首先介紹一篇AVPlayer 的文章：
http://www.cnblogs.com/mzds/p/3711867.html
然后我再寫寫我在實(shí)際項(xiàng)目中遇到的問題

1.__ 然后監(jiān)聽playerItem的status和loadedTimeRange屬性，status有三種狀態(tài)__ ==>這是原文中的話，但是后面列出的屬性卻是AVPlayer 的status（應(yīng)該是作者筆誤），其實(shí)AVPlayerItem和AVPlayer 都有status 屬性的，而且可以使用KVO監(jiān)聽的。
文檔中枚舉類型如下：

typedef NS_ENUM(NSInteger, AVPlayerItemStatus) {
 AVPlayerItemStatusUnknown,
 AVPlayerItemStatusReadyToPlay,
 AVPlayerItemStatusFailed
};  

而AVPlayer 的status 枚舉類型如下：

typedef NS_ENUM(NSInteger, AVPlayerStatus) {
 AVPlayerStatusUnknown,
 AVPlayerStatusReadyToPlay,
 AVPlayerStatusFailed
};

看清楚啊！！！看清楚這兩個(gè)之間的區(qū)別，這里我主要想說明：
AVPlayerItemStatus是代表當(dāng)前播放資源item 的狀態(tài)（可以理解成這url鏈接or視頻文件。。。可以播放成功/失敗）
AVPlayerStatus是代表當(dāng)前播放器的狀態(tài)。

我在編程的時(shí)候遇到一個(gè)問題就是AVPlayer 的status 為
AVPlayerStatusReadyToPlay，但是視頻就是播放不成功，后來將KVO的監(jiān)聽換成了AVPlayerItem ，返回了AVPlayerItemStatusFailed。

編程的時(shí)候最好使用item 的status，會(huì)準(zhǔn)確點(diǎn)。
2.addPeriodicTimeObserverForInterval
給AVPlayer 添加time Observer 有利于我們?nèi)z測(cè)播放進(jìn)度
但是添加以后一定要記得移除，其實(shí)不移除程序不會(huì)崩潰，但是這個(gè)線程是不會(huì)釋放的，會(huì)占用你大量的內(nèi)存資源（當(dāng)時(shí)發(fā)現(xiàn)這個(gè)問題的時(shí)候我搞了一上午，說實(shí)話當(dāng)時(shí)我根本不知道是哪里出現(xiàn)了問題，自己對(duì)AVPlayer 根本不了解）
蘋果文檔中的注釋：

@result
An object conforming to the NSObject protocol. You must retain this returned value as long as you want the time observer to be invoked by the player.
Pass this object to -removeTimeObserver: to cancel time observation.

3.CMTime 結(jié)構(gòu)體
連接的教程里面 給的參數(shù)是CMTimeMake(1, 1)，其實(shí)就是1s調(diào)用一下block，
打個(gè)比方CMTimeMake(a, b)就是a/b秒之后調(diào)用一下block
介紹一個(gè)網(wǎng)站有關(guān)這個(gè)結(jié)構(gòu)體的:
https://zwo28.wordpress.com/2015/03/06/%E8%A7%86%E9%A2%91%E5%90%88%E6%88%90%E4%B8%ADcmtime%E7%9A%84%E7%90%86%E8%A7%A3%EF%BC%8C%E4%BB%A5%E5%8F%8A%E5%88%A9%E7%94%A8cmtime%E5%AE%9E%E7%8E%B0%E8%BF%87%E6%B8%A1%E6%95%88%E6%9E%9C/

4.拖動(dòng)slider 播放跳躍播放，要使用AVPlayer 對(duì)象的seekToTime:方法,
舉個(gè)最簡(jiǎn)單的例子：假如一個(gè)video視頻有20s，想要跳到10s進(jìn)行播放(_palyer 為AVPlayer 對(duì)象)
[_player seekToTime:CMTimeMake(10,1)];后面的參數(shù)寫1，前面的參數(shù)寫將要播放的秒數(shù)，我試驗(yàn)得出的結(jié)果，不要問我問什么，需要自己理解。
5.播放到結(jié)尾怎么回到開頭呢？
[_player seekToTime:kCMTimeZero];

