版本记录

前言

APP中的视图就是进行人机交互的，各种点击等手势都是通过与屏幕视图进行交互完成的。这里面很重要的一个就是HitTest方法，它用于寻找响应者以及传递事件，接下来我们就一起研究下HitTest方法的调用机制。

hitTest方法

hitTest:withEvent:是UIView的一个方法，该方法会被系统调用，作用就是用于选择最合适的视图来响应触摸事件。

事件的声明周期

每一个事件从产生到结束是有一个过程的，也就是事件的声明周期。下面看一张图。

几个重要的类

1. UITouch

• 一个手指一次触摸屏幕，就对应生成一个UITouch对象。多个手指同时触摸，生成多个UITouch对象。
• 多个手指先后触摸，系统会根据触摸的位置判断是否更新同一个UITouch对象。若两个手指一前一后触摸同一个位置（即双击），那么第一次触摸时生成一个UITouch对象，第二次触摸更新这个UITouch对象（UITouch对象的 tap count 属性值从1变成2）；若两个手指一前一后触摸的位置不同，将会生成两个UITouch对象，两者之间没有联系。
• 每个UITouch对象记录了触摸的一些信息，包括触摸时间、位置、阶段、所处的视图、窗口等信息。
• 手指离开屏幕一段时间后，确定该UITouch对象不会再被更新将被释放。

/#import <Foundation/Foundation.h>
#import <CoreGraphics/CoreGraphics.h>
#import <UIKit/UIKitDefines.h>

NS_ASSUME_NONNULL_BEGIN

@class UIWindow, UIView, UIGestureRecognizer;

typedef NS_ENUM(NSInteger, UITouchPhase) {
    UITouchPhaseBegan,             // whenever a finger touches the surface.
    UITouchPhaseMoved,             // whenever a finger moves on the surface.
    UITouchPhaseStationary,        // whenever a finger is touching the surface but hasn't moved since the previous event.
    UITouchPhaseEnded,             // whenever a finger leaves the surface.
    UITouchPhaseCancelled,         // whenever a touch doesn't end but we need to stop tracking (e.g. putting device to face)
};

typedef NS_ENUM(NSInteger, UIForceTouchCapability) {
    UIForceTouchCapabilityUnknown = 0,
    UIForceTouchCapabilityUnavailable = 1,
    UIForceTouchCapabilityAvailable = 2
};

typedef NS_ENUM(NSInteger, UITouchType) {
    UITouchTypeDirect,                       // A direct touch from a finger (on a screen)
    UITouchTypeIndirect,                     // An indirect touch (not a screen)
    UITouchTypeStylus NS_AVAILABLE_IOS(9_1), // A touch from a stylus
} NS_ENUM_AVAILABLE_IOS(9_0);

typedef NS_OPTIONS(NSInteger, UITouchProperties) {
    UITouchPropertyForce = (1UL << 0),
    UITouchPropertyAzimuth = (1UL << 1),
    UITouchPropertyAltitude = (1UL << 2),
    UITouchPropertyLocation = (1UL << 3), // For predicted Touches
} NS_AVAILABLE_IOS(9_1);

NS_CLASS_AVAILABLE_IOS(2_0) @interface UITouch : NSObject

@property(nonatomic,readonly) NSTimeInterval      timestamp;
@property(nonatomic,readonly) UITouchPhase        phase;
@property(nonatomic,readonly) NSUInteger          tapCount;   // touch down within a certain point within a certain amount of time
@property(nonatomic,readonly) UITouchType         type NS_AVAILABLE_IOS(9_0);

// majorRadius and majorRadiusTolerance are in points
// The majorRadius will be accurate +/- the majorRadiusTolerance
@property(nonatomic,readonly) CGFloat majorRadius NS_AVAILABLE_IOS(8_0);
@property(nonatomic,readonly) CGFloat majorRadiusTolerance NS_AVAILABLE_IOS(8_0);

@property(nullable,nonatomic,readonly,strong) UIWindow                        *window;
@property(nullable,nonatomic,readonly,strong) UIView                          *view;
@property(nullable,nonatomic,readonly,copy)   NSArray <UIGestureRecognizer *> *gestureRecognizers NS_AVAILABLE_IOS(3_2);

