- (void)setURL:(NSURL*)URL

    if (mURL != URL)

        mURL = [URL copy];

/*

         Create an asset for inspection of a resource referenced by a given URL.

         Load the values for the asset key "playable".

*/

        AVURLAsset *asset = [AVURLAsset URLAssetWithURL:mURL options:nil];

        NSArray *requestedKeys = @[@"playable"];

        /* Tells the asset to load the values of any of the specified keys that are not already loaded. */

        [asset loadValuesAsynchronouslyForKeys:requestedKeys completionHandler:

^{

             dispatch_async( dispatch_get_main_queue(),

^{

                                /* IMPORTANT: Must dispatch to main queue in order to operate on the AVPlayer and AVPlayerItem. */

                                [self prepareToPlayAsset:asset withKeys:requestedKeys];

});

}];

- (NSURL*)URL

    return mURL;

#pragma mark -

#pragma mark Movie controller methods

#pragma mark

#pragma mark Button Action Methods

- (IBAction)play:(id)sender

    /* If we are at the end of the movie, we must seek to the beginning first

        before starting playback. */

    if (YES == seekToZeroBeforePlay)

        seekToZeroBeforePlay = NO;

        [self.mPlayer seekToTime:kCMTimeZero];

    [self.mPlayer play];

    [self showStopButton];

- (IBAction)pause:(id)sender

    [self.mPlayer pause];

    [self showPlayButton];

/* Display AVMetadataCommonKeyTitle and AVMetadataCommonKeyCopyrights metadata. */

- (IBAction)showMetadata:(id)sender

    AVPlayerDemoMetadataViewController* metadataViewController = [[AVPlayerDemoMetadataViewController alloc] init];

    [metadataViewController setMetadata:[[[self.mPlayer currentItem] asset] commonMetadata]];

    [self presentViewController:metadataViewController animated:YES completion:NULL];

#pragma mark -

#pragma mark Play, Stop buttons

/* Show the stop button in the movie player controller. */

-(void)showStopButton

    NSMutableArray *toolbarItems = [NSMutableArray arrayWithArray:[self.mToolbar items]];

    [toolbarItems replaceObjectAtIndex:0 withObject:self.mStopButton];

    self.mToolbar.items = toolbarItems;

/* Show the play button in the movie player controller. */

-(void)showPlayButton

    NSMutableArray *toolbarItems = [NSMutableArray arrayWithArray:[self.mToolbar items]];

    [toolbarItems replaceObjectAtIndex:0 withObject:self.mPlayButton];

    self.mToolbar.items = toolbarItems;

/* If the media is playing, show the stop button; otherwise, show the play button. */

- (void)syncPlayPauseButtons

    if ([self isPlaying])

        [self showStopButton];

    else

        [self showPlayButton];

-(void)enablePlayerButtons

    self.mPlayButton.enabled = YES;

    self.mStopButton.enabled = YES;

-(void)disablePlayerButtons

    self.mPlayButton.enabled = NO;

    self.mStopButton.enabled = NO;

#pragma mark -

#pragma mark Movie scrubber control

/* ---------------------------------------------------------

**  Methods to handle manipulation of the movie scrubber control

** ------------------------------------------------------- */

/* Requests invocation of a given block during media playback to update the movie scrubber control. */

-(void)initScrubberTimer

    double interval = .1f;

    CMTime playerDuration = [self playerItemDuration];

    if (CMTIME_IS_INVALID(playerDuration))

        return;

    double duration = CMTimeGetSeconds(playerDuration);

    if (isfinite(duration))

        CGFloat width = CGRectGetWidth([self.mScrubber bounds]);

        interval = 0.5f * duration / width;

    /* Update the scrubber during normal playback. */

    __weak AVPlayerDemoPlaybackViewController *weakSelf = self;

    mTimeObserver = [self.mPlayer addPeriodicTimeObserverForInterval:CMTimeMakeWithSeconds(interval, NSEC_PER_SEC)

                                queue:NULL /* If you pass NULL, the main queue is used. */

                                usingBlock:^(CMTime time)

                                                [weakSelf syncScrubber];

}];

/* Set the scrubber based on the player current time. */

- (void)syncScrubber

    CMTime playerDuration = [self playerItemDuration];

    if (CMTIME_IS_INVALID(playerDuration))

        mScrubber.minimumValue = 0.0;

        return;

    double duration = CMTimeGetSeconds(playerDuration);

    if (isfinite(duration))

        float minValue = [self.mScrubber minimumValue];

        float maxValue = [self.mScrubber maximumValue];

        double time = CMTimeGetSeconds([self.mPlayer currentTime]);

