实现tableView回到顶部的代码

#import <Foundation/Foundation.h>

@interface XMGTopWindow : NSObject

+ (void)show;

+ (void)hide;

@end

#import "XMGTopWindow.h"

@implementation XMGTopWindow

static UIWindow *window_;

+ (void)initialize{

 window_ = [[UIWindow alloc] init];

 window_.frame = CGRectMake(0, 0, XQMScreenW, 20);

 window_.windowLevel = UIWindowLevelAlert;

 window_.backgroundColor = [UIColor clearColor];

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

@end

上面使用 [subview setContentOffset:offset animated:YES]; 滚动到顶部，如果出现tableView滚动了一部分后停留（特别是在加载更多数据 reloadData 之后），使用下面方法

if([subview isKindOfClass:[UITabelView class]]){
      UITableView *tableView = (UITableView *)subview;
      NSIndexPath *indexPath = [NSIndexPath indexPathForRow:0 inSection:0];
      [tableView scrollToRowAtIndexPath:indexPath atScrollPosition:UITableViewScrollPositionBottom animated:YES];
}else{
     CGPoint offset = subview.contentOffset;
     offset.y = - subview.contentInset.top;
     [subview setContentOffset:offset animated:YES];
}

判断视图是否显示在屏幕中

- (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;

}