- (CGPoint)locationInView:(nullable UIView *)view;
- (CGPoint)previousLocationInView:(nullable UIView *)view;

// Use these methods to gain additional precision that may be available from touches.
// Do not use precise locations for hit testing. A touch may hit test inside a view, yet have a precise location that lies just outside.
- (CGPoint)preciseLocationInView:(nullable UIView *)view NS_AVAILABLE_IOS(9_1);
- (CGPoint)precisePreviousLocationInView:(nullable UIView *)view NS_AVAILABLE_IOS(9_1);

// Force of the touch, where 1.0 represents the force of an average touch
@property(nonatomic,readonly) CGFloat force NS_AVAILABLE_IOS(9_0);
// Maximum possible force with this input mechanism
@property(nonatomic,readonly) CGFloat maximumPossibleForce NS_AVAILABLE_IOS(9_0);

// Azimuth angle. Valid only for stylus touch types. Zero radians points along the positive X axis.
// Passing a nil for the view parameter will return the azimuth relative to the touch's window.
- (CGFloat)azimuthAngleInView:(nullable UIView *)view NS_AVAILABLE_IOS(9_1);
// A unit vector that points in the direction of the azimuth angle. Valid only for stylus touch types.
// Passing nil for the view parameter will return a unit vector relative to the touch's window.
- (CGVector)azimuthUnitVectorInView:(nullable UIView *)view NS_AVAILABLE_IOS(9_1);

// Altitude angle. Valid only for stylus touch types.
// Zero radians indicates that the stylus is parallel to the screen surface,
// while M_PI/2 radians indicates that it is normal to the screen surface.
@property(nonatomic,readonly) CGFloat altitudeAngle NS_AVAILABLE_IOS(9_1);

// An index which allows you to correlate updates with the original touch.
// Is only guaranteed non-nil if this UITouch expects or is an update.
@property(nonatomic,readonly) NSNumber * _Nullable estimationUpdateIndex NS_AVAILABLE_IOS(9_1);
// A set of properties that has estimated values
// Only denoting properties that are currently estimated
@property(nonatomic,readonly) UITouchProperties estimatedProperties NS_AVAILABLE_IOS(9_1);
// A set of properties that expect to have incoming updates in the future.
// If no updates are expected for an estimated property the current value is our final estimate.
// This happens e.g. for azimuth/altitude values when entering from the edges
@property(nonatomic,readonly) UITouchProperties estimatedPropertiesExpectingUpdates NS_AVAILABLE_IOS(9_1);


@end

NS_ASSUME_NONNULL_END

2. UIEvent

这个是事件的真身。触摸的目的就是生成触摸事件供响应者响应，一个触摸事件对应一个UIEvent对象，其中的type属性标识了事件的类型。如果一个触摸事件可能是由多个手指同时触摸产生的，触摸对象集合通过allTouches属性获取。

#import <Foundation/Foundation.h>
#import <CoreGraphics/CoreGraphics.h>
#import <UIKit/UIKitDefines.h>

NS_ASSUME_NONNULL_BEGIN

@class UIWindow, UIView, UIGestureRecognizer, UITouch;

typedef NS_ENUM(NSInteger, UIEventType) {
    UIEventTypeTouches,
    UIEventTypeMotion,
    UIEventTypeRemoteControl,
    UIEventTypePresses NS_ENUM_AVAILABLE_IOS(9_0),
};

typedef NS_ENUM(NSInteger, UIEventSubtype) {
    // available in iPhone OS 3.0
    UIEventSubtypeNone                              = 0,
    
    // for UIEventTypeMotion, available in iPhone OS 3.0
    UIEventSubtypeMotionShake                       = 1,
    
    // for UIEventTypeRemoteControl, available in iOS 4.0
    UIEventSubtypeRemoteControlPlay                 = 100,
    UIEventSubtypeRemoteControlPause                = 101,
    UIEventSubtypeRemoteControlStop                 = 102,
    UIEventSubtypeRemoteControlTogglePlayPause      = 103,
    UIEventSubtypeRemoteControlNextTrack            = 104,
    UIEventSubtypeRemoteControlPreviousTrack        = 105,
    UIEventSubtypeRemoteControlBeginSeekingBackward = 106,
    UIEventSubtypeRemoteControlEndSeekingBackward   = 107,
    UIEventSubtypeRemoteControlBeginSeekingForward  = 108,
    UIEventSubtypeRemoteControlEndSeekingForward    = 109,
};


