Make scrollbar always visible on UIScrollView?

问题:

I need to make a scrollbar always visible on viewDidLoad so that the user can understand that there is content to scroll. I did the following:

[myscrollView flashScrollIndicators];

But then the scrollbars only appear for some time after viewDidLoad and disappear again only to reappear when the user touches the screen..

I need to make scrollbars always visible. How can I do it?

回答1:

Apple indirectly discourage constantly displaying scroll indicators in their iOS Human Interface Guidelines but guidelines are just guidelines for a reason, they don't account for every scenario and sometimes you may need to politely ignore them.

The scroll indicators of any content views are UIImageView subviews of those content views. This means you can access the scroll indicators of a UIScrollView as you would any of its other subviews (i.e. myScrollView.subviews) and modify the scroll indicators as you would any UIImageView (e.g. scrollIndicatorImageView.backgroundColor = [UIColor redColor];).

The most popular solution appears to be the following code:

#define noDisableVerticalScrollTag 836913
#define noDisableHorizontalScrollTag 836914

@implementation UIImageView (ForScrollView) 

- (void) setAlpha:(float)alpha {

    if (self.superview.tag == noDisableVerticalScrollTag) {
        if (alpha == 0 && self.autoresizingMask == UIViewAutoresizingFlexibleLeftMargin) {
            if (self.frame.size.width < 10 && self.frame.size.height > self.frame.size.width) {
                UIScrollView *sc = (UIScrollView*)self.superview;
                if (sc.frame.size.height < sc.contentSize.height) {
                    return;
                }
            }
        }
    }

    if (self.superview.tag == noDisableHorizontalScrollTag) {
        if (alpha == 0 && self.autoresizingMask == UIViewAutoresizingFlexibleTopMargin) {
            if (self.frame.size.height < 10 && self.frame.size.height < self.frame.size.width) {
                UIScrollView *sc = (UIScrollView*)self.superview;
                if (sc.frame.size.width < sc.contentSize.width) {
                    return;
                }
            }
        }
    }

    [super setAlpha:alpha];
}

@end

Which is originally credited to this source.

This defines a category for UIImageView that defines a custom setter for the alpha property. This works because at some point in the underlying code for the UIScrollView, it will set its scroll indicator's alpha property to 0 in order to hide it. At this point it will run through our category and, if the hosting UIScrollView has the right tag, it will ignore the value being set, leaving it displayed.

In order to use this solution ensure your UIScrollView has the appropriate tag e.g.

If you want to display the scroll indicator from the moment its UIScrollView is visible simply flash the scroll indicators when the view appears .e.g

- (void)viewDidAppear:(BOOL)animate
{
    [super viewDidAppear:animate];
    [self.scrollView flashScrollIndicators];
}

Additional SO references:

• UIScrollView - showing the scroll bar
• UIScrollView indicator always show?
• Scroll Indicators Visibility
• Make scrollbars always visible in uiscrollview

回答2:

I want to offer my solution. I don't like the most popular variant with category (overriding methods in category can be the reason of some indetermination what method should be called in runtime, since there is two methods with the same selector). I use swizzling instead. And also I don't need to use tags.

Add this method to your view controller, where you have scroll view (self.categoriesTableView in my case)

- (void)viewDidAppear:(BOOL)animated {
    [super viewDidAppear:animated];
    // Do swizzling to turn scroll indicator always on
    // Search correct subview with scroll indicator image across tableView subviews
    for (UIView * view in self.categoriesTableView.subviews) {
        if ([view isKindOfClass:[UIImageView class]]) {
            if (view.alpha == 0 && view.autoresizingMask == UIViewAutoresizingFlexibleLeftMargin) {
                if (view.frame.size.width < 10 && view.frame.size.height > view.frame.size.width) {
                    if (self.categoriesTableView.frame.size.height < self.categoriesTableView.contentSize.height) {
                        // Swizzle class for found imageView, that should be scroll indicator
                        object_setClass(view, [AlwaysOpaqueImageView class]);
                        break;
                    }
                }
            }
        }
    }
    // Ask to flash indicator to turn it on
   [self.categoriesTableView flashScrollIndicators];
}

Add new class

@interface AlwaysOpaqueImageView : UIImageView
@end

@implementation AlwaysOpaqueImageView

- (void)setAlpha:(CGFloat)alpha {
    [super setAlpha:1.0];
}

@end

The scroll indicator (vertical scroll indicator in this case) will be always at the screen.

回答3:

I dont know whether this will work or not. But just a hint for you.

Scrollbar inside the Scrollview is a Imageview. Which is a subview of UIScrollview

So get the Scrollbar Imageview of the UIscrollview. Then try to set that image property hidden to NO or Change Alpha value

static const int UIScrollViewHorizontalBarIndexOffset = 0;
static const int UIScrollViewVerticalBarIndexOffset = 1;
-(UIImageView *)scrollbarImageViewWithIndex:(int)indexOffset 
{
    int viewsCount = [[yourScrollview subviews] count];
    UIImageView *scrollBar = [[yourScrollview subviews] objectAtIndex:viewsCount - indexOffset - 1];
    return scrollBar;
}

-(void) viewDidLoad
{
    //Some Code
    //Get Scrollbar
    UIImageView *scrollBar = [self scrollbarImageViewWithIndex: UIScrollViewVerticalBarIndexOffset];

    //The try setting hidden property/ alpha value
    scrollBar.hidden=NO;
}

Got reference from here