Why does UITableView make so many calls to its delegate & datasource?

Anyone care to shed some light on why UITableView makes so many repeat calls to its delegate & datasource as it's being setup? Just looking at one I'm working on now I see that numberOfSectionsInTableView is called 3 times and then viewForHeaderInSection cycles through 3 more times for each section (I have 2 sections so total of 6 times before being displayed for first time)...all before the first screen is even rendered.

I realize that the API is private but wondering if someone might be willing to offer up some insight into this design pattern and what might be going on behind the scenes so I might learn a thing or two about why so much repetition is considered acceptable (or even advantageous) in this case...or, more likely, how I've managed to foul up applying it.

Edit: Added stack traces before RootViewController tableView is shown for first time

1st time calling viewForHeaderInSection:

#0  0x000086cc in -[RootViewController tableView:viewForHeaderInSection:] at RootViewController.m:444
#1  0x30c901a8 in -[UITableView(UITableViewInternal) _delegateWantsHeaderForSection:]
#2  0x3开发者_如何学运维0c8f6f4 in -[UISectionRowData refreshWithSection:tableView:tableViewRowData:]
#3  0x30c8f5a8 in -[UITableViewRowData rectForFooterInSection:]
#4  0x30c3d430 in -[UITableViewRowData heightForTable]
#5  0x30c05430 in -[UITableView(_UITableViewPrivate) _updateContentSize]
#6  0x30c3ce0c in -[UITableView noteNumberOfRowsChanged]
#7  0x30c3c7c0 in -[UITableView reloadData]
#8  0x30e50e60 in -[UITableViewController viewWillAppear:]
#9  0x30c78810 in -[UINavigationController _startTransition:fromViewController:toViewController:]
#10 0x30c783b0 in -[UINavigationController _startDeferredTransitionIfNeeded]
#11 0x30c782a0 in -[UINavigationController viewWillLayoutSubviews]
#12 0x30c5c874 in -[UILayoutContainerView layoutSubviews]
#13 0x30c382d8 in -[UIView(CALayerDelegate) _layoutSublayersOfLayer:]
#14 0x33e871c0 in -[CALayer layoutSublayers]
#15 0x33e86edc in CALayerLayoutIfNeeded
#16 0x33e86844 in CA::Context::commit_transaction
#17 0x33e86474 in CA::Transaction::commit
#18 0x33e8e5dc in CA::Transaction::observer_callback
#19 0x32d5c830 in __CFRunLoopDoObservers
#20 0x32da4346 in CFRunLoopRunSpecific
#21 0x32da3c1e in CFRunLoopRunInMode
#22 0x31bb9374 in GSEventRunModal
#23 0x30bf3c30 in -[UIApplication _run]
#24 0x30bf2230 in UIApplicationMain
#25 0x00002260 in main at main.m:14

2nd call:

#0  0x000086cc in -[RootViewController tableView:viewForHeaderInSection:] at RootViewController.m:444
#1  0x30c901a8 in -[UITableView(UITableViewInternal) _delegateWantsHeaderForSection:]
#2  0x30c8f6f4 in -[UISectionRowData refreshWithSection:tableView:tableViewRowData:]
#3  0x30c90628 in -[UITableViewRowData(UITableViewRowDataPrivate) _ensureSectionOffsetIsValidForSection:]
#4  0x30c8f5d0 in -[UITableViewRowData rectForFooterInSection:]
#5  0x30c3d430 in -[UITableViewRowData heightForTable]
#6  0x30c05430 in -[UITableView(_UITableViewPrivate) _updateContentSize]
#7  0x30c3ce0c in -[UITableView noteNumberOfRowsChanged]
#8  0x30c3c7c0 in -[UITableView reloadData]
#9  0x30e50e60 in -[UITableViewController viewWillAppear:]
#10 0x30c78810 in -[UINavigationController _startTransition:fromViewController:toViewController:]
#11 0x30c783b0 in -[UINavigationController _startDeferredTransitionIfNeeded]
#12 0x30c782a0 in -[UINavigationController viewWillLayoutSubviews]
#13 0x30c5c874 in -[UILayoutContainerView layoutSubviews]
#14 0x30c382d8 in -[UIView(CALayerDelegate) _layoutSublayersOfLayer:]
#15 0x33e871c0 in -[CALayer layoutSublayers]
#16 0x33e86edc in CALayerLayoutIfNeeded
#17 0x33e86844 in CA::Context::commit_transaction
#18 0x33e86474 in CA::Transaction::commit
#19 0x33e8e5dc in CA::Transaction::observer_callback
#20 0x32d5c830 in __CFRunLoopDoObservers
#21 0x32da4346 in CFRunLoopRunSpecific
#22 0x32da3c1e in CFRunLoopRunInMode
#23 0x31bb9374 in GSEventRunModal
#24 0x30bf3c30 in -[UIApplication _run]
#25 0x30bf2230 in UIApplicationMain
#26 0x00002260 in main at main.m:14