NS_CLASS_AVAILABLE_IOS(2_0) @interface UIEvent : NSObject

@property(nonatomic,readonly) UIEventType     type NS_AVAILABLE_IOS(3_0);
@property(nonatomic,readonly) UIEventSubtype  subtype NS_AVAILABLE_IOS(3_0);

@property(nonatomic,readonly) NSTimeInterval  timestamp;

#if UIKIT_DEFINE_AS_PROPERTIES
@property(nonatomic, readonly, nullable) NSSet <UITouch *> *allTouches;
#else
- (nullable NSSet <UITouch *> *)allTouches;
#endif
- (nullable NSSet <UITouch *> *)touchesForWindow:(UIWindow *)window;
- (nullable NSSet <UITouch *> *)touchesForView:(UIView *)view;
- (nullable NSSet <UITouch *> *)touchesForGestureRecognizer:(UIGestureRecognizer *)gesture NS_AVAILABLE_IOS(3_2);

// An array of auxiliary UITouch’s for the touch events that did not get delivered for a given main touch. This also includes an auxiliary version of the main touch itself.
- (nullable NSArray <UITouch *> *)coalescedTouchesForTouch:(UITouch *)touch NS_AVAILABLE_IOS(9_0);

// An array of auxiliary UITouch’s for touch events that are predicted to occur for a given main touch. These predictions may not exactly match the real behavior of the touch as it moves, so they should be interpreted as an estimate.
- (nullable NSArray <UITouch *> *)predictedTouchesForTouch:(UITouch *)touch NS_AVAILABLE_IOS(9_0);

@end

NS_ASSUME_NONNULL_END

3. UIReponder

先看一下API

#import <Foundation/Foundation.h>
#import <UIKit/UIKitDefines.h>
#import <UIKit/UIEvent.h>
#import <UIKit/UIPasteConfigurationSupporting.h>

NS_ASSUME_NONNULL_BEGIN

@class UIPress;
@class UIPressesEvent;

@protocol UIResponderStandardEditActions <NSObject>
@optional
- (void)cut:(nullable id)sender NS_AVAILABLE_IOS(3_0);
- (void)copy:(nullable id)sender NS_AVAILABLE_IOS(3_0);
- (void)paste:(nullable id)sender NS_AVAILABLE_IOS(3_0);
- (void)select:(nullable id)sender NS_AVAILABLE_IOS(3_0);
- (void)selectAll:(nullable id)sender NS_AVAILABLE_IOS(3_0);
- (void)delete:(nullable id)sender NS_AVAILABLE_IOS(3_2);
- (void)makeTextWritingDirectionLeftToRight:(nullable id)sender NS_AVAILABLE_IOS(5_0);
- (void)makeTextWritingDirectionRightToLeft:(nullable id)sender NS_AVAILABLE_IOS(5_0);
- (void)toggleBoldface:(nullable id)sender NS_AVAILABLE_IOS(6_0);
- (void)toggleItalics:(nullable id)sender NS_AVAILABLE_IOS(6_0);
- (void)toggleUnderline:(nullable id)sender NS_AVAILABLE_IOS(6_0);

- (void)increaseSize:(nullable id)sender NS_AVAILABLE_IOS(7_0);
- (void)decreaseSize:(nullable id)sender NS_AVAILABLE_IOS(7_0);

@end

NS_CLASS_AVAILABLE_IOS(2_0) @interface UIResponder : NSObject <UIResponderStandardEditActions>

#if UIKIT_DEFINE_AS_PROPERTIES
@property(nonatomic, readonly, nullable) UIResponder *nextResponder;
#else
- (nullable UIResponder*)nextResponder;
#endif

#if UIKIT_DEFINE_AS_PROPERTIES
@property(nonatomic, readonly) BOOL canBecomeFirstResponder;    // default is NO
#else
- (BOOL)canBecomeFirstResponder;    // default is NO
#endif
- (BOOL)becomeFirstResponder;

#if UIKIT_DEFINE_AS_PROPERTIES
@property(nonatomic, readonly) BOOL canResignFirstResponder;    // default is YES
#else
- (BOOL)canResignFirstResponder;    // default is YES
#endif
- (BOOL)resignFirstResponder;