        [self.mScrubber setValue:(maxValue - minValue) * time / duration + minValue];

/* The user is dragging the movie controller thumb to scrub through the movie. */

- (IBAction)beginScrubbing:(id)sender

    mRestoreAfterScrubbingRate = [self.mPlayer rate];

    [self.mPlayer setRate:0.f];

    /* Remove previous timer. */

    [self removePlayerTimeObserver];

/* Set the player current time to match the scrubber position. */

- (IBAction)scrub:(id)sender

    if ([sender isKindOfClass:[UISlider class]] && !isSeeking)

        isSeeking = YES;

        UISlider* slider = sender;

        CMTime playerDuration = [self playerItemDuration];

        if (CMTIME_IS_INVALID(playerDuration)) {

            return;

        double duration = CMTimeGetSeconds(playerDuration);

        if (isfinite(duration))

            float minValue = [slider minimumValue];

            float maxValue = [slider maximumValue];

            float value = [slider value];

            double time = duration * (value - minValue) / (maxValue - minValue);

            [self.mPlayer seekToTime:CMTimeMakeWithSeconds(time, NSEC_PER_SEC) completionHandler:^(BOOL finished) {

                dispatch_async(dispatch_get_main_queue(), ^{

                    isSeeking = NO;

});

}];

/* The user has released the movie thumb control to stop scrubbing through the movie. */

- (IBAction)endScrubbing:(id)sender

    if (!mTimeObserver)

        CMTime playerDuration = [self playerItemDuration];

        if (CMTIME_IS_INVALID(playerDuration))

            return;

        double duration = CMTimeGetSeconds(playerDuration);

        if (isfinite(duration))

            CGFloat width = CGRectGetWidth([self.mScrubber bounds]);

            double tolerance = 0.5f * duration / width;

            __weak AVPlayerDemoPlaybackViewController *weakSelf = self;

            mTimeObserver = [self.mPlayer addPeriodicTimeObserverForInterval:CMTimeMakeWithSeconds(tolerance, NSEC_PER_SEC) queue:NULL usingBlock:

            ^(CMTime time)

                [weakSelf syncScrubber];

}];

    if (mRestoreAfterScrubbingRate)

        [self.mPlayer setRate:mRestoreAfterScrubbingRate];

        mRestoreAfterScrubbingRate = 0.f;

- (BOOL)isScrubbing

    return mRestoreAfterScrubbingRate != 0.f;

-(void)enableScrubber

    self.mScrubber.enabled = YES;

-(void)disableScrubber

    self.mScrubber.enabled = NO;

#pragma mark

#pragma mark View Controller

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil

    if ((self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil]))

        [self setPlayer:nil];

        [self setEdgesForExtendedLayout:UIRectEdgeAll];

    return self;

- (id)init

    if (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad)

        return [self initWithNibName:@"AVPlayerDemoPlaybackView-iPad" bundle:nil];

    else

        return [self initWithNibName:@"AVPlayerDemoPlaybackView" bundle:nil];

- (void)viewDidUnload

    self.mPlaybackView = nil;

    self.mToolbar = nil;

    self.mPlayButton = nil;

    self.mStopButton = nil;

    self.mScrubber = nil;

    [super viewDidUnload];

- (void)viewDidLoad

    [self setPlayer:nil];

    UIView* view  = [self view];

    UISwipeGestureRecognizer* swipeUpRecognizer = [[UISwipeGestureRecognizer alloc] initWithTarget:self action:@selector(handleSwipe:)];

    [swipeUpRecognizer setDirection:UISwipeGestureRecognizerDirectionUp];

    [view addGestureRecognizer:swipeUpRecognizer];

    UISwipeGestureRecognizer* swipeDownRecognizer = [[UISwipeGestureRecognizer alloc] initWithTarget:self action:@selector(handleSwipe:)];

    [swipeDownRecognizer setDirection:UISwipeGestureRecognizerDirectionDown];

    [view addGestureRecognizer:swipeDownRecognizer];

    UIBarButtonItem *scrubberItem = [[UIBarButtonItem alloc] initWithCustomView:self.mScrubber];

    UIBarButtonItem *flexItem = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemFlexibleSpace target:nil action:nil];

