Xamarin.Mac事件和线程似乎无法一起使用

Question

I'm having loads of difficulty with Xamarin.Mac at the moment.

Basically all I want to do is to POST and Grab some JSON to/from a webserver, and then call an event which bubbles up through delegates to (what will eventually be a) UI change event (at the moment it's just a debug.writeline call).

I have two PCL libraries, one is the 'core' and the other is a library the core references, and a WPF app and a OS X app - the WPF app works fine. I'm also using MVVMCross.

At first I tried using a preexisting class I had which used HTTPWebRequest, and it worked fine, for the first button click (which fires this call). On the second button click the OS X app crashes. I tried switching to HttpClient, no dice.

After reading this thread: https://bugzilla.xamarin.com/show_bug.cgi?id=19906 I tried, using interfaces switching out the HTTP class for the OS X app to the native implementation of NSUrlConnection.

Now this is the situaiton I find myself: If I have a NSUrlConnection ASync request with an anonymous function passed to it it works forever click after click If I take that out and call a FireEvent() function which just raises events (which may raise more events down the chain) then it works forever, click after click If I have both the ASync with anonymous function and the FireEvent, on the third click of the button that fires this off, it throws a native crash.

If I take out all the async stuff, it will work fine, but that's not ideal.

Any ideas? I'm at my wits end.

        public void ExecuteAsync() 
    {
        var request = new NSUrlRequest(new NSUrl("http://www.example.com"));

        asyncRunning = true;

        NSUrlConnection.SendAsynchronousRequest(request, NSOperationQueue.MainQueue, delegate(NSUrlResponse response, NSData data, NSError error) {

        var responseStr = data.ToString();

            var blah = new HttpResponseCompleteEventArgs("{TESTJSONThatIveRemovedForReadability}");

            System.Diagnostics.Debug.WriteLine("Inside IsMainThread: " + NSThread.IsMain.ToString());

            response.InvokeOnMainThread ( delegate {
                FireEvent(blah);
                System.Diagnostics.Debug.WriteLine("Inside IsMainThread invoke: " + NSThread.IsMain.ToString());
            });



        });

        System.Diagnostics.Debug.WriteLine("Outside IsMainThread: " + NSThread.IsMain.ToString());


    }

And the crash

Stacktrace:

  at <unknown> <0xffffffff>
  at (wrapper managed-to-native) MonoMac.AppKit.NSApplication.NSApplicationMain (int,string[]) <IL 0x0009d, 0xffffffff>
  at MonoMac.AppKit.NSApplication.Main (string[]) [0x00041] in /Users/builder/data/lanes/xamcore-lion-1.8-branch/b8b75fd4/source/xamcore/src/AppKit/NSApplication.cs:105
  at FreshlySqueezed.Mac.MainClass.Main (string[]) [0x00007] in /Users/blah/Development/Development/FreshlySqueezed/FreshlySqueezed.Mac/Main.cs:14
  at (wrapper runtime-invoke) <Module>.runtime_invoke_void_object (object,intptr,intptr,intptr) <IL 0x00050, 0xffffffff>

Native stacktrace:


Debug info from gdb:

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quProcess 2693 stopped
* thread #1: tid = 0x1e2f7, 0x91b0cfed libsystem_kernel.dylib`__wait4 + 5, queue = 'com.apple.main-thread', stop reason = signal SIGSTOP
  thread #2: tid = 0x1e30e, 0x91b0d992 libsystem_kernel.dylib`kevent64 + 10, queue = 'com.apple.libdispatch-manager'
  thread #3: tid = 0x1e30f, 0x91b0d046 libsystem_kernel.dylib`__workq_kernreturn + 10
  thread #4: tid = 0x1e310, 0x91b0d046 libsystem_kernel.dylib`__workq_kernreturn + 10
  thread #5: tid = 0x1e311, 0x91b0d046 libsystem_kernel.dylib`__workq_kernreturn + 10
  thread #6: tid = 0x1e314, 0x91b07fb6 libsystem_kernel.dylib`semaphore_wait_trap + 10
  thread #7: tid = 0x1e315, 0x91b0ca26 libsystem_kernel.dylib`__recvfrom + 10
  thread #8: tid = 0x1e32c, 0x91b07f7a libsystem_kernel.dylib`mach_msg_trap + 10
  thread #9: tid = 0x1e32e, 0x91b0d046 libsystem_kernel.dylib`__workq_kernreturn + 10
(lldb) quit