#if UIKIT_DEFINE_AS_PROPERTIES
@property(nonatomic, readonly) BOOL isFirstResponder;
#else
- (BOOL)isFirstResponder;
#endif

// Generally, all responders which do custom touch handling should override all four of these methods.
// Your responder will receive either touchesEnded:withEvent: or touchesCancelled:withEvent: for each
// touch it is handling (those touches it received in touchesBegan:withEvent:).
// *** You must handle cancelled touches to ensure correct behavior in your application.  Failure to
// do so is very likely to lead to incorrect behavior or crashes.
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(nullable UIEvent *)event;
- (void)touchesMoved:(NSSet<UITouch *> *)touches withEvent:(nullable UIEvent *)event;
- (void)touchesEnded:(NSSet<UITouch *> *)touches withEvent:(nullable UIEvent *)event;
- (void)touchesCancelled:(NSSet<UITouch *> *)touches withEvent:(nullable UIEvent *)event;
- (void)touchesEstimatedPropertiesUpdated:(NSSet<UITouch *> *)touches NS_AVAILABLE_IOS(9_1);

// Generally, all responders which do custom press handling should override all four of these methods.
// Your responder will receive either pressesEnded:withEvent or pressesCancelled:withEvent: for each
// press it is handling (those presses it received in pressesBegan:withEvent:).
// pressesChanged:withEvent: will be invoked for presses that provide an analog value
// (like thumbsticks or analog push buttons)
// *** You must handle cancelled presses to ensure correct behavior in your application.  Failure to
// do so is very likely to lead to incorrect behavior or crashes.
- (void)pressesBegan:(NSSet<UIPress *> *)presses withEvent:(nullable UIPressesEvent *)event NS_AVAILABLE_IOS(9_0);
- (void)pressesChanged:(NSSet<UIPress *> *)presses withEvent:(nullable UIPressesEvent *)event NS_AVAILABLE_IOS(9_0);
- (void)pressesEnded:(NSSet<UIPress *> *)presses withEvent:(nullable UIPressesEvent *)event NS_AVAILABLE_IOS(9_0);
- (void)pressesCancelled:(NSSet<UIPress *> *)presses withEvent:(nullable UIPressesEvent *)event NS_AVAILABLE_IOS(9_0);

- (void)motionBegan:(UIEventSubtype)motion withEvent:(nullable UIEvent *)event NS_AVAILABLE_IOS(3_0);
- (void)motionEnded:(UIEventSubtype)motion withEvent:(nullable UIEvent *)event NS_AVAILABLE_IOS(3_0);
- (void)motionCancelled:(UIEventSubtype)motion withEvent:(nullable UIEvent *)event NS_AVAILABLE_IOS(3_0);

- (void)remoteControlReceivedWithEvent:(nullable UIEvent *)event NS_AVAILABLE_IOS(4_0);

- (BOOL)canPerformAction:(SEL)action withSender:(nullable id)sender NS_AVAILABLE_IOS(3_0);
// Allows an action to be forwarded to another target. By default checks -canPerformAction:withSender: to either return self, or go up the responder chain.
- (nullable id)targetForAction:(SEL)action withSender:(nullable id)sender NS_AVAILABLE_IOS(7_0);

@property(nullable, nonatomic,readonly) NSUndoManager *undoManager NS_AVAILABLE_IOS(3_0);

@end

typedef NS_OPTIONS(NSInteger, UIKeyModifierFlags) {
    UIKeyModifierAlphaShift     = 1 << 16,  // This bit indicates CapsLock
    UIKeyModifierShift          = 1 << 17,
    UIKeyModifierControl        = 1 << 18,
    UIKeyModifierAlternate      = 1 << 19,
    UIKeyModifierCommand        = 1 << 20,
    UIKeyModifierNumericPad     = 1 << 21,
} NS_ENUM_AVAILABLE_IOS(7_0);

NS_CLASS_AVAILABLE_IOS(7_0) @interface UIKeyCommand : NSObject <NSCopying, NSSecureCoding>

- (instancetype)init NS_DESIGNATED_INITIALIZER;
- (nullable instancetype)initWithCoder:(NSCoder *)aDecoder NS_DESIGNATED_INITIALIZER;