    UIButton *infoButton = [UIButton buttonWithType:UIButtonTypeInfoLight];

    [infoButton addTarget:self action:@selector(showMetadata:) forControlEvents:UIControlEventTouchUpInside];

    UIBarButtonItem *infoItem = [[UIBarButtonItem alloc] initWithCustomView:infoButton];

    self.mToolbar.items = @[self.mPlayButton, flexItem, scrubberItem, infoItem];

    isSeeking = NO;

    [self initScrubberTimer];

    [self syncPlayPauseButtons];

    [self syncScrubber];

    [super viewDidLoad];

- (void)viewWillDisappear:(BOOL)animated

    [self.mPlayer pause];

    [super viewWillDisappear:animated];

- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation

    return YES;

-(void)setViewDisplayName

    /* Set the view title to the last component of the asset URL. */

    self.title = [mURL lastPathComponent];

    /* Or if the item has a AVMetadataCommonKeyTitle metadata, use that instead. */

    for (AVMetadataItem* item in ([[[self.mPlayer currentItem] asset] commonMetadata]))

        NSString* commonKey = [item commonKey];

        if ([commonKey isEqualToString:AVMetadataCommonKeyTitle])

            self.title = [item stringValue];

- (void)handleSwipe:(UISwipeGestureRecognizer *)gestureRecognizer

    UIView* view = [self view];

    UISwipeGestureRecognizerDirection direction = [gestureRecognizer direction];

    CGPoint location = [gestureRecognizer locationInView:view];

    if (location.y < CGRectGetMidY([view bounds]))

        if (direction == UISwipeGestureRecognizerDirectionUp)

            [UIView animateWithDuration:0.2f animations:

^{

                [[self navigationController] setNavigationBarHidden:YES animated:YES];

            } completion:

            ^(BOOL finished)

                [[UIApplication sharedApplication] setStatusBarHidden:YES withAnimation:UIStatusBarAnimationSlide];

}];

        if (direction == UISwipeGestureRecognizerDirectionDown)

            [UIView animateWithDuration:0.2f animations:

^{

                [[UIApplication sharedApplication] setStatusBarHidden:NO withAnimation:UIStatusBarAnimationSlide];

            } completion:

            ^(BOOL finished)

                [[self navigationController] setNavigationBarHidden:NO animated:YES];

}];

    else

        if (direction == UISwipeGestureRecognizerDirectionDown)

            if (![self.mToolbar isHidden])

                [UIView animateWithDuration:0.2f animations:

^{

                    [self.mToolbar setTransform:CGAffineTransformMakeTranslation(0.f, CGRectGetHeight([self.mToolbar bounds]))];

                } completion:

                ^(BOOL finished)

                    [self.mToolbar setHidden:YES];

}];

        else if (direction == UISwipeGestureRecognizerDirectionUp)

            if ([self.mToolbar isHidden])

                [self.mToolbar setHidden:NO];

                [UIView animateWithDuration:0.2f animations:

^{

                    [self.mToolbar setTransform:CGAffineTransformIdentity];

                } completion:^(BOOL finished){}];

- (void)dealloc

    [self removePlayerTimeObserver];

    [self.mPlayer removeObserver:self forKeyPath:@"rate"];

    [mPlayer.currentItem removeObserver:self forKeyPath:@"status"];

    [self.mPlayer pause];

@end

@implementation AVPlayerDemoPlaybackViewController (Player)

#pragma mark Player Item

- (BOOL)isPlaying

    return mRestoreAfterScrubbingRate != 0.f || [self.mPlayer rate] != 0.f;

/* Called when the player item has played to its end time. */

- (void)playerItemDidReachEnd:(NSNotification *)notification

    /* After the movie has played to its end time, seek back to time zero

        to play it again. */

    seekToZeroBeforePlay = YES;

/* ---------------------------------------------------------

 **  Get the duration for a AVPlayerItem.

 ** ------------------------------------------------------- */

- (CMTime)playerItemDuration

    AVPlayerItem *playerItem = [self.mPlayer currentItem];

    if (playerItem.status == AVPlayerItemStatusReadyToPlay)

        return([playerItem duration]);

    return(kCMTimeInvalid);

/* Cancels the previously registered time observer. */

-(void)removePlayerTimeObserver

    if (mTimeObserver)

        [self.mPlayer removeTimeObserver:mTimeObserver];

        mTimeObserver = nil;