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qu* thread #1: tid = 0x1e2f7, 0x91b0cfed libsystem_kernel.dylib`__wait4 + 5, queue = 'com.apple.main-thread', stop reason = signal SIGSTOP
  * frame #0: 0x91b0cfed libsystem_kernel.dylib`__wait4 + 5
    frame #1: 0x92c46ec5 libsystem_c.dylib`waitpid$UNIX2003 + 48
    frame #2: 0x018ab5f9 libmono-2.0.dylib`mono_handle_native_sigsegv(signal=11, ctx=0x006e1fe0) + 489 at mini-exceptions.c:2305
    frame #3: 0x018fd9d5 libmono-2.0.dylib`mono_arch_handle_altstack_exception(sigctx=0x006e1fe0, fault_addr=0x608962a8, stack_ovf=0) + 149 at exceptions-x86.c:1170
    frame #4: 0x01804351 libmono-2.0.dylib`mono_sigsegv_signal_handler(_dummy=11, info=0x006e1fa0, context=0x006e1fe0) + 369 at mini.c:6842
    frame #5: 0x94fd1deb libsystem_platform.dylib`_sigtramp + 43

  thread #2: tid = 0x1e30e, 0x91b0d992 libsystem_kernel.dylib`kevent64 + 10, queue = 'com.apple.libdispatch-manager'
    frame #0: 0x91b0d992 libsystem_kernel.dylib`kevent64 + 10
    frame #1: 0x9a415899 libdispatch.dylib`_dispatch_mgr_invoke + 238
    frame #2: 0x9a415532 libdispatch.dylib`_dispatch_mgr_thread + 52

  thread #3: tid = 0x1e30f, 0x91b0d046 libsystem_kernel.dylib`__workq_kernreturn + 10
    frame #0: 0x91b0d046 libsystem_kernel.dylib`__workq_kernreturn + 10
    frame #1: 0x91cfbdcf libsystem_pthread.dylib`_pthread_wqthread + 372

  thread #4: tid = 0x1e310, 0x91b0d046 libsystem_kernel.dylib`__workq_kernreturn + 10
    frame #0: 0x91b0d046 libsystem_kernel.dylib`__workq_kernreturn + 10
    frame #1: 0x91cfbdcf libsystem_pthread.dylib`_pthread_wqthread + 372

  thread #5: tid = 0x1e311, 0x91b0d046 libsystem_kernel.dylib`__workq_kernreturn + 10
    frame #0: 0x91b0d046 libsystem_kernel.dylib`__workq_kernreturn + 10
    frame #1: 0x91cfbdcf libsystem_pthread.dylib`_pthread_wqthread + 372

  thread #6: tid = 0x1e314, 0x91b07fb6 libsystem_kernel.dylib`semaphore_wait_trap + 10
    frame #0: 0x91b07fb6 libsystem_kernel.dylib`semaphore_wait_trap + 10
    frame #1: 0x01a1891e libmono-2.0.dylib`mono_sem_wait(sem=<unavailable>, alertable=<unavailable>, sem=0x01b1ac20, alertable=1) + 30 at mono-semaphore.c:121
    frame #2: 0x019c43ea libmono-2.0.dylib`finalizer_thread(unused=0x00000000) + 74 at gc.c:1073
    frame #3: 0x0199c654 libmono-2.0.dylib`start_wrapper [inlined] start_wrapper_internal(data=0x00193bd0) + 442 at threads.c:647
    frame #4: 0x0199c49a libmono-2.0.dylib`start_wrapper(data=0x00193bd0) + 26 at threads.c:692
    frame #5: 0x01a1de1d libmono-2.0.dylib`inner_start_thread(arg=0xbffff2c0) + 253 at mono-threads-posix.c:94
    frame #6: 0x01a3eebd libmono-2.0.dylib`GC_start_routine(arg=0x006e2f60) + 93 at pthread_support.c:1502
    frame #7: 0x91cfa5fb libsystem_pthread.dylib`_pthread_body + 144
    frame #8: 0x91cfa485 libsystem_pthread.dylib`_pthread_start + 130