下面放上我的寫的代碼：寫的不好請(qǐng)指正，多謝。

AVViewController.h

#import <UIKit/UIKit.h> 
@interface AVViewController : UIViewController 
@end

AVViewController.m

#import "AVViewController.h"
#import "VideoView.h"

@interface AVViewController () <VideoSomeDelegate>

@property (nonatomic ,strong) VideoView *videoView;

@property (nonatomic ,strong) NSMutableArray<NSLayoutConstraint *> *array;

@property (nonatomic ,strong) UISlider *videoSlider;

@property (nonatomic ,strong) NSMutableArray<NSLayoutConstraint *> *sliderArray;

@end

@implementation AVViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    [self.view setBackgroundColor:[UIColor whiteColor]];
    [self initVideoView];
    
}

- (void)initVideoView {
    
    //NSString *path = [[NSBundle mainBundle] pathForResource:@"some" ofType:@"mp4"];//這個(gè)時(shí)播放本地的，播放本地的時(shí)候還需要改VideoView.m中的代碼
    NSString *path = @"http://static.tripbe.com/videofiles/20121214/9533522808.f4v.mp4";
    _videoView = [[VideoView alloc] initWithUrl:path delegate:self];
    _videoView.someDelegate = self;
    [_videoView setTranslatesAutoresizingMaskIntoConstraints:NO];
    [self.view addSubview:_videoView];
    [self initVideoSlider];
    
    if (self.traitCollection.verticalSizeClass == UIUserInterfaceSizeClassCompact) {
        [self installLandspace];
    } else {
        [self installVertical];
    }
}
- (void)installVertical {
    if (_array != nil) {
        [self.view removeConstraints:_array];
        [_array removeAllObjects];
        [self.view removeConstraints:_sliderArray];
        [_sliderArray removeAllObjects];
    } else {
        _array = [NSMutableArray array];
        _sliderArray = [NSMutableArray array];
    }
    id topGuide = self.topLayoutGuide;
    NSDictionary *dic = @{@"top":@100,@"height":@180,@"edge":@20,@"space":@80};
    [_array addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[_videoView]|" options:0 metrics:nil views:NSDictionaryOfVariableBindings(_videoView)]];
    [_array addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-(edge)-[_videoSlider]-(edge)-|" options:0 metrics:dic views:NSDictionaryOfVariableBindings(_videoSlider)]];
    [_array addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[topGuide]-(top)-[_videoView(==height)]-(space)-[_videoSlider]" options:0 metrics:dic views:NSDictionaryOfVariableBindings(_videoView,topGuide,_videoSlider)]];
    [self.view addConstraints:_array];
    
    
    
}
- (void)installLandspace {
    if (_array != nil) {
        
        [self.view removeConstraints:_array];
        [_array removeAllObjects];
        
        [self.view removeConstraints:_sliderArray];
        [_sliderArray removeAllObjects];
    } else {
        
        _array = [NSMutableArray array];
        _sliderArray = [NSMutableArray array];
    }
    
    id topGuide = self.topLayoutGuide;
    NSDictionary *dic = @{@"edge":@20,@"space":@30};
    
    [_array addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[_videoView]|" options:0 metrics:nil views:NSDictionaryOfVariableBindings(_videoView)]];
    [_array addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[topGuide][_videoView]|" options:0 metrics:nil views:NSDictionaryOfVariableBindings(_videoView,topGuide)]];
    [self.view addConstraints:_array];
    
    [_sliderArray addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-(edge)-[_videoSlider]-(edge)-|" options:0 metrics:dic views:NSDictionaryOfVariableBindings(_videoSlider)]];
    [_sliderArray addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"V:[_videoSlider]-(space)-|" options:0 metrics:dic views:NSDictionaryOfVariableBindings(_videoSlider)]];
    [self.view addConstraints:_sliderArray];
}
- (void)initVideoSlider {
    
    _videoSlider = [[UISlider alloc] init];
    [_videoSlider setTranslatesAutoresizingMaskIntoConstraints:NO];
    [_videoSlider setThumbImage:[UIImage imageNamed:@"sliderButton"] forState:UIControlStateNormal];
    [self.view addSubview:_videoSlider];
    