#pragma mark -

#pragma mark Loading the Asset Keys Asynchronously

#pragma mark -

#pragma mark Error Handling - Preparing Assets for Playback Failed

/* --------------------------------------------------------------

 **  Called when an asset fails to prepare for playback for any of

 **  the following reasons:

**

 **  1) values of asset keys did not load successfully,

 **  2) the asset keys did load successfully, but the asset is not

 **     playable

 **  3) the item did not become ready to play.

 ** ----------------------------------------------------------- */

-(void)assetFailedToPrepareForPlayback:(NSError *)error

    [self removePlayerTimeObserver];

    [self syncScrubber];

    [self disableScrubber];

    [self disablePlayerButtons];

    /* Display the error. */

    UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:[error localizedDescription]

                                                        message:[error localizedFailureReason]

                                                       delegate:nil

                                              cancelButtonTitle:@"OK"

                                              otherButtonTitles:nil];

    [alertView show];

#pragma mark Prepare to play asset, URL

/*

 Invoked at the completion of the loading of the values for all keys on the asset that we require.

 Checks whether loading was successfull and whether the asset is playable.

 If so, sets up an AVPlayerItem and an AVPlayer to play the asset.

*/

- (void)prepareToPlayAsset:(AVURLAsset *)asset withKeys:(NSArray *)requestedKeys

    /* Make sure that the value of each key has loaded successfully. */

    for (NSString *thisKey in requestedKeys)

        NSError *error = nil;

        AVKeyValueStatus keyStatus = [asset statusOfValueForKey:thisKey error:&error];

        if (keyStatus == AVKeyValueStatusFailed)

            [self assetFailedToPrepareForPlayback:error];

            return;

        /* If you are also implementing -[AVAsset cancelLoading], add your code here to bail out properly in the case of cancellation. */

    /* Use the AVAsset playable property to detect whether the asset can be played. */

    if (!asset.playable)

        /* Generate an error describing the failure. */

        NSString *localizedDescription = NSLocalizedString(@"Item cannot be played", @"Item cannot be played description");

        NSString *localizedFailureReason = NSLocalizedString(@"The assets tracks were loaded, but could not be made playable.", @"Item cannot be played failure reason");

        NSDictionary *errorDict = [NSDictionary dictionaryWithObjectsAndKeys:

                                   localizedDescription, NSLocalizedDescriptionKey,

                                   localizedFailureReason, NSLocalizedFailureReasonErrorKey,

                                   nil];

        NSError *assetCannotBePlayedError = [NSError errorWithDomain:@"StitchedStreamPlayer" code:0 userInfo:errorDict];

        /* Display the error to the user. */

        [self assetFailedToPrepareForPlayback:assetCannotBePlayedError];

        return;

    /* At this point we're ready to set up for playback of the asset. */

    /* Stop observing our prior AVPlayerItem, if we have one. */

    if (self.mPlayerItem)

        /* Remove existing player item key value observers and notifications. */

        [self.mPlayerItem removeObserver:self forKeyPath:@"status"];

        [[NSNotificationCenter defaultCenter] removeObserver:self

                                                        name:AVPlayerItemDidPlayToEndTimeNotification

                                                      object:self.mPlayerItem];

    /* Create a new instance of AVPlayerItem from the now successfully loaded AVAsset. */

    self.mPlayerItem = [AVPlayerItem playerItemWithAsset:asset];

    /* Observe the player item "status" key to determine when it is ready to play. */

    [self.mPlayerItem addObserver:self

                      forKeyPath:@"status"

                         options:NSKeyValueObservingOptionInitial | NSKeyValueObservingOptionNew

                         context:AVPlayerDemoPlaybackViewControllerStatusObservationContext];

    /* When the player item has played to its end time we'll toggle

     the movie controller Pause button to be the Play button */

    [[NSNotificationCenter defaultCenter] addObserver:self

                                             selector:@selector(playerItemDidReachEnd:)

                                                 name:AVPlayerItemDidPlayToEndTimeNotification

                                               object:self.mPlayerItem];

    seekToZeroBeforePlay = NO;

    /* Create new player, if we don't already have one. */

    if (!self.mPlayer)

        /* Get a new AVPlayer initialized to play the specified player item. */

        [self setPlayer:[AVPlayer playerWithPlayerItem:self.mPlayerItem]];

