从iOS10开始，苹果官方添加点击状态栏，让UIScrollView回滚到顶部，但是iOS9之前的版本需要程序员写代码实现该功能。

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实现思路

---

1，状态栏处添加一个UIWindow

创建一个UIWindow对象，并添加点击手势，而且windowLevel大于UIWindowLevelStatusBar

+ (void)initialize
{
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(0.25 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
    window_ = [[UIWindow alloc] init];
    window_.windowLevel = UIWindowLevelAlert;
    window_.frame = [UIApplication sharedApplication].statusBarFrame;
      
    window_.x = 0;
    window_.width = window_.width - window_.x;
    window_.backgroundColor = [UIColor clearColor];
    window_.hidden = NO;
    [window_ addGestureRecognizer:[[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(windowClick)]];
        
    // 添加通知刷新
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(windowClick) name:FOFTopWindowNeedClickNotification object:nil];;
    });
}

对外提供show和hide类方法

+ (void)show
{
    
    window_.hidden = NO;
}

+ (void)hide
{
    window_.hidden = YES;
}

2，递归寻找展示在keyWindow上的UIScrollView控件

+ (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];
        }
        
        // 继续查找子控件
        [self searchScrollViewInView:subview];
    }
}

3，判断该scrollView是否和keyWindow重叠（难点）

- (BOOL)isShowingOnKeyWindow
{
    // 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:self.frame fromView:self.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 !self.isHidden && self.alpha > 0.01 && self.window == keyWindow && intersects;
}

利用UIView对象方法转换坐标系

- (CGRect)convertRect:(CGRect)rect toView:(nullable UIView *)view;
- (CGRect)convertRect:(CGRect)rect fromView:(nullable UIView *)view;

4，在AppDelegate添加show方法

- (void)applicationDidBecomeActive:(UIApplication *)application {
    // Restart any tasks that were paused (or not yet started) while the application was inactive. If the application was previously in the background, optionally refresh the user interface.
    
    // 添加一个window, 点击这个window, 可以让屏幕上的scrollView滚到最顶部
    [FOFTopWindow show];
}

** Demo代码查看git **
https://github.com/tangbin583085/TBTopWindow