Feature Allow Default value on OAPH generator (#28)

Author: ChrisPulman

README.md

@@ -81,7 +81,15 @@ using ReactiveUI.SourceGenerators;
 public partial class MyReactiveClass : ReactiveObject
 {
     [ObservableAsProperty]
-    private string _myProperty;
+    private string _myProperty = "Default Value";
+
+    public MyReactiveClass()
+    {
+        _myPrpertyHelper = MyPropertyObservable()
+            .ToProperty(this, x => x.MyProperty);
+    }
+
+    IObservable<string> MyPropertyObservable() => Observable.Return("Test Value");
 }
 ```
 

src/ReactiveUI.SourceGenerators.Execute/TestViewModel.cs

@@ -21,7 +21,7 @@ public partial class TestViewModel : ReactiveObject
     [JsonInclude]
     [DataMember]
     [ObservableAsProperty]
-    private double _test2Property;
+    private double _test2Property = 1.1d;
 
     [JsonInclude]
     [Reactive]
@@ -53,10 +53,11 @@ public TestViewModel()
         Test6ArgOnlyCommand?.Execute("Hello World").Subscribe();
         Test7ObservableCommand?.Execute().Subscribe();
 
+        Console.Out.WriteLine($"Test2Property default Value: {Test2Property}");
         _test2PropertyHelper = Test8ObservableCommand!.ToProperty(this, x => x.Test2Property);
 
         Test8ObservableCommand?.Execute(100).Subscribe(Console.Out.WriteLine);
-        Console.Out.WriteLine($"Test2Property Value: {Test2}");
+        Console.Out.WriteLine($"Test2Property Value: {Test2Property}");
         Console.Out.WriteLine($"Test2Property underlying Value: {_test2Property}");
 
         Test9AsyncCommand?.ThrownExceptions.Subscribe(Console.Out.WriteLine);

src/ReactiveUI.SourceGenerators/CodeAnalyzers/PropertyToReactiveFieldAnalyzer.cs

@@ -34,9 +34,9 @@ public class PropertyToReactiveFieldAnalyzer : DiagnosticAnalyzer
         /// <param name="context">The context.</param>
         public override void Initialize(AnalysisContext context)
         {
-            if (context == null)
+            if (context is null)
             {
-                return;
+                throw new System.ArgumentNullException(nameof(context));
             }
 
             context.ConfigureGeneratedCodeAnalysis(GeneratedCodeAnalysisFlags.None);

src/ReactiveUI.SourceGenerators/Diagnostics/DiagnosticDescriptors.cs

@@ -137,7 +137,7 @@ internal static class DiagnosticDescriptors
         category: typeof(ReactiveCommandGenerator).FullName,
         defaultSeverity: DiagnosticSeverity.Error,
         isEnabledByDefault: true,
-        description: "All asynchronous methods annotated with [ReactiveCommand] should return a Task type, to benefit from the additional support provided by ReactiveCommand and ReactiveCommand<T>.",
+        description: "All asynchronous methods annotated with [ReactiveCommand] should return a Task type, to benefit from the additional support provided by ReactiveCommand.FromTask.",
         helpLinkUri: "https://www.reactiveui.net/errors/RXUISG0008");
 
     /// <summary>

src/ReactiveUI.SourceGenerators/ObservableAsProperty/ObservableAsPropertyGenerator.Execute.cs

@@ -64,7 +64,7 @@ internal static ImmutableArray<MemberDeclarationSyntax> GetPropertySyntax(Proper
                 getterFieldIdentifierName = propertyInfo.FieldName;
             }
 
-            var getterArrowExpression = ArrowExpressionClause(ParseExpression($"{getterFieldIdentifierName} = {getterFieldIdentifierName + "Helper"}.Value"));
+            var getterArrowExpression = ArrowExpressionClause(ParseExpression($"{getterFieldIdentifierName} = ({getterFieldIdentifierName}Helper?.Value ?? {getterFieldIdentifierName})"));
 
             // Prepare the forwarded attributes, if any
             var forwardedAttributes =