        /* Observe the AVPlayer "currentItem" property to find out when any

         AVPlayer replaceCurrentItemWithPlayerItem: replacement will/did

         occur.*/

        [self.player addObserver:self

                      forKeyPath:@"currentItem"

                         options:NSKeyValueObservingOptionInitial | NSKeyValueObservingOptionNew

                         context:AVPlayerDemoPlaybackViewControllerCurrentItemObservationContext];

        /* Observe the AVPlayer "rate" property to update the scrubber control. */

        [self.player addObserver:self

                      forKeyPath:@"rate"

                         options:NSKeyValueObservingOptionInitial | NSKeyValueObservingOptionNew

                         context:AVPlayerDemoPlaybackViewControllerRateObservationContext];

    /* Make our new AVPlayerItem the AVPlayer's current item. */

    if (self.player.currentItem != self.mPlayerItem)

        /* Replace the player item with a new player item. The item replacement occurs

         asynchronously; observe the currentItem property to find out when the

         replacement will/did occur

         If needed, configure player item here (example: adding outputs, setting text style rules,

         selecting media options) before associating it with a player

*/

        [self.mPlayer replaceCurrentItemWithPlayerItem:self.mPlayerItem];

        [self syncPlayPauseButtons];

    [self.mScrubber setValue:0.0];

#pragma mark -

#pragma mark Asset Key Value Observing

#pragma mark

#pragma mark Key Value Observer for player rate, currentItem, player item status

/* ---------------------------------------------------------

**  Called when the value at the specified key path relative

**  to the given object has changed.

**  Adjust the movie play and pause button controls when the

**  player item "status" value changes. Update the movie

**  scrubber control when the player item is ready to play.

**  Adjust the movie scrubber control when the player item

**  "rate" value changes. For updates of the player

**  "currentItem" property, set the AVPlayer for which the

**  player layer displays visual output.

**  NOTE: this method is invoked on the main queue.

** ------------------------------------------------------- */

- (void)observeValueForKeyPath:(NSString*) path

            ofObject:(id)object

            change:(NSDictionary*)change

            context:(void*)context

    /* AVPlayerItem "status" property value observer. */

    if (context == AVPlayerDemoPlaybackViewControllerStatusObservationContext)

        [self syncPlayPauseButtons];

        AVPlayerItemStatus status = [[change objectForKey:NSKeyValueChangeNewKey] integerValue];

        switch (status)

            /* Indicates that the status of the player is not yet known because

             it has not tried to load new media resources for playback */

            case AVPlayerItemStatusUnknown:

                [self removePlayerTimeObserver];

                [self syncScrubber];

                [self disableScrubber];

                [self disablePlayerButtons];

            break;

            case AVPlayerItemStatusReadyToPlay:

                /* Once the AVPlayerItem becomes ready to play, i.e.

                 [playerItem status] == AVPlayerItemStatusReadyToPlay,

                 its duration can be fetched from the item. */

                [self initScrubberTimer];

                [self enableScrubber];

                [self enablePlayerButtons];

            break;

            case AVPlayerItemStatusFailed:

                AVPlayerItem *playerItem = (AVPlayerItem *)object;

                [self assetFailedToPrepareForPlayback:playerItem.error];

            break;

    /* AVPlayer "rate" property value observer. */

    else if (context == AVPlayerDemoPlaybackViewControllerRateObservationContext)

        [self syncPlayPauseButtons];

    /* AVPlayer "currentItem" property observer.

        Called when the AVPlayer replaceCurrentItemWithPlayerItem:

        replacement will/did occur. */

    else if (context == AVPlayerDemoPlaybackViewControllerCurrentItemObservationContext)

        AVPlayerItem *newPlayerItem = [change objectForKey:NSKeyValueChangeNewKey];

        /* Is the new player item null? */

        if (newPlayerItem == (id)[NSNull null])

            [self disablePlayerButtons];

            [self disableScrubber];

        else /* Replacement of player currentItem has occurred */

            /* Set the AVPlayer for which the player layer displays visual output. */

            [self.mPlaybackView setPlayer:mPlayer];

            [self setViewDisplayName];

            /* Specifies that the player should preserve the video’s aspect ratio and

             fit the video within the layer’s bounds. */

            [self.mPlaybackView setVideoFillMode:AVLayerVideoGravityResizeAspect];

            [self syncPlayPauseButtons];

    else

        [super observeValueForKeyPath:path ofObject:object change:change context:context];

@end

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