  thread #7: tid = 0x1e315, 0x91b0ca26 libsystem_kernel.dylib`__recvfrom + 10
    frame #0: 0x91b0ca26 libsystem_kernel.dylib`__recvfrom + 10
    frame #1: 0x92c470c9 libsystem_c.dylib`recv$UNIX2003 + 54
    frame #2: 0x018d58c0 libmono-2.0.dylib`socket_transport_recv(buf=0xb039aeed, len=11) + 160 at debugger-agent.c:1085
    frame #3: 0x018d1c8d libmono-2.0.dylib`debugger_thread(arg=0x00000000) + 21485 at debugger-agent.c:1475
    frame #4: 0x01a1de1d libmono-2.0.dylib`inner_start_thread(arg=0xbffff2c0) + 253 at mono-threads-posix.c:94
    frame #5: 0x01a3eebd libmono-2.0.dylib`GC_start_routine(arg=0x006e2f60) + 93 at pthread_support.c:1502
    frame #6: 0x91cfa5fb libsystem_pthread.dylib`_pthread_body + 144
    frame #7: 0x91cfa485 libsystem_pthread.dylib`_pthread_start + 130

  thread #8: tid = 0x1e32c, 0x91b07f7a libsystem_kernel.dylib`mach_msg_trap + 10
    frame #0: 0x91b07f7a libsystem_kernel.dylib`mach_msg_trap + 10
    frame #1: 0x91b0716c libsystem_kernel.dylib`mach_msg + 68
    frame #2: 0x9bea0bf9 CoreFoundation`__CFRunLoopServiceMachPort + 169
    frame #3: 0x9bea01d1 CoreFoundation`__CFRunLoopRun + 1393
    frame #4: 0x9be9f9ea CoreFoundation`CFRunLoopRunSpecific + 394
    frame #5: 0x9be9f84b CoreFoundation`CFRunLoopRunInMode + 123
    frame #6: 0x90de9b88 AppKit`_NSEventThread + 283
    frame #7: 0x91cfa5fb libsystem_pthread.dylib`_pthread_body + 144
    frame #8: 0x91cfa485 libsystem_pthread.dylib`_pthread_start + 130

  thread #9: tid = 0x1e32e, 0x91b0d046 libsystem_kernel.dylib`__workq_kernreturn + 10
    frame #0: 0x91b0d046 libsystem_kernel.dylib`__workq_kernreturn + 10
    frame #1: 0x91cfbdcf libsystem_pthread.dylib`_pthread_wqthread + 372
Executing commands in '/tmp/mono-gdb-commands.J45EfM'.
(lldb)  process attach --pid 2693
Process 2693 stopped

Anyone got any idea? I'm stumped.

Thanks

Answer 1

So after hours of head scratching I seem to have stumbled across the solution.

From early on I had suspicions that this had something to do with Garbage Collection, or an Event losing it's binding as the app crashes before any of my code is hit after hitting the button the third time, and from reading other stackoverflow issues with Xamarin/OS X my symptoms sounded similar to a over-eager Garbage Collector.

So at first I thought I'd raise an event every time I get the response from the webserver and the event has bubbled up to the ViewModel, so essentially bubbling it up to the ViewController in the OSX project. The intention was to try and use this event to rebind the button binding after every click - however I tried running it with an empty Event Handler, and I discovered it fixed the initial issue!

So despite the fact the ViewController already has a reference to the ViewModel through this.ViewModel, it seems OS X/XamMac was being over-eager and disposing of the ViewModel, therefore bringing the App to it's knees. Interestingly, I only found adding and handling an event to fix this issue, trying to assign this.ViewModel to another class variable had no effect.

So here's the code that fixed it:

In the viewmodel I added this:

public event System.EventHandler GoCommandFinished;

public void CallbackFromWebRequest(SqueezeBase value){
                System.Diagnostics.Debug.WriteLine("hmmm");
            Hello = "whey?" + x;
            if (GoCommandFinished != null) {
                GoCommandFinished (null, null);
            }
        }

and in the view controller on the OS X side I added this to the ViewDidLoad:

((FirstViewModel)this.ViewModel).GoCommandFinished += GoCommandFinished;

and then created an empty function to handle this:

private void GoCommandFinished(object sender, EventArgs e){

        }

And volia! I'd be interested to know a cleaner way of sorting this out, as this feels very hacky to me... but for now, I don't care, it works! But if anyone from Xamarin could explain how to calm down the Garbage Collection a bit more elegantly, that would be great!

Thanks