AVFoundation 框架初探究（四）

叨叨两句

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      动手写这篇总结时候也是二月底过完年回来上班了，又开始新的一年了，今年会是什么样子？这问题可能得年底再回答自己了。在家窝了那么久，上班还是的接着看我们要看的东西，毕竟我们要做的事还真的太多的。

      总结第五章的内容，这两天把后面几章的内容大概的翻着看了看，知道了下后面几章的内容大致讲的都是那些内容。这里就先开始总结书本中第五章的内容。前面第四章的内容视频播放我们再前面的确也总过了，就不在这里再去重复总结。

   

一：AVPlayerViewController

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      在第五章的最开始讲述的就是AVPlayerViewController，这个控制器在前面也没有好好说过，不过苹果给我们的关于AVPlayerViewController的API也就那么多，我们在这里看看它的头文件，以及它的一些使用。下面就先看看AVPlayerViewController这个类的头文件的方法，我们对它的属性进行一个解释说明：

File:  AVPlayerViewController.h
Framework:  AVKit
Copyright © 2014-2017 Apple Inc. All rights reserved.
导入库头文件
#import <UIKit/UIKit.h>
#import <AVFoundation/AVFoundation.h>

NS_ASSUME_NONNULL_BEGIN
//AVPlayerViewController有这个代理，这个代理方法就在下面
@protocol AVPlayerViewControllerDelegate;

@class           AVPlayerViewController
这个摘要说明了AVPlayerViewController这个控制器的基本特征
@abstract        AVPlayerViewController is a subclass of UIViewController that can be used to display the visual content of an AVPlayer object and the standard playback controls.

// 8.0之后有的AVPlayerViewController
API_AVAILABLE(ios(8.0))
@interface AVPlayerViewController : UIViewController
 
// 简单的播放器AVPlayer属性
@property        player
@abstract        The player from which to source the media content for the view controller.
@property (nonatomic, strong, nullable) AVPlayer *player;

// 是否展示添加在上面的子控件
@property        showsPlaybackControls
@abstract        Whether or not the receiver shows playback controls. Default is YES.
@discussion        Clients can set this property to NO when they don't want to have any playback controls on top of the visual content (e.g. for a game splash screen).
@property (nonatomic) BOOL showsPlaybackControls;

// 这个属性字面意思是视频重力  其实它是用来确定在承载层的范围内视频可以拉伸或者缩放的程度
// 具体的在下面@discussion部分有讨论，我们再总结一下它三个值分别代表的含义
// AVLayerVideoGravityResizeAspect 会在承载层的范围内缩放视频的大小来保持视频的原始比例宽高，默认值
// AVLayerVideoGravityResizeAspectFill 保留视频的宽高比，并且通过缩放填满整个播放界面
// AVLayerVideoGravityResize 会将视频内容拉伸匹配承载层的范围，一般不常用，因为会把图片扭曲导致变形
@property        videoGravity
@abstract        A string defining how the video is displayed within an AVPlayerLayer bounds rect.
@discussion      Options are AVLayerVideoGravityResizeAspect, AVLayerVideoGravityResizeAspectFill and AVLayerVideoGravityResize. AVLayerVideoGravityResizeAspect is default.
 See <AVFoundation/AVAnimation.h> for a description of these options.
@property (nonatomic, copy) NSString *videoGravity;

// 通过这个bool类型的值确定视频是否已经准备好展示
@property        readyForDisplay
@abstract        Boolean indicating that the first video frame has been made ready for display for the current item of the associated AVPlayer.
@property (nonatomic, readonly, getter = isReadyForDisplay) BOOL readyForDisplay;

// 视频的尺寸
@property        videoBounds
@abstract        The current size and position of the video image as displayed within the receiver's view's bounds.
@property (nonatomic, readonly) CGRect videoBounds;

// 有自定义的控件可以添加在这里UIView上
@property        contentOverlayView
@abstract        Use the content overlay view to add additional custom views between the video content and the controls.
@property (nonatomic, readonly, nullable) UIView *contentOverlayView;

// 是否允许使用画中画播放模式，这个画中画播放在下面会写Demo  9.0之后的属性
@property        allowsPictureInPicturePlayback
@abstract        Whether or not the receiver allows Picture in Picture playback. Default is YES.
@property (nonatomic) BOOL allowsPictureInPicturePlayback API_AVAILABLE(ios(9.0));

// 10.0之后的属性
@property        updatesNowPlayingInfoCenter
@abstract        Whether or not the now playing info center should be updated. Default is YES.
@property (nonatomic) BOOL updatesNowPlayingInfoCenter API_AVAILABLE(ios(10.0));

// 理解摘要的意思是是否允许点击播放之后自动全屏播放视频  默认值是NO
@property        entersFullScreenWhenPlaybackBegins
@abstract        Whether or not the receiver automatically enters full screen when the play button is tapped. Default is NO.
@discussion      If YES, the receiver will show a user interface tailored to this behavior.
@property (nonatomic) BOOL entersFullScreenWhenPlaybackBegins API_AVAILABLE(ios(11.0));

