以下是效果图：（该功能需要使用真机才能看到效果）

锁屏效果

第一步：需要先打开后台播放，否则在后台模式下，音乐播放不出

设置后台模式

第二步：在AppDelegate中写入以下代码：

// 将进入后台
-(void)applicationWillResignActive:(UIApplication *)application
{
  // 接受远程控制
  [self becomeFirstResponder];
  [[UIApplication sharedApplication] 
}  

// 将进入前台
-(void)applicationWillEnterForeground:(UIApplication *)application {
    // 取消远程控制
    [self resignFirstResponder];
    [[UIApplication sharedApplication] endReceivingRemoteControlEvents];
}

第三步：在AppDelegate中实现锁屏下音频界面的操作：

// 锁屏播放界面的操作事件
- (void)remoteControlReceivedWithEvent:(UIEvent *)event
{
  // 判断是否为远程控制
  if (event.type == UIEventTypeRemoteControl) {  
      NSInteger type = -1;
      switch (event.subtype) {
        case  UIEventSubtypeRemoteControlPlay: // 播放         
                type = UIEventSubtypeRemoteControlPlay;
                break;
        case UIEventSubtypeRemoteControlPause: // 暂停        
                type = UIEventSubtypeRemoteControlPause;
                break;
        case UIEventSubtypeRemoteControlNextTrack: // 下一首
                type = UIEventSubtypeRemoteControlNextTrack;
                break;
        case UIEventSubtypeRemoteControlPreviousTrack: // 上一首
                type = UIEventSubtypeRemoteControlPreviousTrack;
                break;
        default:
                break;
        }
        // 发送通知，操作音频
        [[NSNotificationCenter defaultCenter] postNotificationName:@"UIEventSubtype" object:@(type)];
    }
}

第四步：在音频所在控制器中写入以下代码

- (void)viewDidLoad
{
    [super viewDidLoad];
    self.view.backgroundColor = [UIColor whiteColor];
    [self createNotification];
}

// 创建通知
- (void)createNotification
{
    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(notificaitonAction:)
                                                 name:@"UIEventSubtype"
                                               object:nil];
}

// 通知事件(该方法自行按需求实现)
- (void)notificaitonAction:(NSNotification *)notification
{
    NSInteger type = [notification.object integerValue];
    switch (type) {
        case  UIEventSubtypeRemoteControlPlay: // 播放
            [_player play];
            break;
        case UIEventSubtypeRemoteControlPause: // 暂停
            [_player pause];
            break;
        case UIEventSubtypeRemoteControlNextTrack: // 下一首
            if (_index == 2) {
                _index = 0;
            }else{
                _index++;
            }
            [self setPlayingInfoWithIndex:_index];
            break;
        case UIEventSubtypeRemoteControlPreviousTrack: // 上一首
            if (_index == 0) {
                _index = 2;
            }else{
                _index--;
            }
            [self setPlayingInfoWithIndex:_index];
            break;
        default:
            break;
    }
}

// 设置锁屏时显示当前播放的音频信息
- (void)setPlayingInfoWithIndex:(NSInteger)index
{
    NSArray * musics = @[[[NSBundle mainBundle] pathForResource:@"那些花儿" ofType:@".mp3"],
                         [[NSBundle mainBundle] pathForResource:@"夜空中最亮的星" ofType:@".mp3"],
                         [[NSBundle mainBundle] pathForResource:@"红豆" ofType:@".mp3"]];
    NSArray * musicName = @[@"那些花儿",@"夜空中最亮的星",@"红豆"];
    NSArray * musicImage = @[[UIImage imageNamed:@"朴树.jpg"],
                             [UIImage imageNamed:@"逃跑计划.jpg"],
                             [UIImage imageNamed:@"王菲.jpg"]];
    NSArray * musicArtist = @[@"朴树",@"逃跑计划",@"王菲"];
    // 设置后台播放
    [[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryPlayback error:nil];

    // 初始化播放器
    NSURL *url = [NSURL fileURLWithPath:musics[index]];
    if (_player) {
        [_player pause];
        _player = nil;
    }
    _player = [[AVPlayer alloc] initWithURL:url];
    [_player play];
    _isPlayingNow = YES;

    // 设置后台播放时显示的东西，例如歌曲名字，图片
    MPMediaItemArtwork *artWork = [[MPMediaItemArtwork alloc] initWithImage:musicImage[index]];


    NSDictionary *dic = @{MPMediaItemPropertyTitle:musicName[index],      // 歌曲名
                          MPMediaItemPropertyArtist:musicArtist[index],   // 演唱者
                          MPMediaItemPropertyArtwork:artWork              // 海报
                          };
   // 进行锁频音乐信息设置
   [[MPNowPlayingInfoCenter defaultCenter] setNowPlayingInfo:dic];
}