DidSelectRowAtIndexPath causes my app to crash

Question

Hi,
I'm just learning Cocoa. I've implemented a tab bar, and the first view (a UIViewController) contains a table view. I've implemented a separate class as the tableview datasource and delegate. Everything works fine, until I add either didSelectRowAtIndexPath or accessoryButtonTappedForRowWithIndexPath. Even if these methods have absolutely no code in them, just adding their implementation to my delegate creates a problem: when I click on a table row or on the disclosure button, my app breaks, and when I "step into" the code, it crashes with a “EXCBADACCESS”. I do not have any symbolic breakpoints defined.
Here's the error I see in gdb:
Loading program into debugger…
GNU gdb 6.3.50-20050815 (Apple version gdb-962) (Sat Jul 26 08:14:40 UTC 2008)
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This GDB was configured as "i386-apple-darwin".warning: Unable to read symbols for "/System/Library/Frameworks/UIKit.framework/UIKit" (file not found).
warning: Unable to read symbols from "UIKit" (not yet mapped into memory).
warning: Unable to read symbols for "/System/Library/Frameworks/CoreGraphics.framework/CoreGraphics" (file not found).
warning: Unable to read symbols from "CoreGraphics" (not yet mapped into memory).
Program loaded.
sharedlibrary apply-load-rules all
Attaching to program: `/Users/Beth/Library/Application Support/iPhone Simulator/User/Applications/24242BF8-FA78-4360-A519-593EC497D8CE/NDQ.app/NDQ', process 237.
Program received signal: “EXCBADACCESS”.
I currently suspect one of two things, but honestly, have no idea what is causing the problem. I suspect that either, because I'm not using a Navigation controller, it's looking for a stack, but not finding one. Or I haven't hooked something up right in IB.
As I mentioned, the table is working fine without these method implementations - that is, it is displaying some simple string data and without those method implementations, I can click on a table row and of course, nothing happens, but it doesn't crash.
Thanks so much in advance for any help.
Elisabeth

Answer 1

Elisabeth Freeman wrote:
I've made a lot of progress since I last posted!
I can see that!!
I have set the View Controllers property (using IB) to the Navigation Controller (topicsTableNavController in the code below) for the first tab.
Neither your code nor your IB conections should set the navigationController property of a view controller. That property is set by the nav controller when the vc is pushed onto its stack. Your code can then use the property to get the nc's address when it wants to push another vc onto the stack.
Then, I have set the view (through IB) of the Navigation Controller to the Table View Controller.
Similarly, you should normally consider nc.view to be read only. The nc's view content is a composite of the nav bar and the top vc's view. It's managed by the nc, so neither your code nor your IB connections should change nc.view.
I think you've simply made a wrong (and very common!) assumption about the relationship between a nav controller and the view controllers it manages: The nc's view is the one that needs to be added to the hierarchy (the tab view in this case). The vc's view is then added to the nc's view, not the other way around. So when we visualize the relationship, the nav controller is always the parent of the view controllers that are pushed on and popped from its stack.
Since you defined your controllers and their connections in IB, the solution you're looking for is in the xib file, not your code. It turns out that IB already knows the correct structure, so it's deceptively easy to build it. That can make it harder to learn what's going on, so you're lucky you managed to scramble the stucture this time, else you might have never learned it.
Try building a new test bed in IB as follows:
1) File->New Project->iPhone OS->Application->Tab Bar Application;
2) Open MainWindow.xib in IB and bring up the window named MainWindow.xib (Window->Document);
3) Switch View Mode (upper left corner) to the center position (2 columns with small-icon tree view to the left);
4) Expand Tab Bar Controller and select (not expand) the first View Controller so it's highlighted in blue;
5) Delete the first View Controller by pressing the Delete key or Edit->Delete while that controller is highlighted;
6) Drag a Navigation Controller to the MainWindow.xib tree (Tools->Library->Objects->Controllers);
7) Position the nav controller over the tab bar controller icon until the tbc is shaded and outlined;
8) Drop the nc so that the tbc becomes its parent--i.e. the nc takes the place of the deleted vc;
9) Expand the nc and expand its child root controller to see the structure;
10) Select the root vc, open the Attributes panel (Tools->Attributes Inspector) and enter a title for the root vc;
11) If you want to define the root vc's view in it's own xib, enter a name for the new xib;
12) else you can now drag a view (e.g. UIView or UITableView from the library onto the root vc's icon;
13) If you want the root vc to be a UITableViewController. go back to step 10 and replace the root UIViewController;
14) After you have the structure you want, right click on each icon to see the connections IB has already made.
Btw, if you decide to make a separate xib in step 11, make sure you set the controller's class id in both MainWindow.xib and the new xib. I.e. both the icon that creates the controller and its proxy need to have the same class (else you get a very common crash).
I'm counting on you to let me know if the above is as clear as mud, ok? I've recently tried to help in two other threads that involved the same misunderstanding about nav controllers. Start reading each of these threads at the quoted paragraph:
The reason your current code isn't producing the transition you expect is that the tab bar has attached the table controller's view to the hierarchy (i.e. using addSubview) instead of the nav controller's view. You want the nav controller's view because that's the one which will change when you push a new controller. [http://discussions.apple.com/thread.jspa?messageID=9592005&#9592005];
It's the nav controller's view that contains the content view, not the other way around. This point is worth taking a moment to see, and it seems to be commonly misunderstood. [http://discussions.apple.com/message.jspa?messageID=9607946#9607946].