UIWindow+AppSwitchScrollStopper.h

## UIWindow+AppSwitchScrollStopper.h
//  UIWindow+AppSwitchScrollStopper.h
//  Created by Tim Johnsen on 3/27/16.

#import <UIKit/UIKit.h>

@interface UIWindow (AppSwitchScrollStopper)

/// Call this early on in your app's lifecycle to avoid
/// scroll-related flashing when your app resumes from the background
- (void)installAppSwitchScrollStopper;

@end

## UIWindow+AppSwitchScrollStopper.m
//  UIWindow+AppSwitchScrollStopper.m
//  Created by Tim Johnsen on 3/27/16.

#import "UIWindow+AppSwitchScrollStopper.h"

@interface UIView (AppSwitchScrollStopper)

- (void)recursiveStopScrolling;

@end

@implementation UIView (AppSwitchScrollStopper)

- (void)recursiveStopScrolling
{
    if ([self isKindOfClass:[UIScrollView class]]) {
        UIScrollView *scrollView = (UIScrollView *)self;

        // Stop the scrolling (http://stackoverflow.com/a/3421387/3943258)
        [scrollView setContentOffset:scrollView.contentOffset animated:NO];

        // Stop the scroll indicators from being captured
        if (scrollView.showsHorizontalScrollIndicator) {
            scrollView.showsHorizontalScrollIndicator = NO;
            scrollView.showsHorizontalScrollIndicator = YES;
        }
        if (scrollView.showsVerticalScrollIndicator) {
            scrollView.showsVerticalScrollIndicator = NO;
            scrollView.showsVerticalScrollIndicator = YES;
        }
    }

    for (UIView *view in self.subviews) {
        [view recursiveStopScrolling];
    }
}

@end


@implementation UIWindow (AppSwitchScrollStopper)

+ (void)load
{
    void (^stopScrollingBlock)(NSNotification *_Nonnull notification) = ^(NSNotification *_Nonnull notification) {
        for (UIWindow *window in [self windowsToStopScrolling]) {
            [window recursiveStopScrolling];
        }
    };
    [[NSNotificationCenter defaultCenter] addObserverForName:UIApplicationDidEnterBackgroundNotification object:nil queue:[NSOperationQueue mainQueue] usingBlock:stopScrollingBlock];
    // Works in iOS extensions too!
    if (&NSExtensionHostDidEnterBackgroundNotification) {
        [[NSNotificationCenter defaultCenter] addObserverForName:NSExtensionHostDidEnterBackgroundNotification object:nil queue:[NSOperationQueue mainQueue] usingBlock:stopScrollingBlock];
    }
}

+ (NSMutableSet<UIWindow *> *)windowsToStopScrolling
{
    static NSMutableSet<UIWindow *> *windowsToStopScrolling = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        windowsToStopScrolling = [NSMutableSet new];
    });
    return windowsToStopScrolling;
}

- (void)installAppSwitchScrollStopper
{
    [[[self class] windowsToStopScrolling] addObject:self];
}

@end