AVPlayer(seekToTime) 拒绝卡顿，畅享丝滑

背景：AVPlayer播放视频时，可通过seekToTime:方法跳转到视频指定时间，通常会绑定进度条通过拖动来实现。细心的人会发现，当拖动是前进方向时，视频画面是非常流畅的，反之拖动后退时，就会出现卡顿，卡顿的程度与视频质量、长度等或有关系。（卡顿原因有待思考，知道的读者望评论告知）

时间进度条

解决方法：我为AVPlayer写了一个分类(SeekSmoothly)，使用ss_seekToTime:替换原本的seekToTime:方法便可。源码如下：

`

AVPlayer+SeekSmoothly.h

#import <AVFoundation/AVFoundation.h>

@interface AVPlayer (SeekSmoothly)

- (void)ss_seekToTime:(CMTime)time;

- (void)ss_seekToTime:(CMTime)time
      toleranceBefore:(CMTime)toleranceBefore
       toleranceAfter:(CMTime)toleranceAfter
    completionHandler:(void (^)(BOOL))completionHandler;

@end

AVPlayer+SeekSmoothly.m

#import "AVPlayer+SeekSmoothly.h"
#import <objc/runtime.h>

@interface AVPlayerSeeker : NSObject
{
    CMTime targetTime;
    BOOL isSeeking;
}

@property (weak, nonatomic) AVPlayer *player;

@end

@implementation AVPlayerSeeker

- (instancetype)initWithPlayer:(AVPlayer *)player {
    self = [super init];
    if (self) {
        self.player = player;
    }
    return self;
}

- (void)seekSmoothlyToTime:(CMTime)time toleranceBefore:(CMTime)toleranceBefore toleranceAfter:(CMTime)toleranceAfter completionHandler:(void (^)(BOOL))completionHandler {
    targetTime = time;
    if (!isSeeking) {
        [self trySeekToTargetTimeWithToleranceBefore:toleranceBefore toleranceAfter:toleranceAfter completionHandler:completionHandler];
    }
}

- (void)trySeekToTargetTimeWithToleranceBefore:(CMTime)toleranceBefore toleranceAfter:(CMTime)toleranceAfter completionHandler:(void (^)(BOOL))completionHandler {
    if (self.player.currentItem.status == AVPlayerItemStatusReadyToPlay) {
        [self seekToTargetTimeToleranceBefore:toleranceBefore toleranceAfter:toleranceAfter completionHandler:completionHandler];
    }
}

- (void)seekToTargetTimeToleranceBefore:(CMTime)toleranceBefore toleranceAfter:(CMTime)toleranceAfter completionHandler:(void (^)(BOOL))completionHandler {
    isSeeking = YES;
    CMTime seekingTime = targetTime;
    [self.player seekToTime:seekingTime toleranceBefore:toleranceBefore
             toleranceAfter:toleranceAfter completionHandler:
     ^(BOOL isFinished) {
         if (CMTIME_COMPARE_INLINE(seekingTime, ==, targetTime)) {
             // seek completed
             isSeeking = NO;
             if (completionHandler) {
                 completionHandler(isFinished);
             }
         } else {
             // targetTime has changed, seek again
             [self trySeekToTargetTimeWithToleranceBefore:toleranceBefore toleranceAfter:toleranceAfter completionHandler:completionHandler];
         }
     }];
}

@end


static NSString *seekerKey = @"ss_seeker";

@implementation AVPlayer (SeekSmoothly)

- (void)ss_seekToTime:(CMTime)time {
    [self ss_seekToTime:time toleranceBefore:kCMTimeZero toleranceAfter:kCMTimeZero completionHandler:nil];
}

- (void)ss_seekToTime:(CMTime)time toleranceBefore:(CMTime)toleranceBefore toleranceAfter:(CMTime)toleranceAfter completionHandler:(void (^)(BOOL))completionHandler {

    AVPlayerSeeker *seeker = objc_getAssociatedObject(self, &seekerKey);
    if (!seeker) {
        seeker = [[AVPlayerSeeker alloc] initWithPlayer:self];
        objc_setAssociatedObject(self, &seekerKey, seeker, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    }

    [self pause];
    [seeker seekSmoothlyToTime:time toleranceBefore:toleranceBefore toleranceAfter:toleranceAfter completionHandler:completionHandler];
}

@end

`

简析源码：

分类中通过运行时关联AVPlayerSeeker对象，
在AVPlayerSeeker中记录seek状态isSeeking，避免AVPlayer seekToTime:方法同时多次调用，
并记录目标时间targetTime，保证最后视频正确跳转。