使滚动条在 UIScrollView 上始终可见？

Question

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?

Answer 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 (ie myScrollView.subviews ) and modify the scroll indicators as you would any UIImageView (eg 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 eg

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

- (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

Answer 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.

Update November, 2019

Starting from iOS 13 UIScrollView subclasses are changed. Now scroll indicators are inherited from UIView and has their own private class called _UIScrollViewScrollIndicator . This means, that they are not subclasses of UIImageView now, so old method won't work anymore.

Also we are not able to implement subclass of _UIScrollViewScrollIndicator because it is private class and we don't have access to it. So the only solution is to use runtime. Now to have support for iOS 13 and earlier implement the next steps:

1. 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;
                    }
                }
            }
        } else if ([NSStringFromClass(view.class) isEqualToString:@"_UIScrollViewScrollIndicator"]) {
            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 scroll indicator, (be sure to create AlwaysOpaqueScrollIndicator in runtime earlier!)
                    // Current implementation is in AlwaysOpaqueScrollTableView class
                    object_setClass(view, NSClassFromString(@"AlwaysOpaqueScrollIndicator"));
                    break;
                }
            }
        }
    }
    // Ask to flash indicator to turn it on
    [self.categoriesTableView flashScrollIndicators];
}

2. Add new class (this is for iOS earlier than 13)

@interface AlwaysOpaqueImageView : UIImageView
@end

@implementation AlwaysOpaqueImageView

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

@end

3. Add these methods somewhere in you code (either the same view controller as in step 1, or to the desired UIScrollView subclass).

+ (void)load {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        // Create child class from _UIScrollViewScrollIndicator since it is private
        Class alwaysOpaqueScrollIndicatorClass =  objc_allocateClassPair(NSClassFromString(@"_UIScrollViewScrollIndicator"), "AlwaysOpaqueScrollIndicator", 0);
        objc_registerClassPair(alwaysOpaqueScrollIndicatorClass);

        // Swizzle setAlpha: method of this class to custom
        Class replacementMethodClass = [self class];

        SEL originalSelector = @selector(setAlpha:);
        SEL swizzledSelector = @selector(alwaysOpaque_setAlpha:);

        Method originalMethod = class_getInstanceMethod(alwaysOpaqueScrollIndicatorClass, originalSelector);
        Method swizzledMethod = class_getInstanceMethod(replacementMethodClass, swizzledSelector);

        BOOL didAddMethod =
            class_addMethod(alwaysOpaqueScrollIndicatorClass,
                originalSelector,
                method_getImplementation(swizzledMethod),
                method_getTypeEncoding(swizzledMethod));

        if (didAddMethod) {
            class_replaceMethod(alwaysOpaqueScrollIndicatorClass,
                swizzledSelector,
                method_getImplementation(originalMethod),
                method_getTypeEncoding(originalMethod));
        } else {
            method_exchangeImplementations(originalMethod, swizzledMethod);
        }
    });
}

#pragma mark - Method Swizzling

- (void)alwaysOpaque_setAlpha:(CGFloat)alpha {
    [self alwaysOpaque_setAlpha:1.0];
}

This step creates the subclass of _UIScrollViewScrollIndicator called AlwaysOpaqueScrollIndicator in runtime and swizzle setAlpha: method implementation to alwaysOpaque_setAlpha: .

Do not forget to add

#import <objc/runtime.h>

to the files you've inserted this code. Thanks to @Smartcat for reminder about this

Answer 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

Answer 4

This is Swift version of @Accid Bright's answer :

class AlwaysOpaqueImageView: UIImageView {

    override var alpha: CGFloat {
        didSet {
            alpha = 1
        }
    }

    static func setScrollbarToAlwaysVisible(from scrollView: UIScrollView) {
        // Do swizzling to turn scroll indicator always on
        // Search correct subview with scroll indicator image across tableView subviews
        for view in scrollView.subviews {
            if view.isKind(of: UIImageView.self),
                view.alpha == 0 && view.autoresizingMask == UIView.AutoresizingMask.flexibleLeftMargin,
                view.frame.size.width < 10 && view.frame.size.height > view.frame.size.width,
                scrollView.frame.size.height < scrollView.contentSize.height {
                // Swizzle class for found imageView, that should be scroll indicator
                object_setClass(view, AlwaysOpaqueImageView.self)
                break
            }
        }

        // Ask to flash indicator to turn it on
        scrollView.flashScrollIndicators()
    }
}

One difference is that setting scrollbar is extracted out as a static method.

Answer 5

The only thing that worked for me was this:

Timer Flash Indicator Fix