}
- (void)willTransitionToTraitCollection:(UITraitCollection *)newCollection withTransitionCoordinator:(id <UIViewControllerTransitionCoordinator>)coordinator {

    [super willTransitionToTraitCollection:newCollection withTransitionCoordinator:coordinator];
    [coordinator animateAlongsideTransition:^(id <UIViewControllerTransitionCoordinatorContext> context) {
        
        if (newCollection.verticalSizeClass == UIUserInterfaceSizeClassCompact) {
            [self installLandspace];
        } else {
            [self installVertical];
        }
        [self.view setNeedsLayout];
    } completion:nil];

}
- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
}
#pragma mark -
- (void)flushCurrentTime:(NSString *)timeString sliderValue:(float)sliderValue {
    _videoSlider.value = sliderValue;
}
/*
#pragma mark - Navigation

// In a storyboard-based application, you will often want to do a little preparation before navigation
- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender {
    // Get the new view controller using [segue destinationViewController].
    // Pass the selected object to the new view controller.
}
*/

@end

VideoView.h

#import <UIKit/UIKit.h>
#import <AVFoundation/AVFoundation.h>

@protocol VideoSomeDelegate <NSObject>

@required

- (void)flushCurrentTime:(NSString *)timeString sliderValue:(float)sliderValue;

//- (void)flushVideoLength:(float)videoLength;




@end

@interface VideoView : UIView

@property (nonatomic ,strong) NSString *playerUrl;


@property (nonatomic ,readonly) AVPlayerItem *item;

@property (nonatomic ,readonly) AVPlayerLayer *playerLayer;

@property (nonatomic ,readonly) AVPlayer *player;

@property (nonatomic ,weak) id <VideoSomeDelegate> someDelegate;

- (id)initWithUrl:(NSString *)path delegate:(id<VideoSomeDelegate>)delegate;


@end

@interface VideoView  (Guester)

- (void)addSwipeView;

@end

VideoView.m

#import "VideoView.h"
#import <AVFoundation/AVFoundation.h>
#import <MediaPlayer/MPVolumeView.h>    
typedef enum  {
    ChangeNone,
    ChangeVoice,
    ChangeLigth,
    ChangeCMTime
}Change;


@interface VideoView ()

@property (nonatomic ,readwrite) AVPlayerItem *item;

@property (nonatomic ,readwrite) AVPlayerLayer *playerLayer;

@property (nonatomic ,readwrite) AVPlayer *player;

@property (nonatomic ,strong)  id timeObser;

@property (nonatomic ,assign) float videoLength;

@property (nonatomic ,assign) Change changeKind;

@property (nonatomic ,assign) CGPoint lastPoint;

//Gesture
@property (nonatomic ,strong) UIPanGestureRecognizer *panGesture;
@property (nonatomic ,strong) MPVolumeView *volumeView;
@property (nonatomic ,weak) UISlider *volumeSlider;
@property (nonatomic ,strong) UIView *darkView;
@end

@implementation VideoView

- (id)initWithUrl:(NSString *)path delegate:(id<VideoSomeDelegate>)delegate {
    if (self = [super init]) {
        _playerUrl = path;
        _someDelegate = delegate;
        [self setBackgroundColor:[UIColor blackColor]];
        [self setUpPlayer];
        [self addSwipeView];
    
    }
    return self;
}
- (void)setUpPlayer {
    //本地視頻
    //NSURL *rul = [NSURL fileURLWithPath:_playerUrl];

     NSURL *url = [NSURL URLWithString:_playerUrl];
     NSLog(@"%@",url);
     
    _item = [[AVPlayerItem alloc] initWithURL:url];
    _player = [AVPlayer playerWithPlayerItem:_item];
    _playerLayer = [AVPlayerLayer playerLayerWithPlayer:_player];
    _playerLayer.videoGravity = AVLayerVideoGravityResizeAspect;
    [self.layer addSublayer:_playerLayer];
    
    [self addVideoKVO];
    [self addVideoTimerObserver];
    [self addVideoNotic];
}