3rd call:

#0  0x000086cc in -[RootViewController tableView:viewForHeaderInSection:] at RootViewController.m:444
#1  0x30c901a8 in -[UITableView(UITableViewInternal) _delegateWantsHeaderForSection:]
#2  0x30c3eebc in -[UITableView(_UITableViewPrivate) _updateVisibleHeadersAndFootersNow]
#3  0x30c3e578 in -[UITableView(_UITableViewPrivate) _updateVisibleCellsNow]
#4  0x30c3c514 in -[UITableView layoutSubviews]
#5  0x30c382d8 in -[UIView(CALayerDelegate) _layoutSublayersOfLayer:]
#6  0x33e871c0 in -[CALayer layoutSublayers]
#7  0x33e86edc in CALayerLayoutIfNeeded
#8  0x33e86844 in CA::Context::commit_transaction
#9  0x33e86474 in CA::Transaction::commit
#10 0x33e8e5dc in CA::Transaction::observer_callback
#11 0x32d5c830 in __CFRunLoopDoObservers
#12 0x32da4346 in CFRunLoopRunSpecific
#13 0x32da3c1e in CFRunLoopRunInMode
#14 0x31bb9374 in GSEventRunModal
#15 0x30bf3c30 in -[UIApplication _run]
#16 0x30bf2230 in UIApplicationMain
#17 0x00002260 in main at main.m:14

4th call:

#0  0x000086cc in -[RootViewController tableView:viewForHeaderInSection:] at RootViewController.m:444
#1  0x30c9712c in -[UITableView(UITableViewInternal) _sectionHeaderViewWithFrame:forSection:opaque:reuseViewIfPossible:]
#2  0x30c3f0b4 in -[UITableView(_UITableViewPrivate) _updateVisibleHeadersAndFootersNow]
#3  0x30c3e578 in -[UITableView(_UITableViewPrivate) _updateVisibleCellsNow]
#4  0x30c3c514 in -[UITableView layoutSubviews]
#5  0x30c382d8 in -[UIView(CALayerDelegate) _layoutSublayersOfLayer:]
#6  0x33e871c0 in -[CALayer layoutSublayers]
#7  0x33e86edc in CALayerLayoutIfNeeded
#8  0x33e86844 in CA::Context::commit_transaction
#9  0x33e86474 in CA::Transaction::commit
#10 0x33e8e5dc in CA::Transaction::observer_callback
#11 0x32d5c830 in __CFRunLoopDoObservers
#12 0x32da4346 in CFRunLoopRunSpecific
#13 0x32da3c1e in CFRunLoopRunInMode
#14 0x31bb9374 in GSEventRunModal
#15 0x30bf3c30 in -[UIApplication _run]
#16 0x30bf2230 in UIApplicationMain
#17 0x00002260 in main at main.m:14

5th call:

