UIView中的hitTest方法

1. 事件响应的过程

在iOS中的view之间逐层叠加，当点击了屏幕上的某个view时，这个点击动作会由硬件层传导到操作系统并生成一个事件（Event），这个事件顺着view的层级由下往上传导，直至找到包含有这个点击点、层级最高、且可与用户交互的view来响应这个事件。

iOS中的事件响应链

2. 响应链中涉及的方法

UIView中的hitTest方法、pointInside方法

// recursively calls -pointInside:withEvent:. point is in the receiver's coordinate system
- (nullable UIView *)hitTest:(CGPoint)point withEvent:(nullable UIEvent *)event;   
 // default returns YES if point is in bounds
- (BOOL)pointInside:(CGPoint)point withEvent:(nullable UIEvent *)event;  

// 点位转换相关方法
- (CGPoint)convertPoint:(CGPoint)point toView:(nullable UIView *)view;
- (CGPoint)convertPoint:(CGPoint)point fromView:(nullable UIView *)view;
- (CGRect)convertRect:(CGRect)rect toView:(nullable UIView *)view;
- (CGRect)convertRect:(CGRect)rect fromView:(nullable UIView *)view;

hitTest方法的实现：

- (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;
}

点击事件会在hitTest、pointInside两个方法的配合下，向下传递；

在hitTest:withEvent:内部首先会判断该视图是否能响应触摸事件，如果不能响应，返回nil，表示该视图不响应此触摸事件。然后再调用pointInside:withEvent:（该方法用来判断点击事件发生的位置是否处于当前视图处理范围）。如果pointInside:withEvent:返回NO，那么hiteTest:withEvent:也直接返回nil；

如果pointInside:withEvent:返回YES，则向当前视图的所有子视图发送hitTest:withEvent:消息，所有子视图的遍历顺序是从最顶层视图一直到到最底层视图，即从subviews数组的末尾向前遍历。直到有子视图返回非空对象或者全部子视图遍历完毕；若第一次有子视图返回非空对象，则 hitTest:withEvent:方法返回此对象，处理结束；如所有子视图都返回非，则hitTest:withEvent:方法返回该视图自身。

UIResponder中的touchesBegan、touchesMoved、touchesEnded等方法

// 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 *)touches withEvent:(nullable UIEvent *)event;
- (void)touchesMoved:(NSSet *)touches withEvent:(nullable UIEvent *)event;
- (void)touchesEnded:(NSSet *)touches withEvent:(nullable UIEvent *)event;
- (void)touchesCancelled:(NSSet *)touches withEvent:(nullable UIEvent *)event;
- (void)touchesEstimatedPropertiesUpdated:(NSSet *)touches NS_AVAILABLE_IOS(9_1);

// 这几个方法比较常用，在此不再敖述；
// 当然，UIResponder中不止这三个响应事件的方法，本文仅以touches的这三个方法为例。

示例

为了使我们更好的理解事件响应过程中，上述UIView与UIResponder这几个方法的执行过程，我们用以下图示例（示例参考文章）进行说明，图中视图ABCDE(UIView型)之间的层次关系是self.view(A(B, C(D, E)))：

测试hitTest方法的执行过程

以下代码是在A视图中都重写我们需要观察的几个父类方法，BCDE中需要重写的代码以此类推：

/*
* 例如：A中重写父类方法的代码，
*/

- (UIView *)hitTest:(CGPoint)point withEvent:(UIEvent *)event {
    
    NSLog(@"AView ---->> hitTest:withEvent: ---");
    UIView * view = [super hitTest:point withEvent:event];
    NSLog(@"AView <<--- hitTest:withEvent: --- /n hitTestView:%@", view);
    return view;
}

- (BOOL)pointInside:(CGPoint)point withEvent:(nullable UIEvent *)event {
    
    NSLog(@"AView --->> pointInside:withEvent: ---");
    BOOL isInside = [super pointInside:point withEvent:event];
    NSLog(@"AView <<--- pointInside:withEvent: --- isInside:%d", isInside);
    return isInside;
}

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    
    NSLog(@"AView touchesBegan");
}

- (void)touchesMoved:(NSSet *)touches withEvent:(nullable UIEvent *)event {
    
    NSLog(@"AView touchesMoved");
}

- (void)touchesEnded:(NSSet *)touches withEvent:(nullable UIEvent *)event {
    
    NSLog(@"AView touchesEnded");
}

当点击了一下B视图所在区域时，Xcode输出log如下：

点击B视图，Xcode输出的log

在示例中可以发现响应链中所涉及方法的执行过程，有以下特点

当UIView中的isUserInteractionEnabled = NO、isHidden = YES、alpha <= 0.01时，hitTest方法不会被调用；

UIResponder 中的touches三个方法都是发生在找到最终的响应事件的view之后；

二是寻找hit-test view的事件链传导了两遍，具体原因不明;

请自行修改两个方法的返回值，来实验。

3. hitTest方法的应用

3.1 改变UIButton的响应热区

具体的说改变视图的响应热区，主要是在pointInside方法中完成的，QiShare关于改变热区的文章中有过描述。但是hitTest、pointInside同属响应链中方法，如果有需求，也可以在hitTest中返回一个确定的view。

3.2 view超出superView的bounds仍能响应事件

如图，在黄色superView上添加一个UIButton，UIButton上半部分超出superView。正常的情况下点击红框区域时，UIButton是无法响应点击事件的，要让红框区域内的UIButton仍能响应点击事件，需要我们重写superView的hitTest方法。

#import "BeyondBoundsOfView.h"

@interface BeyondBoundsOfView ()

@property (nonatomic, strong) UIButton *button;

@end

@implementation BeyondBoundsOfView

- (instancetype)initWithFrame:(CGRect)frame {
    
    self = [super initWithFrame:frame];
    if (self) {
        _button = [UIButton buttonWithType:UIButtonTypeSystem];
        [_button setTitleColor:[UIColor whiteColor] forState:UIControlStateNormal];
        [_button setTitle:@"UIButton" forState:UIControlStateNormal];
        [_button setBackgroundColor:[UIColor lightGrayColor]];
        _button.frame = CGRectMake(0, 0, 80, 80);
        [self addSubview:_button];
    }
    return self;
}

- (void)layoutSubviews {
    [super layoutSubviews];
    
    CGSize size = self.frame.size;
    _button.center = CGPointMake(size.width / 2, 0);
}


- (UIView *)hitTest:(CGPoint)point withEvent:(UIEvent *)event {
    
    if (!self.isUserInteractionEnabled || self.isHidden || self.alpha <= 0.01) {
        return nil;
    }
    
    for (UIView *subview in self.subviews) {
        CGPoint convertedPoint = [subview convertPoint:point fromView:self];
        UIView *hitTestView = [subview hitTest:convertedPoint withEvent:event];
        if (hitTestView) {
            return hitTestView;
        }
    }
    return nil;
}

@end

上面代码中关键的一行：
CGPoint convertedPoint = [subview convertPoint:point fromView:self];
获取到convertedPoint对我们循环调用子view的hitTest很关键。

工程源码GitHub地址