When does a UITableView's contentSize get set?

I have a non scrolling UITableView in a UIScrollView. I set the frame size of the UITableView to its content size.

When I add a row to the UITableView, I call insertRowsAtIndexPaths:withRowAnimation: on the UITableView. Then I call a method to resize the frame of the UITableView:

- (void)resizeTableViewFrameHeight
{
    // Table view does not scroll, so its frame height should be equal to its contentSize height
    CGRect frame = self.tableView.frame;
    frame.size = self.tableView.contentSize;
    self.tableView.frame = frame;
}

It seems though that the contentSize hasn't been updated at this point. If I manually calculate the frame in the above method based on the number of rows and sections, then the method works properly.

My question is, how can I get the UITableView to update its contentSize? I suppose I could call reloadData and that would probably do it, but it seems inefficient to reload the entire table when I'm just inserting one cell.

You can make the UITableView calculate the size of the content immediately by calling layoutIfNeeded on the UITableView. This will run all the necessary calculations to layout the UITableView.

Example for a UITableViewController subclass that you want to put in a container view with variable size:

Objective-C

- (CGSize)preferredContentSize
{
    // Force the table view to calculate its height
    [self.tableView layoutIfNeeded];
    return self.tableView.contentSize;
}

Swift

override var preferredContentSize: CGSize {
    get {
        // Force the table view to calculate its height
        self.tableView.layoutIfNeeded()
        return self.tableView.contentSize
    }
    set {}
}

While I don't love KVO (Key-Value Observing), it's a fairly good way to know exactly when your table's contentSize has changed (rather than just calling your update methods in a bunch of random places). It's rather simple to use (though somewhat cryptic and implicit). To observe changes on your table's contentSize, do the following:

1) Become an observer of your table's contentSize property like so:

[self.tableView addObserver:self forKeyPath:@"contentSize" options:NSKeyValueObservingOptionNew context:NULL];

This is usually done in the view controller that holds the tableView (like in viewDidLoad:, for example).

2) Implement the KVO observing method and make the changes you need:

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *,id> *)change context:(void *)context {
    if(object == self.tableView && [keyPath isEqualToString:@"contentSize"]) {
        // perform your updates here
    }
}

3) Remove your view controller as an observer at some logical point (I call it in dealloc). You do this like so:

- (void)dealloc {
    [self.tableView removeObserver:self forKeyPath:@"contentSize"];
}

Try this:

- (void)resizeTableViewFrameHeight
{
    UITableView *tableView = self.tableView;
    CGRect frame = tableView.frame;
    frame.size.height = [tableView sizeThatFits:CGSizeMake(frame.size.width, HUGE_VALF)].height;
    tableView.frame = frame;
}

1. Add observer (in my sample in viewDidLoad

   tableView.addObserver(self, forKeyPath: "contentSize", options: .new, context: nil)
   

2. Observe value

   override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
       if let obj = object as? UITableView {
           if obj == self.tableView && keyPath == "contentSize" {
               if let newSize = change?[NSKeyValueChangeKey.newKey] as? CGSize {
                   //do stuff here
               }
           }
       }
   }
   

3. Remove observer when not needed

   deinit {
       self.tableView.removeObserver(self, forKeyPath: "contentSize")
   }
   

as @Farthen suggested you should always call yourTableView.layoutIfNeeded() to recalculate content size first.

By calling layoutIfNeeded UITableView will calculate contentSize again and then you can update the frame or constant whatever it is.

Simple two line of code will save much more efforts.

tableView.layoutIfNeeded()
tableViewHeight.constant = tableView.contentSize.height