@property (nullable,nonatomic,readonly) NSString *input;
@property (nonatomic,readonly) UIKeyModifierFlags modifierFlags;
@property (nullable,nonatomic,copy) NSString *discoverabilityTitle NS_AVAILABLE_IOS(9_0);

// The action for UIKeyCommands should accept a single (id)sender, as do the UIResponderStandardEditActions above

// Creates an key command that will _not_ be discoverable in the UI.
+ (UIKeyCommand *)keyCommandWithInput:(NSString *)input modifierFlags:(UIKeyModifierFlags)modifierFlags action:(SEL)action;

// Key Commands with a discoverabilityTitle _will_ be discoverable in the UI.
+ (UIKeyCommand *)keyCommandWithInput:(NSString *)input modifierFlags:(UIKeyModifierFlags)modifierFlags action:(SEL)action discoverabilityTitle:(NSString *)discoverabilityTitle NS_AVAILABLE_IOS(9_0);

@end

@interface UIResponder (UIResponderKeyCommands)
@property (nullable,nonatomic,readonly) NSArray<UIKeyCommand *> *keyCommands NS_AVAILABLE_IOS(7_0); // returns an array of UIKeyCommand objects<
@end

@class UIInputViewController;
@class UITextInputMode;
@class UITextInputAssistantItem;

@interface UIResponder (UIResponderInputViewAdditions)

// Called and presented when object becomes first responder.  Goes up the responder chain.
@property (nullable, nonatomic, readonly, strong) __kindof UIView *inputView NS_AVAILABLE_IOS(3_2);
@property (nullable, nonatomic, readonly, strong) __kindof UIView *inputAccessoryView NS_AVAILABLE_IOS(3_2);

/// This method is for clients that wish to put buttons on the Shortcuts Bar, shown on top of the keyboard.
/// You may modify the returned inputAssistantItem to add to or replace the existing items on the bar.
/// Modifications made to the returned UITextInputAssistantItem are reflected automatically.
/// This method should not be overriden. Goes up the responder chain.
@property (nonnull, nonatomic, readonly, strong) UITextInputAssistantItem *inputAssistantItem NS_AVAILABLE_IOS(9_0) __TVOS_PROHIBITED __WATCHOS_PROHIBITED;

// For viewController equivalents of -inputView and -inputAccessoryView
// Called and presented when object becomes first responder.  Goes up the responder chain.
@property (nullable, nonatomic, readonly, strong) UIInputViewController *inputViewController NS_AVAILABLE_IOS(8_0);
@property (nullable, nonatomic, readonly, strong) UIInputViewController *inputAccessoryViewController NS_AVAILABLE_IOS(8_0);

/* When queried, returns the current UITextInputMode, from which the keyboard language can be determined.
 * When overridden it should return a previously-queried UITextInputMode object, which will attempt to be
 * set inside that app, but not persistently affect the user's system-wide keyboard settings. */
@property (nullable, nonatomic, readonly, strong) UITextInputMode *textInputMode NS_AVAILABLE_IOS(7_0);
/* When the first responder changes and an identifier is queried, the system will establish a context to
 * track the textInputMode automatically. The system will save and restore the state of that context to
 * the user defaults via the app identifier. Use of -textInputMode above will supersede use of -textInputContextIdentifier. */
@property (nullable, nonatomic, readonly, strong) NSString *textInputContextIdentifier NS_AVAILABLE_IOS(7_0);
// This call is to remove stored app identifier state that is no longer needed.
+ (void)clearTextInputContextIdentifier:(NSString *)identifier NS_AVAILABLE_IOS(7_0);

// If called while object is first responder, reloads inputView, inputAccessoryView, and textInputMode.  Otherwise ignored.
- (void)reloadInputViews NS_AVAILABLE_IOS(3_2);

@end

// These are pre-defined constants for use with the input property of UIKeyCommand objects.
UIKIT_EXTERN NSString *const UIKeyInputUpArrow         NS_AVAILABLE_IOS(7_0);
UIKIT_EXTERN NSString *const UIKeyInputDownArrow       NS_AVAILABLE_IOS(7_0);
UIKIT_EXTERN NSString *const UIKeyInputLeftArrow       NS_AVAILABLE_IOS(7_0);
UIKIT_EXTERN NSString *const UIKeyInputRightArrow      NS_AVAILABLE_IOS(7_0);
UIKIT_EXTERN NSString *const UIKeyInputEscape          NS_AVAILABLE_IOS(7_0);