// 也是理解摘要，是否允许退出全屏播放在播放结束之后
@property        exitsFullScreenWhenPlaybackEnds
@abstract        Whether or not the receiver automatically exits full screen when playback ends. Default is NO.
@discussion      If multiple player items have been enqueued, the receiver exits fullscreen once no more items are remaining in the queue.
@property (nonatomic) BOOL exitsFullScreenWhenPlaybackEnds API_AVAILABLE(ios(11.0));

// AVPlayerViewControllerDelegate代理
@property        delegate
@abstract        The receiver's delegate.
@property (nonatomic, weak, nullable) id <AVPlayerViewControllerDelegate> delegate API_AVAILABLE(ios(9.0));
@end

// 下面就是AVPlayerViewControllerDelegate代理方法  下面的代理方法都是optional可选的
@protocol        AVPlayerViewControllerDelegate
@abstract        A protocol for delegates of AVPlayerViewController.
@protocol AVPlayerViewControllerDelegate <NSObject>
@optional

@method          playerViewControllerWillStartPictureInPicture:
@param           playerViewController The player view controller.
@abstract        Delegate can implement this method to be notified when Picture in Picture will start.
// 即将开始画中画播放
- (void)playerViewControllerWillStartPictureInPicture:(AVPlayerViewController *)playerViewController;

@method          playerViewControllerDidStartPictureInPicture:
@param           playerViewController  The player view controller.
@abstract        Delegate can implement this method to be notified when Picture in Picture did start.
// 已经开始了画中画播放模式
- (void)playerViewControllerDidStartPictureInPicture:(AVPlayerViewController *)playerViewController;

@method          playerViewController:failedToStartPictureInPictureWithError:
@param           playerViewController  The player view controller.
@param           error An error describing why it failed.
@abstract        Delegate can implement this method to be notified when Picture in Picture failed to start.
// 这个摘要已经给我们说的比较清楚了，当画中画模式播放失败之后就会走这个代理方法
- (void)playerViewController:(AVPlayerViewController *)playerViewController failedToStartPictureInPictureWithError:(NSError *)error;

@method          playerViewControllerWillStopPictureInPicture:
@param           playerViewController The player view controller.
@abstract        Delegate can implement this method to be notified when Picture in Picture will stop.
// 画中画播放模式即将结束
- (void)playerViewControllerWillStopPictureInPicture:(AVPlayerViewController *)playerViewController;

@method          playerViewControllerDidStopPictureInPicture:
@param           playerViewController  The player view controller.
@abstract        Delegate can implement this method to be notified when Picture in Picture did stop.
// 画中画播放已经结束
- (void)playerViewControllerDidStopPictureInPicture:(AVPlayerViewController *)playerViewController;

@method          playerViewControllerShouldAutomaticallyDismissAtPictureInPictureStart:
@param           playerViewController   The player view controller.
@abstract        Delegate can implement this method and return NO to prevent（阻止） player view controller from automatically being dismissed when Picture in Picture starts.
// 要是在画中画开始播放的时候 播放的底层控制器要是消失就返回NO
- (BOOL)playerViewControllerShouldAutomaticallyDismissAtPictureInPictureStart:(AVPlayerViewController *)playerViewController;

@method          playerViewController:restoreUserInterfaceForPictureInPictureStopWithCompletionHandler:
@param           playerViewController  The player view controller.
@param           completionHandler  The completion handler the delegate needs to call after restore.
@abstract        Delegate can implement this method to restore（恢复） the user interface（交互） before Picture in Picture stops.
//  画中画播放结束恢复用户交互
- (void)playerViewController:(AVPlayerViewController *)playerViewController restoreUserInterfaceForPictureInPictureStopWithCompletionHandler:(void (^)(BOOL restored))completionHandler;

@end
NS_ASSUME_NONNULL_END

就这两点了

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      书中的第四五章的内容在总结也就还剩这么两点，由于还有部分涉及到的是Mac OS 系统的部分知识，我们就在这里不提了，就说说还需要我们理解的两点：

      一： CMTime  也是对第四章一点内容的补充

             我们简单的看看这个CMTime的一般算数运算

#pragma mark --
#pragma mark -- CMTime的简单的使用
-(void)CMTimeCalculate{
        
        CMTime timeO = CMTimeMake(1,10);
        CMTime timeT = CMTimeMake(1,5);
        
        // 加
        CMTime timeA = CMTimeAdd(timeO,timeT);
        CMTimeShow(timeA);
        
        //减
        CMTime timeS = CMTimeSubtract(timeO,timeT);
        CMTimeShow(timeS);
        
        //乘 整形
        CMTime timeB = CMTimeMake(1,10);
        CMTime timeI = CMTimeMultiply(timeB,5);
        CMTimeShow(timeI);
        
        //乘 浮点型
        CMTime timeF = CMTimeMultiplyByFloat64(timeB,5.6);
        CMTimeShow(timeF);
        
        // 比较  1是大于  0是相等  -1是小于
        int timeC = CMTimeCompare(timeO,timeT);
        NSLog(@"比较得到大的是%d",timeC); // -1
        
