Technical Q&A QA1820

How do I achieve smooth video scrubbing with AVPlayer seekToTime:?

Q:  My app allows the user to scrub video files using a slider control in combination with AVPlayer seekToTime: but there is a considerable lag in the display of the video frames. How can I achieve smoother scrubbing?

A: Avoid making calls to AVPlayer seekToTime: in rapid succession. This will cancel the seeks in progress, resulting in a lot of seeking and not a lot of displaying of the target frames. Instead, use the completion handler variant of AVPlayer seekToTime:, and wait for a seek in progress to complete first before issuing another. Listing 1 and Listing 2 give examples of this technique (Note: these examples assume a valid player object has been created and the player's current item status is being maintained via key-value observing. See the AV Foundation Programming Guide for more information):

Listing 1  Using the completion handler variant of AVPlayer seekToTime: for smoother scrubbing (Swift).

import AVFoundation
 
class MyClass {
 
    var isSeekInProgress = false
    let player = <#A valid player object #>
    var chaseTime = kCMTimeZero
    // your player.currentItem.status
    var playerCurrentItemStatus:AVPlayerItemStatus = .Unknown
 
    ...
 
    func stopPlayingAndSeekSmoothlyToTime(newChaseTime:CMTime)
    {
        player.pause()
 
        if CMTimeCompare(newChaseTime, chaseTime) != 0
        {
            chaseTime = newChaseTime;
 
            if !isSeekInProgress
            {
                trySeekToChaseTime()
            }
        }
    }
 
    func trySeekToChaseTime()
    {
        if playerCurrentItemStatus == .Unknown
        {
            // wait until item becomes ready (KVO player.currentItem.status)
        }
        else if playerCurrentItemStatus == .ReadyToPlay
        {
            actuallySeekToTime()
        }
    }
 
    func actuallySeekToTime()
    {
        isSeekInProgress = true
        let seekTimeInProgress = chaseTime
        player.seekToTime(seekTimeInProgress, toleranceBefore: kCMTimeZero,
                toleranceAfter: kCMTimeZero, completionHandler:
        { (isFinished:Bool) -> Void in
 
            if CMTimeCompare(seekTimeInProgress, chaseTime) == 0
            {
                isSeekInProgress = false
            }
            else
            {
                trySeekToChaseTime()
            }
        })
    }
 
}

Listing 2  Using the completion handler variant of AVPlayer seekToTime: for smoother scrubbing (Objective-C).

@import AVFoundation;
 
@interface MyClass ()
{
    AVPlayer player;
    BOOL isSeekInProgress;
    CMTime chaseTime;
    AVPlayerStatus playerCurrentItemStatus; // your player.currentItem.status
    ...
}
 
@implementation MyClass
 
...
 
- (void)stopPlayingAndSeekSmoothlyToTime:(CMTime)newChaseTime
{
    [self->player pause];
 
    if (CMTIME_COMPARE_INLINE(newChaseTime, !=, self->chaseTime))
    {
        self->chaseTime = newChaseTime;
 
        if (!self->isSeekInProgress)
            [self trySeekToChaseTime];
    }
}
 
- (void)trySeekToChaseTime
{
    if (playerCurrentItemStatus == AVPlayerItemStatusUnknown)
    {
        // wait until item becomes ready (KVO player.currentItem.status)
    }
    else if (playerCurrentItemStatus == AVPlayerItemStatusReadyToPlay)
    {
        [self actuallySeekToTime];
    }
}
 
- (void)actuallySeekToTime
{
    self->isSeekInProgress = YES;
    CMTime seekTimeInProgress = self->chaseTime;
    [self->player seekToTime:seekTimeInProgress toleranceBefore:kCMTimeZero
    toleranceAfter:kCMTimeZero completionHandler:
     ^(BOOL isFinished)
     {
         if (CMTIME_COMPARE_INLINE(seekTimeInProgress, ==, self->chaseTime))
             self->isSeekInProgress = NO;
         else
             [self trySeekToChaseTime];
     }];
}
 
@end

Document Revision History


DateNotes
2016-03-23

New document that how to achieve smooth video scrubbing with AVPlayer seekToTime:




Copyright © 2016 Apple Inc. All Rights Reserved. Terms of Use | Privacy Policy | Updated: 2016-03-23