@interface UIResponder (ActivityContinuation)
@property (nullable, nonatomic, strong) NSUserActivity *userActivity NS_AVAILABLE_IOS(8_0);
- (void)updateUserActivityState:(NSUserActivity *)activity NS_AVAILABLE_IOS(8_0);
- (void)restoreUserActivityState:(NSUserActivity *)activity NS_AVAILABLE_IOS(8_0);
@end

#if TARGET_OS_IOS
@interface UIResponder (UIPasteConfigurationSupporting) <UIPasteConfigurationSupporting>
@end
#endif

NS_ASSUME_NONNULL_END

我们的UIView、UIViewController、UIApplication、AppDelegate等很多类都继承它，所以就可以响应事件。响应事件主要靠下面的几个方法。

//手指触碰屏幕，触摸开始
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(nullable UIEvent *)event;

//手指在屏幕上移动
- (void)touchesMoved:(NSSet<UITouch *> *)touches withEvent:(nullable UIEvent *)event;

//手指离开屏幕，触摸结束
- (void)touchesEnded:(NSSet<UITouch *> *)touches withEvent:(nullable UIEvent *)event;

//触摸结束前，某个系统事件中断了触摸，例如电话呼入
- (void)touchesCancelled:(NSSet<UITouch *> *)touches withEvent:(nullable UIEvent *)event;

寻找合适响应视图的过程

寻找过程

touch->UIApplication->UIWindow->UIViewController.view->subViews->...->view

• 用户点击屏幕产生一个触摸事件，加入到UIApplication管理的事件队列中。
• UIApplication会从事件队列中取出最早的事件进行分发处理，先发送事件给应用程序的主窗口UIWindow。
• 主窗口会调用其hitTest:withEvent:方法在视图(UIView)层次结构中找到一个最合适的UIView来处理触摸事件。

这里需要说明的是，在寻找响应UIView过程中大致的原理。

- (UIView *)hitTest:(CGPoint)point withEvent:(UIEvent *)event 
{
    //系统默认会忽略isUserInteractionEnabled设置为NO、隐藏、alpha小于等于0.01的视图
    if (!self.isUserInteractionEnabled || self.isHidden || self.alpha <= 0.01) {
        return nil;
    }
    if ([self pointInside:point withEvent:event]) {
        for (UIView *subview in [self.subviews reverseObjectEnumerator]) {
            CGPoint convertedPoint = [subview convertPoint:point fromView:self];
            UIView *hitTestView = [subview hitTest:convertedPoint withEvent:event];
            if (hitTestView) {
                return hitTestView;
            }
        }
        return self;
    }
    return nil;
}

1. 首先在当前视图hitTest方法中调用pointInside方法，判断触摸点是否在当前视图内部。
2. 如果pointInside方法返回NO，当hitTest方法返回的是nil，该视图不处理该事件。
3. 如果pointInside方法返回YES，说明触摸点在该视图内部，则从最上层的子视图开始遍历，为什么从最上层呢？因为subviews数组后添加的视图就是在数组的末尾，遍历当前视图的所有子视图，调用hitTest方法，重复1 - 3 步骤。
4. 直到有的子视图hitTest方法返回非空对象或者全部子视图遍历完毕，则结束该视图的循环遍历。若第一次有子视图的hitTest方法返回非空对象，则当前视图的hitTest方法就返回此对象，处理结束；若所有子视图的hitTest方法都返回nil，则当前视图的hitTest方法返回当前视图本身，最终由该对象处理触摸事件。

下面我们就借用一个例子说明一下这个寻找的过程。这里例子是别人写的，已经进行了引用标注，在下面列出来参考文章。

这里首先假设：ViewB在ViewC上层，ViewD在ViewE上层，即ViewB的addSubView：执行在ViewC之后，ViewD的addSubView：执行在ViewE之后。

当我们点击屏幕的时候，也就是ViewE的红点，hitTest执行顺序如下所示：下面是pointInside为YES部分，X部分执行pointInside为NO部分，最终hitTest返回ViewE。

