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.

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

Answer 2

This is possible, but you'd have to subclass UIScrollView and override all the touch handling. It wouldn't be my project of choice - it would take a good bit of work to get it right.

Answer 3

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;

Answer 4

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

scrollView.decelerationRate = UIScrollViewDecelerationRateFast;

Answer 5

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;
}