這個類的主要功能就是獲取一個視頻里面任何一幀的圖片。

1. 創(chuàng)建AVAssetImageGenerator

AVAssetImageGenerator的初始化方法有兩個，一個是類方法， 一個是對象方法， 要闖入的參數(shù)都是一個AVAsset對象

- (instancetype)initWithAsset:(AVAsset *)asset;
+ (instancetype)assetImageGeneratorWithAsset:(AVAsset *)asset;

2.實際使用

1.獲取某個特定時間的視頻幀（圖像）

主要用到的方法是：- (nullable CGImageRef)copyCGImageAtTime:(CMTime)requestedTime actualTime:(nullable CMTime *)actualTime error:(NSError * _Nullable * _Nullable)outError

其中CMTime是一個結(jié)構(gòu)體，

typedef struct
{
    CMTimeValue value;      /*! @field value The value of the CMTime. value/timescale = seconds. */
    CMTimeScale timescale;  /*! @field timescale The timescale of the CMTime. value/timescale = seconds.  */
    CMTimeFlags flags;      /*! @field flags The flags, eg. kCMTimeFlags_Valid, kCMTimeFlags_PositiveInfinity, etc. */
    CMTimeEpoch epoch;      /*! @field epoch Differentiates between equal timestamps that are actually different because
                                                 of looping, multi-item sequencing, etc.  
                                                 Will be used during comparison: greater epochs happen after lesser ones. 
                                                 Additions/subtraction is only possible within a single epoch,
                                                 however, since epoch length may be unknown/variable. */
} CMTime;

C語言的API, 需要注意的是： 我們想要的時間 = value/timescale；例如：1s = CMTimeMake(1, 1);
0.5s = CMTimeMake(1, 2);

所以我們獲取某一時刻的視頻幀

-(void)getImageWithTime:(CMTime)time {
    NSURL *videoUrl = [[NSBundle mainBundle] URLForResource:@"hubblecast.m4v" withExtension:nil];
    AVAsset *videoAsset = [AVAsset assetWithURL:videoUrl];
    self.videoAsset = videoAsset;

    AVAssetImageGenerator *imageGenerator = [AVAssetImageGenerator assetImageGeneratorWithAsset:self.videoAsset];
    self.imageGenerator = imageGenerator;
    imageGenerator.maximumSize = CGSizeMake(200, 0);//按比例生成， 不指定會默認視頻原來的格式大小
    CMTime actualTime;//獲取到圖片確切的時間
    NSError *error = nil;
   CGImageRef CGImage = [imageGenerator copyCGImageAtTime:time actualTime:&actualTime error:&error];
    if (!error) {
        
        UIImage *image = [UIImage imageWithCGImage:CGImage];
        self.imageView.image = image;
        CMTimeShow(actualTime);   //{111600/90000 = 1.240}
        CMTimeShow(time); // {1/1 = 1.000}
    }
    
}


//調(diào)用 獲取第一幀圖片
 [self getImageWithTime:CMTimeMake(1, 1)];

上面發(fā)現(xiàn)想要獲取的時間（time）和實際獲取到的時間的幀（actualTime）不一樣
所以得設(shè)置
//防止時間出現(xiàn)偏差
imageGenerator.requestedTimeToleranceBefore = kCMTimeZero;
imageGenerator.requestedTimeToleranceAfter = kCMTimeZero;
得到的

      CMTimeShow(actualTime);   //{90000/90000 = 1.000}
       CMTimeShow(time); // {1/1 = 1.000}

獲取多個時刻的視頻幀

主要用到這個方法， 根據(jù)多個CMTime對象轉(zhuǎn)化為NSValue, 用數(shù)組包裝傳進去，然后根據(jù)傳進去的NSValue個數(shù)進行回調(diào)， 回調(diào)的次數(shù)等于NSValue的個數(shù)；
- (void)generateCGImagesAsynchronouslyForTimes:(NSArray<NSValue *> *)requestedTimes completionHandler:(AVAssetImageGeneratorCompletionHandler)handler;

 self.imageGenerator =                                                   // 1
    [AVAssetImageGenerator assetImageGeneratorWithAsset:self.videoAsset];
    
    // Generate the @2x equivalent
    self.imageGenerator.maximumSize = CGSizeMake(200.0f, 0.0f);             // 2
    
    CMTime duration = self.videoAsset.duration;
    
    NSMutableArray *times = [NSMutableArray array];                         // 3
    CMTimeValue increment = duration.value / 20;
    CMTimeValue currentValue = 2.0 * duration.timescale;
    while (currentValue <= duration.value) {
        CMTime time = CMTimeMake(currentValue, duration.timescale);
        [times addObject:[NSValue valueWithCMTime:time]];
        currentValue += increment;
    }
    
    __block NSUInteger imageCount = times.count;                            // 4
    __block NSMutableArray *images = [NSMutableArray array];
    
    AVAssetImageGeneratorCompletionHandler handler;                         // 5
    
    handler = ^(CMTime requestedTime,
                CGImageRef imageRef,
                CMTime actualTime,
                AVAssetImageGeneratorResult result,
                NSError *error) {
        
        if (result == AVAssetImageGeneratorSucceeded) {                     // 6
            UIImage *image = [UIImage imageWithCGImage:imageRef];
            
            [images addObject:image];
            NSLog(@"%@", image);
        } else {
            NSLog(@"Error: %@", [error localizedDescription]);
        }
        
        // If the decremented image count is at 0, we're all done.
        if (--imageCount == 0) {                                            // 7
            dispatch_async(dispatch_get_main_queue(), ^{
                //獲取完畢， 作出相應(yīng)的操作
            });
        }
    };
    
    [self.imageGenerator generateCGImagesAsynchronouslyForTimes:times       // 8
                                              completionHandler:handler];