Why does MonoTouch crash on this simple code?

I totally do NOT understand why MonoTouch is crashing on this code. Let me outline 开发者_运维问答my steps to get to this point:

1. New Project
2. New > iPhone View with Controller (called TestScreen)
3. In Main.cs, added: window.AddSubview(new TestScreen().View);
4. Added button to the TestScreen UIView, and added an outlet called testButton on 'File's Owner' (which should be my TestScreen controller), and linked up the button to that outlet.

Then, I added the following code:

public override void ViewDidLoad ()
        {
            base.ViewDidLoad ();

            this.testButton.SetTitle("Test", UIControlState.Normal); // this works!
            this.testButton.TouchUpInside += HandleTestButtonhandleTouchUpInside;
        }

        int i = 0;
        void HandleTestButtonhandleTouchUpInside (object sender, EventArgs e)
        {
            i += 1;
            this.testButton.SetTitle("Testing " + i.ToString(), UIControlState.Normal); // this crashes!
        }

Question is: Why does the this.testButton.SetTitle() method cause a crash?

Here's the application output at time of crash:

Stacktrace:

at (wrapper managed-to-native) MonoTouch.UIKit.UIApplication.UIApplicationMain (int,string[],intptr,intptr) at MonoTouch.UIKit.UIApplication.Main (string[],string,string) [0x00038] in /Users/plasma/Source/iphone/monotouch/UIKit/UIApplication.cs:26

at MonoTouch.UIKit.UIApplication.Main (string[]) [0x00000] in /Users/plasma/Source/iphone/monotouch/UIKit/UIApplication.cs:31 at MyApp.Application.Main (string[]) [0x00000] in /Users/username/Projects/MyApp/MyApp/Main.cs:14 at (wrapper runtime-invoke) .runtime_invoke_void_object (object,intptr,intptr,intptr)

Native stacktrace:

0   MyApp               0x000d1965 mono_handle_native_sigsegv + 343
1   MyApp               0x0000ffb4 mono_sigsegv_signal_handler + 322
2   libSystem.B.dylib                   0x9476845b _sigtramp + 43
3   ???                                 0xffffffff 0x0 + 4294967295
4   UIKit                               0x01c981b5 -[UIControl sendAction:to:forEvent:] + 67
5   UIKit                               0x01c9a647 -[UIControl(Internal) _sendActionsForEvents:withEvent:] + 527
6   UIKit                               0x01c991f4 -[UIControl touchesEnded:withEvent:] + 458
7   UIKit                               0x01c2e0d1 -[UIWindow _sendTouchesForEvent:] + 567
8   UIKit                               0x01c0f37a -[UIApplication sendEvent:] + 447
9   UIKit                               0x01c14732 _UIApplicationHandleEvent + 7576
10  GraphicsServices                    0x03edfa36 PurpleEventCallback + 1550
11  CoreFoundation                      0x00e21064 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__ + 52
12  CoreFoundation                      0x00d816f7 __CFRunLoopDoSource1 + 215
13  CoreFoundation                      0x00d7e983 __CFRunLoopRun + 979
14  CoreFoundation                      0x00d7e240 CFRunLoopRunSpecific + 208
15  CoreFoundation                      0x00d7e161 CFRunLoopRunInMode + 97
16  GraphicsServices                    0x03ede268 GSEventRunModal + 217
17  GraphicsServices                    0x03ede32d GSEventRun + 115
18  UIKit                               0x01c1842e UIApplicationMain + 1160
19  ???                                 0x09a7714b 0x0 + 161968459
20  ???                                 0x09a76f1c 0x0 + 161967900
21  ???                                 0x09a7651c 0x0 + 161965340
22  ???                                 0x09a76374 0x0 + 161964916
23  ???                                 0x09a764c6 0x0 + 161965254
24  MyApp               0x0000fd6f mono_jit_runtime_invoke + 1332
25  MyApp               0x001ee239 mono_runtime_invoke + 137
26  MyApp               0x001f0920 mono_runtime_exec_main + 669
27  MyApp               0x001efd0a mono_runtime_run_main + 843
28  MyApp               0x000a3c62 mono_jit_exec + 200
29  MyApp               0x002a25eb main + 3838
30  MyApp               0x000030c9 _start + 208
31  MyApp               0x00002ff8 start + 40

Debug info from gdb:

/tmp/mono-gdb-commands.DGcT7h:1: Error in sourced command file: unable to debug self

================================================================= 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.

---

The error turned out to be in my Step 3.

My new TestScreen() was going out of scope and getting disposed for some reason. Assigned it to a variable and then passed in variable to AddSubview() and that fixed it.

---

this.InvokeOnMainThread(delegate{
              i += 1;
                          ((UIButton) sender).SetTitle("Testing " + i.ToString(), UIControlState.Normal);

            });

---

Try to cast the sender to a UIButton. I hope that helps.

void HandleTestButtonhandleTouchUpInside (object sender, EventArgs e)
    {
        i += 1;
        ((UIButton) sender).SetTitle("Testing " + i.ToString(), UIControlState.Normal);
    }