#pragma mark - KVO
- (void)addVideoKVO
{
    //KVO
    [_item addObserver:self forKeyPath:@"status" options:NSKeyValueObservingOptionNew context:nil];
    [_item addObserver:self forKeyPath:@"loadedTimeRanges" options:NSKeyValueObservingOptionNew context:nil];
    [_item addObserver:self forKeyPath:@"playbackBufferEmpty" options:NSKeyValueObservingOptionNew context:nil];
}
- (void)removeVideoKVO {
    [_item removeObserver:self forKeyPath:@"status"];
    [_item removeObserver:self forKeyPath:@"loadedTimeRanges"];
    [_item removeObserver:self forKeyPath:@"playbackBufferEmpty"];
}
- (void)observeValueForKeyPath:(nullable NSString *)keyPath ofObject:(nullable id)object change:(nullable NSDictionary<NSString*, id> *)change context:(nullable void *)context {

    if ([keyPath isEqualToString:@"status"]) {
        AVPlayerItemStatus status = _item.status;
        switch (status) {
            case AVPlayerItemStatusReadyToPlay:
            {
                NSLog(@"AVPlayerItemStatusReadyToPlay");
                [_player play];
                _videoLength = floor(_item.asset.duration.value * 1.0/ _item.asset.duration.timescale);
            }
                break;
            case AVPlayerItemStatusUnknown:
            {
                NSLog(@"AVPlayerItemStatusUnknown");
            }
                break;
            case AVPlayerItemStatusFailed:
            {
                NSLog(@"AVPlayerItemStatusFailed");
                NSLog(@"%@",_item.error);
            }
                break;
                
            default:
                break;
        }
    } else if ([keyPath isEqualToString:@"loadedTimeRanges"]) {
    
    } else if ([keyPath isEqualToString:@"playbackBufferEmpty"]) {
        
    }
}
#pragma mark - Notic
- (void)addVideoNotic {
    
    //Notification
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(movieToEnd:) name:AVPlayerItemDidPlayToEndTimeNotification object:nil];
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(movieJumped:) name:AVPlayerItemTimeJumpedNotification object:nil];
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(movieStalle:) name:AVPlayerItemPlaybackStalledNotification object:nil];
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(backGroundPauseMoive) name:UIApplicationDidEnterBackgroundNotification object:nil];
    
}
- (void)removeVideoNotic {
    //
    [[NSNotificationCenter defaultCenter] removeObserver:self name:AVPlayerItemDidPlayToEndTimeNotification object:nil];
    [[NSNotificationCenter defaultCenter] removeObserver:self name:AVPlayerItemPlaybackStalledNotification object:nil];
    [[NSNotificationCenter defaultCenter] removeObserver:self name:AVPlayerItemTimeJumpedNotification object:nil];
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}

- (void)movieToEnd:(NSNotification *)notic {
    NSLog(@"%@",NSStringFromSelector(_cmd));
}
- (void)movieJumped:(NSNotification *)notic {
    NSLog(@"%@",NSStringFromSelector(_cmd));
}
- (void)movieStalle:(NSNotification *)notic {
    NSLog(@"%@",NSStringFromSelector(_cmd));
}
- (void)backGroundPauseMoive {
    NSLog(@"%@",NSStringFromSelector(_cmd));
}

#pragma mark - TimerObserver
- (void)addVideoTimerObserver {
    __weak typeof (self)self_ = self;
    _timeObser = [_player addPeriodicTimeObserverForInterval:CMTimeMake(1, 1) queue:NULL usingBlock:^(CMTime time) {
        float currentTimeValue = time.value*1.0/time.timescale/self_.videoLength;
        NSString *currentString = [self_ getStringFromCMTime:time];

        if ([self_.someDelegate respondsToSelector:@selector(flushCurrentTime:sliderValue:)]) {
            [self_.someDelegate flushCurrentTime:currentString sliderValue:currentTimeValue];
        } else {
            NSLog(@"no response");
        }
        NSLog(@"%@",self_.someDelegate);
    }];
}
- (void)removeVideoTimerObserver {
    NSLog(@"%@",NSStringFromSelector(_cmd));
    [_player removeTimeObserver:_timeObser];
}


#pragma mark - Utils
- (NSString *)getStringFromCMTime:(CMTime)time
{
    float currentTimeValue = (CGFloat)time.value/time.timescale;//得到當(dāng)前的播放時(shí)
    