        //求绝对值
        CMTime timeAB = CMTimeAbsoluteValue(timeS);
        CMTimeShow(timeAB);
}

      CMTimeRange也是属于CMTime范畴，下面是在我们的iOS源代码对于它的定义：

/*!
	@typedef	CMTimeRange
	@abstract	A time range represented as two CMTime structures.
*/
typedef struct
{
	CMTime			start;		/*! @field start The start time of the time range. */
	CMTime			duration;	/*! @field duration The duration of the time range. */
} CMTimeRange;

      通过这个定义我们就了解了它的组成，在Demo中我们已经是简单的使用过它了，具体点的我们可以在代码中去查看。

      关于CMTime还有一点值得我们注意,那就是它和秒之间的转换函数： Float64 CMTimeGetSeconds(CMTime time)  通过这个函数，你就可以把一个CMTime实例转换成Float64位对象。

AVAssetExportSession

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      我们先看看在我们的Demo里面我们使用到的关于AVAssetExportSession的代码，我们在这里使用它的时候只是利用它进行了一下视频的压缩：

#pragma mark --
#pragma mark -- 视频压缩方法
-(void)compressVideoWithFileUrl:(NSURL *)fileUrl{
    
    /* 
      这里需要注意的一点就是在重复的路径上保存文件是不行的，可以选择在点击开始的时候删除之前的
      也可以这样按照时间命名不同的文件保存
      在后面的 AVAssetWriter 也要注意这一点
     */
    // 压缩后的视频的方法命名
    NSDateFormatter * formatter = [[NSDateFormatter alloc]init];
    [formatter setDateFormat:@"yyyy-MM-dd-HH:mm:ss"];
        
    // 压缩后的文件路径
    self.videoPath = [NSString stringWithFormat:@"%@/%@.mov",NSTemporaryDirectory(),[formatter stringFromDate:[NSDate date]]];
    
    // 先根据你传入的文件的路径穿件一个AVAsset
    AVAsset * asset = [AVAsset  assetWithURL:fileUrl];
    /*
     
     根据urlAsset创建AVAssetExportSession压缩类
     第二个参数的意义：常用 压缩中等质量  AVAssetExportPresetMediumQuality
     AVF_EXPORT NSString *const AVAssetExportPresetLowQuality        NS_AVAILABLE_IOS(4_0);
     AVF_EXPORT NSString *const AVAssetExportPresetMediumQuality     NS_AVAILABLE_IOS(4_0);
     AVF_EXPORT NSString *const AVAssetExportPresetHighestQuality    NS_AVAILABLE_IOS(4_0);
    
    */
    AVAssetExportSession * exportSession = [[AVAssetExportSession alloc]initWithAsset:asset presetName:AVAssetExportPresetMediumQuality];
    
    // 优化压缩，这个属性能使压缩的质量更好
    exportSession.shouldOptimizeForNetworkUse = YES;
    // 到处的文件的路径
    exportSession.outputURL =  [NSURL fileURLWithPath:self.videoPath];
    // 导出的文件格式
    /*!
      @constant  AVFileTypeMPEG4  mp4格式的   AVFileTypeQuickTimeMovie mov格式的
      @abstract A UTI for the MPEG-4 file format.
      @discussion
      The value of this UTI is @"public.mpeg-4".
      Files are identified with the .mp4 extension.
      可以看看这个outputFileType格式，比如AVFileTypeMPEG4也可以写成public.mpeg-4，其他类似
    */
    exportSession.outputFileType = AVFileTypeQuickTimeMovie;
    
    NSLog(@"视频压缩后的presetName: %@",exportSession.presetName);
    // 压缩的方法  export 导出  Asynchronously 异步
    [exportSession exportAsynchronouslyWithCompletionHandler:^{
        
        /*
         exportSession.status 枚举属性
         typedef NS_ENUM(NSInteger, AVAssetExportSessionStatus) {
         AVAssetExportSessionStatusUnknown,
         AVAssetExportSessionStatusWaiting,
         AVAssetExportSessionStatusExporting,
         AVAssetExportSessionStatusCompleted,
         AVAssetExportSessionStatusFailed,
         AVAssetExportSessionStatusCancelled
         };
         */
        int exportStatus = exportSession.status;
        switch (exportStatus) {
            case AVAssetExportSessionStatusFailed:
                
                NSLog(@"压缩失败");
                break;
            case AVAssetExportSessionStatusCompleted:
            {
                /*
                 压缩后的大小
                 也可以利用exportSession的progress属性，随时监测压缩的进度
                 */
                NSData * data = [NSData dataWithContentsOfFile:self.videoPath];
                float dataSize = (float)data.length/1024/1024;
                NSLog(@"视频压缩后大小 %f M", dataSize);
               
            }
                break;
            default:
                break;
        }
    }];
}

      总结一下：

      通过补充上面的这两点就算是总结完了书中前五章大概的内容，通过后面这两点的学习，能总结出来的就是多看API文件！摘要虽然都是英文的，有些同行可能因为不太好就不会去看，但读懂一些基本的英文文档也是我们的基本技能，通过看API可以学到许多东西！