iphone - 自定义 UISlider，如音乐应用

Question

我构建了一个自定义 slider 来显示音乐轨道的播放进度并允许在轨道内滑动。

两者都运行良好，但一旦停止拖动并重新定位 slider ，就会有轻微的滞后(和跳动)——Apple Music 应用 slider 是无缝的。

_scrubberSlider = [[ScrubberSlider alloc] initWithFrame:CGRectMake(0, 0, 300, 30)];
_scrubberSlider.continuous = YES;
_scrubberSlider.maximumValue = 1.0f;
_scrubberSlider.minimumValue = 0.0f;
 [_scrubberSlider addTarget:self action:@selector(handleSliderMove:) forControlEvents:UIControlEventValueChanged];

- (void) handleSliderMove:(UISlider*)sender
{
    CGFloat currentPlayback = sender.value * [[_theMusicPlayer.nowPlayingItem valueForKey:MPMediaItemPropertyPlaybackDuration] floatValue];
    _theMusicPlayer.currentPlaybackTime = currentPlayback;
}

-(void) handleTrackTime
{
    if (!trackTimer)
    {
    NSNumber *playBackTime = [NSNumber numberWithFloat: _musicPlayer.currentPlaybackTime];
    trackTimer = [NSTimer scheduledTimerWithTimeInterval:0.1f target:self selector:@selector(timeIntervalFinished:) userInfo:@{@"playbackTime" : playBackTime} repeats:YES];
    }
}

-(void) timeIntervalFinished:(NSTimer*)sender
{
     [self playbackTimeUpdated:_musicPlayer.currentPlaybackTime];
}

-(void) playbackTimeUpdated:(CGFloat)playbackTime 
{
    // Update time label
    [self updatePosition];
}

-(void) updatePosition
{
   if (!_scrubberSlider.isScrubbing)
   {
   CGFloat percent = _theMusicPlayer.currentPlaybackTime / [[_theMusicPlayer.nowPlayingItem valueForKey:MPMediaItemPropertyPlaybackDuration] floatValue];
   _scrubberSlider.value = percent;
   }
}

自定义 slider

@interface ScrubberSlider : UISlider

@property(nonatomic) BOOL isScrubbing;

@end

@implementation ScrubberSlider

- (id)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self)
    {
    }
    return self;
}

-(BOOL) beginTrackingWithTouch:(UITouch *)touch withEvent:(UIEvent *)event
{
    [super beginTrackingWithTouch:touch withEvent:event];

    _isScrubbing = YES;

    return YES;
}

- (void)endTrackingWithTouch:(UITouch *)touch withEvent:(UIEvent *)event
{
    [super endTrackingWithTouch:touch withEvent:event];

    _isScrubbing = NO;
}

@end

Answer 1

也许您的处理方式有误。不要尝试与轨道时间完全同步(这会导致一些非常讨厌的竞争条件，并且正是为什么您的 slider 在“抛出”而不是拖动，按住然后释放时无法正确更新的原因)，请尝试使用您拥有的计时器作为“滴答声”。您可以将射速降低到，比如说，每半秒一次，然后使用与现在基本相同的逻辑，但不是试图在 -updatePosition 中获得精确的小数值，您向前“勾选” slider 。

-(void) updatePosition
{
   if (!_scrubberSlider.isScrubbing)
   {
       _scrubberSlider.value += .5f;
   }
}

它并不完美，但话又说回来，它比尝试使用 NSTimer 将自己与音乐轨道的时间同步(对于精确运动来说可能非常不准确)要高效和无缝得多。这还要求您的 slider 的最大值等于轨道的最大值，因此开始播放的示例方法将包括如下内容:

- (IBAction)startPlayback:(id)sender {
    //... Handle however you wish to play the track
    _scrubberSlider.maximumValue = [[_theMusicPlayer.nowPlayingItem valueForKey:MPMediaItemPropertyPlaybackDuration] floatValue];
    [self handleTrackTime];
}