• 首先调用A的hitTest方法，触摸点在其内部，pointInside返回YES，遍历子视图，按照先后顺序调用B和C的hitTest方法，记住B在C的上层，首先调用。
• 执行B的hitTest方法，触摸点不在B的内部，pointInside返回NO，hitTest返回nil。
• 执行C的hitTest方法，触摸点在C的内部，pointInside返回YES，遍历子视图，依次调用D和E的hitTest方法。
• D执行hitTest方法，触摸点不在D的内部，pointInside返回NO，hitTest方法返回nil
• E执行hitTest方法，触摸点在E的内部，pointInside返回YES，E没有子视图了，则hitTest返回的就是E本身，这样就找到了需要显示的视图。

事件的传递过程

事件的传递和上边寻找响应视图的hitTest中pointInside返回YES的视图的执行顺序是相反的，都是从最上层的视图开始传递，一直到UIApplication，借用上面的例子就是E来响应事件，E如果不处理，就会传递给下一个响应者C，C如果也不处理，就会传递给A，A如果也不处理，则系统就会忽略该事件了。

下面就是官方文档给出的响应过程。

1. 响应者对事件的操作方式

响应者对于事件的拦截以及传递都是通过touchesBegan:withEvent:
方法控制的，该方法的默认实现是将事件沿着默认的响应链往下传递。

响应者对于接收到的事件有3种操作：

• 不拦截，这个是默认操作

  • 事件会自动沿着默认的响应链往下传递

• 拦截，不再往下分发事件

  • 重写touchesBegan:withEvent:进行事件处理，不调用父类的touchesBegan:withEvent:

• 拦截，继续往下分发事件

  • 重写touchesBegan:withEvent:进行事件处理，同时调用父类的touchesBegan:withEvent:将事件往下传递

2. 响应链中的事件传递规则

每一个响应者对象（UIResponder对象）都有一个nextResponder方法，用于获取响应链中当前对象的下一个响应者。因此，一旦事件的最佳响应者确定了，这个事件所处的响应链就确定了。

• UIView
  • 如果视图是根控制器的根视图，那么nextResponder就是根控制器对象；否则，其nextResponder为父视图。
• UIViewController
  • 若控制器的视图是window的根视图，则其nextResponder为窗口对象；若控制器是从别的控制器present出来的，则其nextResponder为presenting view controller。
• UIWindow
  • nextResponder为UIApplication对象。
• UIApplication
  • 若当前应用的app delegate是一个UIResponder对象，且不是UIView、UIViewController或app本身，则UIApplication的nextResponder为app delegate。

下面看一张图

responderChain

可以用以下方式打印一个响应链中的每一个响应对象，在最佳响应者的touchBegin:withEvent:方法中调用即可（别忘了调用父类的方法）。

- (void)printResponderChain
{
      UIResponder *responder = self; 
      printf("%s",[NSStringFromClass([responder class]) UTF8String]); 
      while (responder.nextResponder) {
           responder = responder.nextResponder;
           printf(" --> %s",[NSStringFromClass([responder class]) UTF8String]); 
      }
}

3. 手势识别器比UIResponder具有更高的事件响应优先级

手势分为离散型手势（discrete gestures）和持续型手势（continuous gesture）。系统提供的离散型手势包括点按手势（UITapGestureRecognizer）和轻扫手势（UISwipeGestureRecognizer ），其余均为持续型手势。

两者主要区别在于状态变化过程：

• 离散型：

  • 识别成功：Possible —> Recognized
  • 识别失败：Possible —> Failed

• 持续型：

  • 完整识别：Possible —> Began —> [Changed] —> Ended
  • 不完整识别：Possible —> Began —> [Changed] —> Cancel

Window在将事件传递给hit-tested view之前，会先将事件传递给相关的手势识别器并由手势识别器优先识别。若手势识别器成功识别了事件，就会取消hit-tested view对事件的响应；若手势识别器没能识别事件，hit-tested view才完全接手事件的响应权。也就是说手势的优先级要比UIResponder高。

在手势的一个子类UIGestureRecognizerSubclass.h中，调用下面几个方法来响应事件。

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event;
- (void)touchesMoved:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event;
- (void)touchesEnded:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event;
- (void)touchesCancelled:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event;

参考文章

1. iOS-使用hitTest控制点击事件的响应对象
2. iOS触摸事件全家桶
3. iOS触摸事件详解

后记

未完，待续~~~