iOS (Monotouch) App Crashes on viewWillAppear

Question

I have an App that loads a DialogViewController . This contains several elements. One of the elements is a RootElement . This RootElement contains an EntryElement .

In viewDidAppear , I am coming back from the EntryElement , and there is a value in the entry element (ie I've just added the value) I want to use that value as the caption in my view.

The View is created in a parent controller :

    public TabBarController ()
    {
        root = new RootElement("My Budget");
        root.UnevenRows = true;

        tab1 = new TestController(root);

        nav = new UINavigationController(tab1);
        nav.Title = "Budget";

        var tabs = new UIViewController []{
            nav
        };

        ViewControllers = tabs;
        SelectedViewController = nav;
    }

Then In the Controller - the magic [should] happen

    public override void ViewWillAppear (bool animated)
    {
        base.ViewWillAppear (animated);
        var caption = "Value..";

        if (BankElement != null && !String.IsNullOrWhiteSpace (BankElement.Value)){
            caption = BankElement.Value;
        }

        MyRoot = new RootElement ("Root");
        MyRoot.Caption = caption;
        BankElement = new EntryElement("Enter", caption, "");

        MyRoot.Add (new Section("") {BankElement});
        rootelement = new RootElement("Root");
        rootelement.Add (new Section("") {MyRoot});

        Root  = rootelement;
    }

However, when the viewDidAppear runs I can a stack Trace. I don't get it every time - just occasionally.

Stacktrace:

  at (wrapper managed-to-native) MonoTouch.UIKit.UIApplication.UIApplicationMain (int,string[],intptr,intptr) <IL 0x0009f, 0xffffffff>
  at MonoTouch.UIKit.UIApplication.Main (string[],string,string) [0x0004c] in /Developer/MonoTouch/Source/monotouch/src/UIKit/UIApplication.cs:38
  at BudgetPlanner.Application.Main (string[]) [0x00000] in /Users/Ashley/github/local/BudgetPlanner/BudgetPlanner/Main.cs:18
  at (wrapper runtime-invoke) <Module>.runtime_invoke_void_object (object,intptr,intptr,intptr) <IL 0x00050, 0xffffffff>

Native stacktrace:

0   BudgetPlanner                       0x00095f7c mono_handle_native_sigsegv + 284
1   BudgetPlanner                       0x0000be98 mono_sigsegv_signal_handler + 248
2   libsystem_c.dylib                   0x908bd86b _sigtramp + 43
3   ???                                 0xffffffff 0x0 + 4294967295
4   UIKit                               0x028014d0 -[UITextField canResignFirstResponder] + 33
5   UIKit                               0x0284761f -[UIResponder resignFirstResponder] + 82
6   UIKit                               0x02801b36 -[UITextField resignFirstResponder] + 141
7   UIKit                               0x0277f517 -[UIView(Hierarchy) _removeFirstResponderFromSubtree] + 209
8   UIKit                               0x0277f873 __UIViewWillBeRemovedFromSuperview + 77
9   UIKit                               0x0277f783 -[UIView(Hierarchy) removeFromSuperview] + 70
10  UIKit                               0x029b73fe -[UINavigationTransitionView _cleanupTransition] + 103
11  UIKit                               0x029b7698 -[UINavigationTransitionView _navigationTransitionDidStop] + 55
12  UIKit                               0x027773b6 -[UIViewAnimationState sendDelegateAnimationDidStop:finished:] + 237
13  UIKit                               0x02777554 -[UIViewAnimationState animationDidStop:finished:] + 68
14  QuartzCore                          0x04bc57d8 _ZN2CA5Layer23run_animation_callbacksEPv + 284
15  libdispatch.dylib                   0x04186014 _dispatch_client_callout + 14
16  libdispatch.dylib                   0x041767d5 _dispatch_main_queue_callback_4CF + 296
17  CoreFoundation                      0x012fcaf5 __CFRunLoopRun + 1925
18  CoreFoundation                      0x012fbf44 CFRunLoopRunSpecific + 276
19  CoreFoundation                      0x012fbe1b CFRunLoopRunInMode + 123
20  GraphicsServices                    0x04d237e3 GSEventRunModal + 88
21  GraphicsServices                    0x04d23668 GSEventRun + 104
22  UIKit                               0x0273965c UIApplicationMain + 1211
23  ???                                 0x0b4ee83c 0x0 + 189720636
24  ???                                 0x0b4eb0a0 0x0 + 189706400
25  ???                                 0x0b4ea7f8 0x0 + 189704184
26  ???                                 0x0b4ea903 0x0 + 189704451
27  BudgetPlanner                       0x00010252 mono_jit_runtime_invoke + 722
28  BudgetPlanner                       0x0017478e mono_runtime_invoke + 126
29  BudgetPlanner                       0x00178be4 mono_runtime_exec_main + 420
30  BudgetPlanner                       0x00178f55 mono_runtime_run_main + 725
31  BudgetPlanner                       0x0006ba65 mono_jit_exec + 149
32  BudgetPlanner                       0x0021f65d main + 2013
33  BudgetPlanner                       0x00003125 start + 53

=================================================================
Got a SIGSEGV while executing native code. This usually indicates
a fatal error in the mono runtime or one of the native libraries 
used by your application.
=================================================================

This is a much simplified version of the controller - with only the root view being displayed - in the real controller there is a lot more data.

Any ideas on this exception? Really starting to confuse me now.

(Running Monotouch 6.0.8 on the Simulator)

Thanks Ash...

Answer 1

I have fixed this myself.

The problem I think is that I was replacing the root element on every reload to refresh it. I have amended the code:

    public override void ViewWillAppear (bool animated)
    {
        base.ViewWillAppear (animated);
        var caption = "Value..";

        if (BankElement != null && !String.IsNullOrWhiteSpace (BankElement.Value)) {
            caption = BankElement.Value;
        }

        if (MyRoot == null) {
            MyRoot = new RootElement ("Root");
            BankElement = new EntryElement("Enter", caption, "");
            MyRoot.Add (new Section("") {BankElement});
            rootelement = new RootElement("Root");
            rootelement.Add (new Section("") {MyRoot});

            Root  = rootelement;
        }


        MyRoot.Caption = caption;
    }

So now I only create a root element and add it to the controller once, then I just update it. Seems good so far!