[UIKit] Wrap the call to UIApplicationMain in a @try/@catch handler. (#15746)

Wrap the call to UIApplicationMain in a @try/@catch handler, and convert any
Objective-C exceptions to a managed exception.

This way the managed Main method (which calls UIApplication.Main) will be able
to catch any Objective-C exceptions.

runtime/bindings.h

@@ -13,6 +13,11 @@
 extern "C" {
 #endif
 
+// The inclusion of "_Nullable" in the signature for xamarin_UIApplicationMain makes clang complain about missing nullability info for other methods in this file.
+// We don't want to fix that right now (but feel free to do so if you're reading this), so ignore the missing nullability info warning for the methods in question.
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wnullability-completeness"
+
 void * xamarin_IntPtr_objc_msgSend_IntPtr (id self, SEL sel, void *a);
 void * xamarin_IntPtr_objc_msgSendSuper_IntPtr (struct objc_super *super, SEL sel, void *a);
 
@@ -56,6 +61,10 @@ void * xamarin_IntPtr_objc_msgSendSuper_IntPtr_UInt32_nint_UInt32 (struct objc_s
 void * xamarin_IntPtr_objc_msgSend_IntPtr_UInt64_nint_UInt64 (id self, SEL sel, void *a, uint64_t b, xm_nint_t c, uint64_t d);
 void * xamarin_IntPtr_objc_msgSendSuper_IntPtr_UInt64_nint_UInt64 (struct objc_super *super, SEL sel, void *a, uint64_t b, xm_nint_t c, uint64_t d);
 
+#pragma clang diagnostic push
+
+int xamarin_UIApplicationMain (int argc, char * _Nullable argv[_Nonnull], NSString * _Nullable principalClassName, NSString * _Nullable delegateClassName, GCHandle *exception_gchandle);
+
 /* Types copied from headers */
 /* We need to do this for now, since we must be able to build XM on older OSXs */
 

runtime/bindings.m

@@ -13,6 +13,10 @@
 #include "bindings.h"
 #include <dlfcn.h>
 
+#if !TARGET_OS_OSX && !TARGET_OS_WATCH
+#include <UIKit/UIKit.h>
+#endif
+
 /*
  * Hand-written bindings to support ObjectiveC exceptions.
  * Reference:
@@ -112,3 +116,17 @@ xamarin_CGPoint__VNImagePointForFaceLandmarkPoint_Vector2_CGRect_nuint_nuint_str
 
 	return func (flp, faceBoundingBox, imageWidth, imageHeight);
 }
+
+/* UIKit bindings */
+#if !TARGET_OS_OSX && !TARGET_OS_WATCH
+int xamarin_UIApplicationMain (int argc, char * _Nullable argv[_Nonnull], NSString * _Nullable principalClassName, NSString * _Nullable delegateClassName, GCHandle *exception_gchandle)
+{
+	@try {
+		*exception_gchandle = INVALID_GCHANDLE;
+		return UIApplicationMain (argc, argv, principalClassName, delegateClassName);
+	} @catch (NSException *e) {
+		xamarin_process_nsexception_using_mode (e, false, exception_gchandle);
+		return 1;
+	}
+}
+#endif // !TARGET_OS_OSX

src/UIKit/UIApplication.cs

@@ -49,8 +49,15 @@ namespace UIKit {
 #if !WATCH
 		// We link with __Internal here so that this function is interposable from third-party native libraries.
 		// See: https://github.com/xamarin/MicrosoftInTune/issues/3 for an example.
-		[DllImport (/*Constants.UIKitLibrary*/ "__Internal")]
-		extern static int UIApplicationMain (int argc, /* char[]* */ string []? argv, /* NSString* */ IntPtr principalClassName, /* NSString* */ IntPtr delegateClassName);
+		[DllImport ("__Internal")]
+		extern static int xamarin_UIApplicationMain (int argc, /* char[]* */ string []? argv, /* NSString* */ IntPtr principalClassName, /* NSString* */ IntPtr delegateClassName, out IntPtr gchandle);
+
+		static int UIApplicationMain (int argc, /* char[]* */ string []? argv, /* NSString* */ IntPtr principalClassName, /* NSString* */ IntPtr delegateClassName)
+		{
+			var rv = xamarin_UIApplicationMain (argc, argv, principalClassName, delegateClassName, out var gchandle);
+			Runtime.ThrowException (gchandle);
+			return rv;
+		}
 #endif
 
 		// called from NSExtension.Initialize (so other, future stuff, can be added if needed)