#0  0x000086cc in -[RootViewController tableView:viewForHeaderInSection:] at RootViewController.m:444
#1  0x30c901a8 in -[UITableView(UITableViewInternal) _delegateWantsHeaderForSection:]
#2  0x30c3eebc in -[UITableView(_UITableViewPrivate) _updateVisibleHeadersAndFootersNow]
#3  0x30c3e578 in -[UITableView(_UITableViewPrivate) _updateVisibleCellsNow]
#4  0x30c3c514 in -[UITableView layoutSubviews]
#5  0x30c382d8 in -[UIView(CALayerDelegate) _layoutSublayersOfLayer:]
#6  0x33e871c0 in -[CALayer layoutSublayers]
#7  0x33e86edc in CALayerLayoutIfNeeded
#8  0x33e86844 in CA::Context::commit_transaction
#9  0x33e86474 in CA::Transaction::commit
#10 0x33e8e5dc in CA::Transaction::observer_callback
#11 0x32d5c830 in __CFRunLoopDoObservers
#12 0x32da4346 in CFRunLoopRunSpecific
#13 0x32da3c1e in CFRunLoopRunInMode
#14 0x31bb9374 in GSEventRunModal
#15 0x30bf3c30 in -[UIApplication _run]
#16 0x30bf2230 in UIApplicationMain
#17 0x00002260 in main at main.m:14

6th call:

#0  0x000086cc in -[RootViewController tableView:viewForHeaderInSection:] at RootViewController.m:444
#1  0x30c9712c in -[UITableView(UITableViewInternal) _sectionHeaderViewWithFrame:forSection:opaque:reuseViewIfPossible:]
#2  0x30c3f0b4 in -[UITableView(_UITableViewPrivate) _updateVisibleHeadersAndFootersNow]
#3  0x30c3e578 in -[UITableView(_UITableViewPrivate) _updateVisibleCellsNow]
#4  0x30c3c514 in -[UITableView layoutSubviews]
#5  0x30c382d8 in -[UIView(CALayerDelegate) _layoutSublayersOfLayer:]
#6  0x33e871c0 in -[CALayer layoutSublayers]
#7  0x33e86edc in CALayerLayoutIfNeeded
#8  0x33e86844 in CA::Context::commit_transaction
#9  0x33e86474 in CA::Transaction::commit
#10 0x33e8e5dc in CA::Transaction::observer_callback
#11 0x32d5c830 in __CFRunLoopDoObservers
#12 0x32da4346 in CFRunLoopRunSpecific
#13 0x32da3c1e in CFRunLoopRunInMode
#14 0x31bb9374 in GSEventRunModal
#15 0x30bf3c30 in -[UIApplication _run]
#16 0x30bf2230 in UIApplicationMain
#17 0x00002260 in main at main.m:14

In my code numberOfSectionsInTableView is only called once, in response to a reloadData call.

You can set a breakpoint in numberOfSectionsInTableView and look at the call stack to see how and why it's being called.

One of the reasons that it calls them a lot is that things can change dramatically each time there's a [... reloadData] or whatever, so it can't make any assumptions that things are as they were last time you called them. This is one of the reasons that they make a big deal out of caching UITableViewCells in the docs for example.

Here's a class demonstrating this:

@interface WinnersViewController : UITableViewController {
}
- (void)setDataSourceAndDelegate;
@end

in my viewDidLoad, I call a method called setDataSourceAndDelegate which sets one of three classes as the datasource and delegate properties on the table view.

- (void)viewDidLoad {
    [self setDataSourceAndDelegate];
    [super viewDidLoad];
}

I have three different objects that act as a DataSource and Delegate (combined) that I use, with an abstract class above them:

WinnersViewDefaultDSD
WinnersViewHasLocationDSD
WinnersViewHasExampleDSD

(I've replaced the actual class name in question with Example for obvious reasons.)

Now these classes return very different things to the table View, and sometimes when the state of my application changes (like having called CoreLocation or the User selecting an instance of class example, the setDataSourceAndDelegate method gets called again, and it can change which object it is on the fly.

So, the reason that UITableView calls so many things on its delegate and datasource so many times is because it needs to support developers doing this sort of thing with the table view. If certain methods were called once and then the answer was assumed unchanging, it wouldn't be as flexible.

Anyway, hope this is a helpful example.

(oh, and if anyone is going to emulate this, notice that the delegate and datasource properties on the TableView are copy and not retain so you have to do an extra retain on the instances of the DSD classes or you'll get a mysterious but frequent crash.)

(Also, I needed to override the viewDidAppear for the various DSD subclasses and ended up needing to do this:)

- (void)viewDidAppear:(BOOL)animated
{
    [[(UITableView*)self.view delegate] delegatedViewDidAppear];
}

(this is a bit of a hack that keeps me from having to track the DSD object in the class.)