当我要求我的表视图滚动到顶部时，为什么会出现NSRangeException？

Question

I am using a subclassed tableView made by someone else and it comes with a method loadObjects that causes the tableView to repopulate its cells, however it doesn't tell the table view to return to the top after the repopulation. I tried to implement this feature with:

- (void)loadObjects{
    [super loadObjects];
    [self.tableView scrollToRowAtIndexPath:[NSIndexPath indexPathForRow:0 inSection:0] atScrollPosition:UITableViewScrollPositionTop animated:YES];
}

However once loadObjects is called (when the table view wants to initially load), i get the following exception:

*** Terminating app due to uncaught exception 'NSRangeException', reason: 
'-[UITableView _contentOffsetForScrollingToRowAtIndexPath:atScrollPosition:]: 
row (0) beyond bounds (0) for section (0).'

I transferred the line I wrote that scrolls the table view to the top to another custom method that is called whenever I pressed a button, and it worked totally fine, so there is a problem in my overwriting of loadObjects . Does anyone understand the exception that was thrown?

Answer 1

在调用scrollToRowAtIndexPath之前测试重新加载tableview。

[self.tableView reloadData];

Answer 2

As some of the commenters pointed out, you'll see this error when attempting to scroll to an index path that does not exist (according to the results of numberOfSectionsInTableView and numberOfRowsInSection).

So the fix is to not attempt to scroll to a non-existent index path. You might achieve this fix by changing the order in which this method is called, relative to loading your data (as the commenters suggested), or by some other means (eg changing the way you compute which index path to scroll to, or only scrolling conditionally if the index path is sure to exist).