Lively trace UIView's hit testing

UIView+HitTestLiveTracing.h

//
//  UIView+LiveHitTestTracing.h
//  UIViewLiveHitTestTracing
//
//  Created by Manfred on 10/28/15.
//
//

@import UIKit;

@interface UIView (LiveHitTestTracing)
+ (void)setLiveHitTestTracingEnabled:(BOOL)liveHitTestTracingEnabled;
+ (BOOL)isLiveHitTestTracingEnabled;
@end

UIView+HitTestLiveTracing.m

//
//  UIView+LiveHitTestTracing.m
//  UIViewLiveHitTestTracing
//
//  Created by Manfred on 10/28/15.
//
//

@import ObjectiveC;

#import "UIView+LiveHitTestTracing.h"

typedef UIView * ObjCRawCGPointUIEvent_UIView(UIView * self,
                                              SEL selector,
                                              CGPoint,
                                              UIEvent *);

typedef ObjCRawCGPointUIEvent_UIView * ObjCRawCGPointUIEvent_UIViewRef;

ObjCRawCGPointUIEvent_UIViewRef original_hitTest_withEvent;
ObjCRawCGPointUIEvent_UIView    swizzled_hitTest_withEvent;

BOOL isLiveHitTestTracingEnabled = NO;

@implementation UIView (LiveHitTestTracing)
+ (BOOL)isLiveHitTestTracingEnabled {
    return isLiveHitTestTracingEnabled;
}

+ (void)setLiveHitTestTracingEnabled:(BOOL)liveHitTestTracingEnabled {
    isLiveHitTestTracingEnabled = liveHitTestTracingEnabled;
}

+ (void)load {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        SEL selectorHitTestWithEvent =
        NSSelectorFromString(@"hitTest:withEvent:");
        
        original_hitTest_withEvent = (ObjCRawCGPointUIEvent_UIViewRef)
        class_getMethodImplementation(self, selectorHitTestWithEvent);
        
        IMP swizzled_hitTest_withEventIMP = (IMP)&swizzled_hitTest_withEvent;
        
        class_replaceMethod(self,
                            selectorHitTestWithEvent,
                            swizzled_hitTest_withEventIMP ,
                            "@:{_struct=CGPoint}@");
    });
}

- (void)showFootprintOfHitTesting {
    UIColor * originalBackgroundColor = self.backgroundColor ?
    self.backgroundColor : [UIColor clearColor];
    
    UIViewKeyframeAnimationOptions animationOptions =
    UIViewKeyframeAnimationOptionCalculationModeCubic
    | UIViewKeyframeAnimationOptionAllowUserInteraction;
    
    NSTimeInterval duration = 1.0;
    
    [UIView animateKeyframesWithDuration: duration
                                   delay: 0.0
                                 options:animationOptions
                              animations:
     ^{
         [UIView addKeyframeWithRelativeStartTime: 0
                                 relativeDuration: duration * 0.5
                                       animations:
          ^{
              CGFloat hue               = ( arc4random() % 256 / 256.0 );
              CGFloat saturation        = ( arc4random() % 128 / 256.0 ) + 0.5;
              CGFloat brightness        = ( arc4random() % 128 / 256.0 ) + 0.5;
              UIColor * aRandomColor    = [UIColor colorWithHue:hue
                                                     saturation:saturation
                                                     brightness:brightness
                                                          alpha:1];
              
              self.backgroundColor      = aRandomColor;
         }];
         [UIView addKeyframeWithRelativeStartTime: duration * 0.5
                                 relativeDuration: duration * 0.5
                                       animations:
          ^{
              self.backgroundColor      = originalBackgroundColor;
         }];
     }
                              completion: nil];
}

@end

UIView * swizzled_hitTest_withEvent(UIView * self,
                                    SEL selector,
                                    CGPoint point,
                                    UIEvent * event)
{
    if (UIView.isLiveHitTestTracingEnabled
        && ![self.superview isKindOfClass:[UIWindow class]])
    {
        [self.superview showFootprintOfHitTesting];
    }
    
    NSCAssert(original_hitTest_withEvent != NULL,
              @"The original implementation of %@ should not be NULL",
              NSStringFromSelector(selector));
    
    UIView * testResult = (* original_hitTest_withEvent)(self,
                                                         selector,
                                                         point,
                                                         event);
    
    
    if (testResult == self
        && UIView.isLiveHitTestTracingEnabled
        && ![self isKindOfClass:[UIWindow class]]) {
        [self showFootprintOfHitTesting];
    }
    
    return testResult;
}