    NSDate * currentDate = [NSDate dateWithTimeIntervalSince1970:currentTimeValue];
    NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSCalendarIdentifierGregorian];
    NSInteger unitFlags = NSCalendarUnitHour | NSCalendarUnitMinute | NSCalendarUnitSecond ;
    NSDateComponents *components = [calendar components:unitFlags fromDate:currentDate];
    
    if (currentTimeValue >= 3600 )
    {
        return [NSString stringWithFormat:@"%ld:%ld:%ld",components.hour,components.minute,components.second];
    }
    else
    {
        return [NSString stringWithFormat:@"%ld:%ld",components.minute,components.second];
    }
}

- (NSString *)getVideoLengthFromTimeLength:(float)timeLength
{
    NSDate * date = [NSDate dateWithTimeIntervalSince1970:timeLength];
    NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSCalendarIdentifierGregorian];
    NSInteger unitFlags = NSCalendarUnitHour | NSCalendarUnitMinute | NSCalendarUnitSecond ;
    NSDateComponents *components = [calendar components:unitFlags fromDate:date];
    
    if (timeLength >= 3600 )
    {
        return [NSString stringWithFormat:@"%ld:%ld:%ld",components.hour,components.minute,components.second];
    }
    else
    {
        return [NSString stringWithFormat:@"%ld:%ld",components.minute,components.second];
    }
}

- (void)layoutSubviews {
    [super layoutSubviews];
    _playerLayer.frame = self.bounds;
}

#pragma mark - release 
- (void)dealloc {
    NSLog(@"%@",NSStringFromSelector(_cmd));
    [self removeVideoTimerObserver];
    [self removeVideoNotic];
    [self removeVideoKVO];
}

@end

#pragma mark - VideoView (Guester)

@implementation VideoView (Guester)

- (void)addSwipeView {
    _panGesture = [[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(swipeAction:)];
    [self addGestureRecognizer:_panGesture];
    [self setUpDarkView];
}
- (void)setUpDarkView {
    _darkView = [[UIView alloc] init];
    [_darkView setTranslatesAutoresizingMaskIntoConstraints:NO];
    [_darkView setBackgroundColor:[UIColor blackColor]];
    _darkView.alpha = 0.0;
    [self addSubview:_darkView];
    
    NSMutableArray *darkArray = [NSMutableArray array];
    [darkArray addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[_darkView]|" options:0 metrics:nil views:NSDictionaryOfVariableBindings(_darkView)]];
    [darkArray addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[_darkView]|" options:0 metrics:nil views:NSDictionaryOfVariableBindings(_darkView)]];
    [self addConstraints:darkArray];
}

- (void)swipeAction:(UISwipeGestureRecognizer *)gesture {
    
    switch (gesture.state) {
        case UIGestureRecognizerStateBegan:
        {
            _changeKind = ChangeNone;
            _lastPoint = [gesture locationInView:self];
        }
            break;
        case  UIGestureRecognizerStateChanged:
        {
            [self getChangeKindValue:[gesture locationInView:self]];
            
        }
            break;
        case UIGestureRecognizerStateEnded:
        {
            if (_changeKind == ChangeCMTime) {
                [self changeEndForCMTime:[gesture locationInView:self]];
            }
            _changeKind = ChangeNone;
            _lastPoint = CGPointZero;
        }
        default:
            break;
    }
    
}
- (void)getChangeKindValue:(CGPoint)pointNow {
    
    switch (_changeKind) {
            
        case ChangeNone:
        {
            [self changeForNone:pointNow];
        }
            break;
        case ChangeCMTime:
        {
            [self changeForCMTime:pointNow];
        }
            break;
        case ChangeLigth:
        {
            [self changeForLigth:pointNow];
        }
            break;
        case ChangeVoice:
        {
            [self changeForVoice:pointNow];
        }
            break;
            
