Swift版

import UIKit 
class TopWindow: UIWindow {
    private static let window_: UIWindow = UIWindow()
    ///  类初始化方法,保证window_只被创建一次
    override class func initialize() {
        window_.frame = CGRectMake(0, 0, global.appWidth, 20)
        window_.windowLevel = UIWindowLevelAlert
        window_.addGestureRecognizer(UITapGestureRecognizer(target: self, action: "topWindowClick"))
    }
    class func topWindowClick() {
        // 遍历当前主窗口所有view,将满足条件的scrollView滚动回原位
        searchAllowScrollViewInView(UIApplication.sharedApplication().keyWindow!)
    }
    private class func searchAllowScrollViewInView(superView: UIView) {
        for subview: UIView in superView.subviews as! [UIView] {
            if subview.isKindOfClass(UIScrollView.self) && superView.viewIsInKeyWindow() {
                // 拿到scrollView的contentOffset
                var offest = (subview as! UIScrollView).contentOffset
                // 将offest的y轴还原成最开始的值
                offest.y = -(subview as! UIScrollView).contentInset.top
                // 重新设置scrollView的内容
                (subview as! UIScrollView).setContentOffset(offest, animated: true)
            }
            // 递归,让子控件再次调用这个方法判断时候还有满足条件的view
            searchAllowScrollViewInView(subview)
        }
    }
    ///  添加topWindow,使手势生效
    class func showTopWindow() {
        window_.hidden = false
    }
    ///  隐藏topWindow,移除手势
    class func hiddenTopWindow() {
        window_.hidden = true
    }
}
///  对UIView的一个扩展
extension UIView {
    ///  判断调用方法的view是否在keyWindow中
    func viewIsInKeyWindow() -> Bool {
        let keyWindow = UIApplication.sharedApplication().keyWindow!
        // 将当前view的坐标系转换到window.bounds
        let viewNewFrame = keyWindow.convertRect(self.frame, fromView: self.superview)
        let keyWindowBounds = keyWindow.bounds
        // 判断当前view是否在keyWindow的范围内
        let isIntersects = CGRectIntersectsRect(viewNewFrame, keyWindowBounds)
        // 判断是否满足所有条件
        return !self.hidden && self.alpha > 0.01 && self.window == keyWindow && isIntersects
    }   
}

在需要的视图控制器中调用TopWindow.showTopWindow()，视图将要消失的时候调用TopWindow.hiddenTopWindow()

Objective-C版

.h文件

#import @interface ZGJTopWindow : NSObject
+ (void)show;
+ (void)hide;
@end

.m文件

#import "ZGJTopWindow.h"
@implementation ZGJTopWindow
static UIWindow *window_;
//初始化window
+ (void)initialize {
  window_ = [[UIWindow alloc] init];
  window_.frame = CGRectMake(0, 0, [UIScreen mainScreen].bounds.size.width, 20);
  window_.windowLevel = UIWindowLevelAlert;
  [window_ addGestureRecognizer:[[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(windowClick)]];
}  
+ (void)show {
  window_.hidden = NO;
}
+ (void)hide {
  window_.hidden = YES;
}
// 监听窗口点击
+ (void)windowClick {
  UIWindow *window = [UIApplication sharedApplication].keyWindow;
  [self searchScrollViewInView:window];
}
+ (void)searchScrollViewInView:(UIView *)superview {
  for (UIScrollView *subview in superview.subviews) {
      // 如果是scrollview, 滚动最顶部
      if ([subview isKindOfClass:[UIScrollView class]] && [subview isShowingOnKeyWindow]) {
          CGPoint offset = subview.contentOffset;
          offset.y = - subview.contentInset.top;
          [subview setContentOffset:offset animated:YES];
      }
      // 递归继续查找子控件
      [self searchScrollViewInView:subview];
  }
}
+ (BOOL)isShowingOnKeyWindow:(UIView *)view {
  // 主窗口
  UIWindow *keyWindow = [UIApplication sharedApplication].keyWindow;
  // 以主窗口左上角为坐标原点, 计算self的矩形框
  CGRect newFrame = [keyWindow convertRect:view.frame fromView:view.superview];
  CGRect winBounds = keyWindow.bounds;
  // 主窗口的bounds 和 self的矩形框 是否有重叠
  BOOL intersects = CGRectIntersectsRect(newFrame, winBounds);
  return !view.isHidden && view.alpha > 0.01 && view.window == keyWindow && intersects;
}
@end

原文：http://www.jianshu.com/p/836cdd481982