如何全局获取当前第一响应者

#import <UIKit/UIKit.h>

@interface UIResponder (FirstResponder)

+ (instancetype)currentFirstResponder;

+ (instancetype)currentSecondResponder;

@end

#import "UIResponder+FirstResponder.h"
static __weak id currentFirstResponder;
static __weak id currentSecodResponder;

@implementation UIResponder (FirstResponder)

+ (instancetype)currentFirstResponder {
    currentFirstResponder = nil;
    currentSecodResponder = nil;
    // Public API 非私有API
    // 通过将target设置为nil，让系统自动遍历响应链
    // 从而响应链当前第一响应者响应我们自定义的方法
    [[UIApplication sharedApplication] sendAction:@selector(findFirstResponder:) to:nil from:nil forEvent:nil];
    return currentFirstResponder;
}

+ (instancetype)currentSecondResponder{
    currentFirstResponder = nil;
    currentSecodResponder = nil;
    [[UIApplication sharedApplication] sendAction:@selector(findFirstResponder:) to:nil from:nil forEvent:nil];
    return currentSecodResponder;
}

- (void)findFirstResponder:(id)sender {
    currentFirstResponder = self;
    [self.nextResponder findSecondResponder:sender];
}

- (void)findSecondResponder:(id)sender{
    currentSecodResponder = self;
}
@end

import UIKit

private weak var wty_currentFirstResponder: AnyObject?

extension UIResponder {

    static func wty_firstResponder() -> AnyObject? {
        wty_currentFirstResponder = nil
        // 通过将target设置为nil，让系统自动遍历响应链
        // 从而响应链当前第一响应者响应我们自定义的方法
        UIApplication.shared.sendAction(#selector(wty_findFirstResponder(_:)), to: nil, from: nil, for: nil)
        return wty_currentFirstResponder
    }
    
    func wty_findFirstResponder(_ sender: AnyObject) {
        // 第一响应者会响应这个方法，并且将静态变量wty_currentFirstResponder设置为自己
        wty_currentFirstResponder = self
    }
}

__weak specifies a reference that does not keep the 
referenced object alive. A weak reference is set to nil when
there are no strong references to the object.

// A powerful feature of blocks is that they can modify 
variables in the same lexical scope. You signal that a block 
can modify a variable using the __block storage type 
modifier. 

// At function level are __block variables. These are mutable
 within the block (and the enclosing scope) and are preserved
if any referencing block is copied to the heap.

UIWindow *keyWindow = [[UIApplication sharedApplication] keyWindow];
UIView *firstResponder = [keyWindow performSelector:@selector(firstResponder)];