        default:
            break;
    }
}
- (void)changeForNone:(CGPoint) pointNow {
    if (fabs(pointNow.x - _lastPoint.x) > fabs(pointNow.y - _lastPoint.y)) {
        _changeKind = ChangeCMTime;
    } else {
        float halfWight = self.bounds.size.width / 2;
        if (_lastPoint.x < halfWight) {
            _changeKind =  ChangeLigth;
        } else {
            _changeKind =   ChangeVoice;
        }
        _lastPoint = pointNow;
    }
}
- (void)changeForCMTime:(CGPoint) pointNow {
    float number = fabs(pointNow.x - _lastPoint.x);
    if (pointNow.x > _lastPoint.x && number > 10) {
        float currentTime = _player.currentTime.value / _player.currentTime.timescale;
        float tobeTime = currentTime + number*0.5;
        NSLog(@"forwart to  changeTo  time:%f",tobeTime);
    } else if (pointNow.x < _lastPoint.x && number > 10) {
        float currentTime = _player.currentTime.value / _player.currentTime.timescale;
        float tobeTime = currentTime - number*0.5;
        NSLog(@"back to  time:%f",tobeTime);
    }
}
- (void)changeForLigth:(CGPoint) pointNow {
    float number = fabs(pointNow.y - _lastPoint.y);
    if (pointNow.y > _lastPoint.y && number > 10) {
        _lastPoint = pointNow;
        [self minLigth];
        
    } else if (pointNow.y < _lastPoint.y && number > 10) {
        _lastPoint = pointNow;
        [self upperLigth];
    }
}
- (void)changeForVoice:(CGPoint)pointNow {
    float number = fabs(pointNow.y - _lastPoint.y);
    if (pointNow.y > _lastPoint.y && number > 10) {
        _lastPoint = pointNow;
        [self minVolume];
    } else if (pointNow.y < _lastPoint.y && number > 10) {
        _lastPoint = pointNow;
        [self upperVolume];
    }
}
- (void)changeEndForCMTime:(CGPoint)pointNow {
    if (pointNow.x > _lastPoint.x ) {
        NSLog(@"end for CMTime Upper");
        float length = fabs(pointNow.x - _lastPoint.x);
        [self upperCMTime:length];
    } else {
        NSLog(@"end for CMTime min");
        float length = fabs(pointNow.x - _lastPoint.x);
        [self mineCMTime:length];
    }
}
- (void)upperLigth {
    
    if (_darkView.alpha >= 0.1) {
        _darkView.alpha =  _darkView.alpha - 0.1;
    }
    
}
- (void)minLigth {
    if (_darkView.alpha <= 1.0) {
        _darkView.alpha =  _darkView.alpha + 0.1;
    }
}

- (void)upperVolume {
    if (self.volumeSlider.value <= 1.0) {
        self.volumeSlider.value =  self.volumeSlider.value + 0.1 ;
    }
    
}
- (void)minVolume {
    if (self.volumeSlider.value >= 0.0) {
        self.volumeSlider.value =  self.volumeSlider.value - 0.1 ;
    }
}
#pragma mark -CMTIME
- (void)upperCMTime:(float)length {

    float currentTime = _player.currentTime.value / _player.currentTime.timescale;
    float tobeTime = currentTime + length*0.5;
    if (tobeTime > _videoLength) {
        [_player seekToTime:_item.asset.duration];
    } else {
        [_player seekToTime:CMTimeMake(tobeTime, 1)];
    }
}
- (void)mineCMTime:(float)length {

    float currentTime = _player.currentTime.value / _player.currentTime.timescale;
    float tobeTime = currentTime - length*0.5;
    if (tobeTime <= 0) {
        [_player seekToTime:kCMTimeZero];
    } else {
        [_player seekToTime:CMTimeMake(tobeTime, 1)];
    }
}

- (MPVolumeView *)volumeView {
    
    if (_volumeView == nil) {
        _volumeView = [[MPVolumeView alloc] init];
        _volumeView.hidden = YES;
        [self addSubview:_volumeView];
    }
    return _volumeView;
}

- (UISlider *)volumeSlider {
    if (_volumeSlider== nil) {
        NSLog(@"%@",[self.volumeView subviews]);
        for (UIView  *subView in [self.volumeView subviews]) {
            if ([subView.class.description isEqualToString:@"MPVolumeSlider"]) {
                _volumeSlider = (UISlider*)subView;
                break;
            }
        }
    }
    return _volumeSlider;
}

@end