Alan Alan - 2 months ago 25
Objective-C Question

How to tell when UITableVIew has completed ReloadData?

I am trying to scroll to the bottom of a UITableView after it does done performing

[self.tableView reloadData]


I originally had

[self.tableView reloadData]
NSIndexPath* indexPath = [NSIndexPath indexPathForRow: ([self.tableView numberOfRowsInSection:([self.tableView numberOfSections]-1)]-1) inSection: ([self.tableView numberOfSections]-1)];

[self.tableView scrollToRowAtIndexPath:indexPath atScrollPosition:UITableViewScrollPositionBottom animated:YES];


But then I read that reloadData is asynchronous, so the scrolling doesn't happen since the
self.tableView
,
[self.tableView numberOfSections]
and
[self.tableView numberOfRowsinSection
are all 0.

Thanks!

What's weird is that I am using:

[self.tableView reloadData];
NSLog(@"Number of Sections %d", [self.tableView numberOfSections]);
NSLog(@"Number of Rows %d", [self.tableView numberOfRowsInSection:([self.tableView numberOfSections]-1)]-1);


In the console it returns Sections = 1, Row = -1;

When I do the exact same NSLogs in
cellForRowAtIndexPath
I get Sections = 1 and Row = 8; (8 is right)

Answer

The reload happens during the next layout pass, which normally happens when you return control to the run loop (after, say, your button action or whatever returns).

So one way to run something after the table view reloads is simply to force the table view to perform layout immediately:

[self.tableView reloadData];
[self.tableView layoutIfNeeded];
 NSIndexPath* indexPath = [NSIndexPath indexPathForRow: ([self.tableView numberOfRowsInSection:([self.tableView numberOfSections]-1)]-1) inSection: ([self.tableView numberOfSections]-1)];
[self.tableView scrollToRowAtIndexPath:indexPath atScrollPosition:UITableViewScrollPositionBottom animated:YES];

Another way is to schedule your after-layout code to run later using dispatch_async:

[self.tableView reloadData];

dispatch_async(dispatch_get_main_queue(), ^{
     NSIndexPath* indexPath = [NSIndexPath indexPathForRow: ([self.tableView numberOfRowsInSection:([self.tableView numberOfSections]-1)]-1) inSection:([self.tableView numberOfSections]-1)];

    [self.tableView scrollToRowAtIndexPath:indexPath atScrollPosition:UITableViewScrollPositionBottom animated:YES];
});

UPDATE

Upon further investigation, I find that the table view sends tableView:numberOfRowsInSection: to its data source before returning from reloadData. If the delegate implements tableView:heightForRowAtIndexPath:, the table view also sends that (for each row) before returning from reloadData.

However, the table view does not send tableView:cellForRowAtIndexPath: until the layout phase, which happens by default when you return control to the run loop.

And I also find that in a tiny test program, the code in your question properly scrolls to the bottom of the table view, without me doing anything special (like sending layoutIfNeeded or using dispatch_async).

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