Introduced MauiExceptions to setup unified UnhandledExceptionEventHandler (#17)

Credits Matt Johnson-Pint

Author: snakefoot

README.md

@@ -72,3 +72,39 @@ Example `NLog.config`-file:
 </rules>
 </nlog>
 ```
+
+### Logging Unhandled Exceptions
+
+The MAUI platform have failed to provide an unified way of tracking unhandled exceptions.
+
+The normal way would be to hook into `AppDomain.CurrentDomain.UnhandledException`, but extra logic needed for iOS and Android:
+
+Exceptions on iOS flows through AppDomain.CurrentDomain.UnhandledException, but one must set [UnwindNativeCode](https://github.com/xamarin/xamarin-macios/issues/15252):
+```csharp
+ObjCRuntime.Runtime.MarshalManagedException += (_, args) =>
+{
+    args.ExceptionMode = ObjCRuntime.MarshalManagedExceptionMode.UnwindNativeCode;
+};
+```
+
+Exceptions on Android only flows through `Android.Runtime.AndroidEnvironment.UnhandledExceptionRaiser`:
+```csharp
+Android.Runtime.AndroidEnvironment.UnhandledExceptionRaiser += (sender, args) =>
+{
+    // Log unhandled args.Exception
+};
+```
+
+NLog.Targets.MauiLog includes unified method to setup `UnhandledExceptionEventHandler`:
+```csharp
+NLog.LogManager.Setup().RegisterMauiLog((sender, ex) => {
+    NLog.LogManager.GetLogger("Application").Fatal(ex, "Unhandled Exception");
+});
+```
+
+One can also consider monitoring unobserved task exceptions:
+```csharp
+TaskScheduler.UnobservedTaskException += (sender, args) => {
+    NLog.LogManager.GetLogger("Application").Error(args.Exception, "Unobserved Task Exception");
+};
+```

appveyor.yml

@@ -1,5 +1,5 @@
 version: 5.0.0.{build}
-image: Visual Studio 2022
+image: Previous Visual Studio 2022
 configuration: Release
 platform: Any CPU
 build_script:

src/NLog.Targets.MauiLog/Config/SetupBuilderExtensions.cs

@@ -1,4 +1,5 @@
-using NLog.Config;
+using System;
+using NLog.Config;
 
 namespace NLog
 {
@@ -15,5 +16,19 @@ public static ISetupBuilder RegisterMauiLog(this ISetupBuilder setupBuilder)
             setupBuilder.SetupExtensions(e => e.RegisterMauiLog());
             return setupBuilder;
         }
+
+        /// <summary>
+        /// Register the NLog.Web LayoutRenderers before loading NLog config
+        /// </summary>
+        public static ISetupBuilder RegisterMauiLog(this ISetupBuilder setupBuilder, UnhandledExceptionEventHandler unhandledException)
+        {
+            if (unhandledException is null)
+                throw new ArgumentNullException(nameof(unhandledException));
+
+            setupBuilder.SetupExtensions(e => e.RegisterMauiLog());
+            NLog.Targets.MauiLog.MauiExceptions.UnhandledException -= unhandledException;   // Avoid double registration
+            NLog.Targets.MauiLog.MauiExceptions.UnhandledException += unhandledException;
+            return setupBuilder;
+        }
     }
 }

src/NLog.Targets.MauiLog/Internal/MauiExceptions.cs

@@ -0,0 +1,69 @@
+// MIT License
+// 
+// Copyright (c) 2022 Matt Johnson-Pint
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+namespace NLog.Targets.MauiLog
+{
+    using System;
+
+    internal static class MauiExceptions
+    {
+        // We'll route all unhandled exceptions through this one event.
+        public static event UnhandledExceptionEventHandler UnhandledException;
+
+        static MauiExceptions()
+        {
+            // This is the normal event expected, and should still be used.
+            // It will fire for exceptions from iOS and Mac Catalyst,
+            // and for exceptions on background threads from WinUI 3.
+
+            AppDomain.CurrentDomain.UnhandledException += (sender, args) =>
+            {
+                UnhandledException?.Invoke(sender, args);
+            };
+
+#if __APPLE__
+
+            // For iOS and Mac Catalyst
+            // Exceptions will flow through AppDomain.CurrentDomain.UnhandledException,
+            // but we need to set UnwindNativeCode to get it to work correctly. 
+            // 
+            // See: https://github.com/xamarin/xamarin-macios/issues/15252
+        
+            ObjCRuntime.Runtime.MarshalManagedException += (_, args) =>
+            {
+                args.ExceptionMode = ObjCRuntime.MarshalManagedExceptionMode.UnwindNativeCode;
+            };
+
+#elif __ANDROID__
+
+            // For Android:
+            // All exceptions will flow through Android.Runtime.AndroidEnvironment.UnhandledExceptionRaiser,
+            // and NOT through AppDomain.CurrentDomain.UnhandledException
+
+            Android.Runtime.AndroidEnvironment.UnhandledExceptionRaiser += (sender, args) =>
+            {
+                UnhandledException?.Invoke(sender, new UnhandledExceptionEventArgs(args.Exception, true));
+            };
+#endif
+        }
+    }
+}