Can UIScrollView's deceleration rate be modified?

Question

Since this question was originally asked, UIScrollView deceleration rate customization has been added via the decelerationRate property introduced in OS 3.0.

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I have a UIScrollView whose deceleration rate I'd like to change. To be clear, I'm talking about when you swipe your finger on a scroll view and the view continues to scroll (but gradually slows) after you lift your finger. I'd like to increase the deceleration rate so that it stops sooner that it does by default.

I've seen some apps where the UIScrollViews seem to decelerate more quickly. There seems to be no API for this in UIScrollView, but I'm wondering if there's an alternative way to do it.

Popish

That link is now: developer.apple.com/library/ios/documentation/UIKit/Reference/… – Impropriate

It's now VERY EASY to do this beyond the two given values. – Dre

Answer 1

You can use UIScrollView's decelerationRate property to control it. Even though its float, its not accepting any value other than UIScrollViewDecelerationRateNormal or UIScrollViewDecelerationRateFast . Look at the following code

NSLog(@"1. decelerationRate %f", scrollview.decelerationRate);

scrollview.decelerationRate = UIScrollViewDecelerationRateNormal;
NSLog(@"2. decelerationRate %f", scrollview.decelerationRate);

scrollview.decelerationRate = UIScrollViewDecelerationRateFast;
NSLog(@"3. decelerationRate %f", scrollview.decelerationRate);

scrollview.decelerationRate = 0.7;
NSLog(@"4. decelerationRate %f", scrollview.decelerationRate);

scrollview.decelerationRate = 0.995;
NSLog(@"5. decelerationRate %f", scrollview.decelerationRate);

Above code gives the following outputs, its very clear we cant not use custom deceleration rate.

2012-01-03 11:59:41.164 testviewv2[10023:707] 1. decelerationRate 0.998000
2012-01-03 11:59:41.172 testviewv2[10023:707] 2. decelerationRate 0.998000
2012-01-03 11:59:41.173 testviewv2[10023:707] 3. decelerationRate 0.990000
2012-01-03 11:59:41.175 testviewv2[10023:707] 4. decelerationRate 0.990000
2012-01-03 11:59:41.176 testviewv2[10023:707] 5. decelerationRate 0.998000

Ten years later! Apple finally stated this in the doco:

https://developer.apple.com/documentation/uikit/uiscrollview/decelerationrate

Joly

I have found the same behaviour – Hoofbeat

confirmed. this must be documentation issue – Seedbed

Unfortunately it's true. I would like to have at least a value between these two, like 0.994, but it's not working. – Dent

Great catch. I was inserting lots of values that were inbetween the two and noticing it didn't feel different. Then I tried 1.1 and 0.5 with the same results. Your code snippet performs the same – Waist

@grzegorzdvipek I happened to see a single line in Apple's demo like this. float decel = UIScrollViewDecelerationRateNormal - (UIScrollViewDecelerationRateNormal - UIScrollViewDecelerationRateFast)/2.0; I logged and found its value is 0.994000 as you wanted. Hope it helps. – Melisamelisande

Answer 2

Yes, I have successfully changed the deceleration rate by doing the following:

scrollView.decelerationRate = UIScrollViewDecelerationRateFast;

Discreet

decelerationRate is actually a float, so you can have more detailed control by entering any float value. – Avow

@Avow It's a float but only works with the UIScrollViewDecelerationRateFast and UIScrollViewDecelerationRateNormal values. – Wojcik

you can ONLY USE the two values supplied. it's not really a float. – Dre

Answer 3

I found that by using KVC to modify the instance variable _decelerationFactor allowed me to change the rate to something other than UIScrollViewDecelerationRateNormal or UIScrollViewDecelerationRateFast. I subclassed UIScrollView and wrapped the whole lot in a try block

- (id)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self) {
        // Initialization code
        @try {
            CGFloat decelerationRate = UIScrollViewDecelerationRateFast +(UIScrollViewDecelerationRateNormal - UIScrollViewDecelerationRateFast) * .52;
            [self setValue:[NSValue valueWithCGSize:CGSizeMake(decelerationRate,decelerationRate)] forKey:@"_decelerationFactor"];

        }
        @catch (NSException *exception) {
            // if they modify the way it works under us.
        }


    }
    return self;
}

Hayfork

Sounds like a quick way to get your app rejected. – Hairpin

I've not had problems with it but it is in a bit of a grey area. In that you can see the variable directly in the header file and don't need to class dump. I feel it falls into the realm of go for it if it improves user experience but don't let it crash your app. – Hayfork

@Hayfork Soooo this was posted in 5 years ago, has anyone gotten this solution into the app store? – Rileyrilievo

Answer 4

It's now very easy to do this:

extension UIScrollView {
    
    ///The usual values are .998 (fast) and .990 (normal).
    ///0.92 for example is a crisp spring effect.
    var customDecelerationFactor: CGFloat {
        set {
            setValue(CGSize(width: newValue, height: newValue),
                     forKey: "_decelerationFactor")
        }
        get { return 0 }
    }
}

For example ...

It's completely unknown (as of 2023) if Apple have "_decelerationFactor" on the list of keywords in code they question when you submit to the app store.

Dre

Answer 5

There is a pretty straightforward approach. Override the setter method.

@interface UIArbitraryDeceleratingScrollView : UIScrollView

@property(nonatomic,assign) CGFloat decelerationRate;

@end

@implementation UIArbitraryDeceleratingScrollView

@synthesize decelerationRate = _decelerationRate;

- (void)setDecelerationRate:(CGFloat)dr
{
    [super setDecelerationRate:dr];
    _decelerationRate = dr;
}

@end

Now assign what you want to, like this for example:

_scroller.decelerationRate = (UIScrollViewDecelerationRateNormal+UIScrollViewDecelerationRateFast)/2.1;

Comprador

This doesn't work. The scroll view is probably directly accessing the private member variable and not the property. – Delores