Fix For CanExecute with IObservable<bool> Fields (#48)

Fix For #44

Author: ChrisPulman

src/ReactiveUI.SourceGenerators.Execute/TestViewModel.cs

@@ -18,6 +18,8 @@ namespace SGReactiveUI.SourceGenerators.Test;
 [DataContract]
 public partial class TestViewModel : ReactiveObject
 {
+    private readonly IObservable<bool> _observable = Observable.Return(true);
+
     [JsonInclude]
     [DataMember]
     [ObservableAsProperty]
@@ -67,6 +69,7 @@ public TestViewModel()
         cancel?.Dispose();
 
         Test10AsyncCommand?.Execute(200).Subscribe(r => Console.Out.WriteLine(r));
+        TestPrivateCanExecuteCommand?.Execute().Subscribe();
 
         Console.ReadLine();
     }
@@ -171,4 +174,7 @@ public TestViewModel()
 
     [ReactiveCommand]
     private async Task<Point> Test10Async(int size, CancellationToken ct) => await Task.FromResult(new Point(size, size));
+
+    [ReactiveCommand(CanExecute = nameof(_observable))]
+    private void TestPrivateCanExecute() => Console.Out.WriteLine("TestPrivateCanExecute");
 }

src/ReactiveUI.SourceGenerators/ReactiveCommand/Models/CanExecuteTypeInfo.cs

@@ -8,5 +8,6 @@ namespace ReactiveUI.SourceGenerators.Input.Models;
 internal enum CanExecuteTypeInfo
 {
     PropertyObservable,
-    MethodObservable
+    MethodObservable,
+    FieldObservable,
 }

src/ReactiveUI.SourceGenerators/ReactiveCommand/ReactiveCommandGenerator.Execute.cs

@@ -316,6 +316,18 @@ internal static bool TryGetCanExecuteExpressionFromSymbol(
 
                 return true;
             }
+            else if (canExecuteSymbol is IFieldSymbol canExecuteFieldSymbol)
+            {
+                // The property type must always be a bool
+                if (!IsObservableBoolType(canExecuteFieldSymbol.Type))
+                {
+                    goto Failure;
+                }
+
+                canExecuteTypeInfo = CanExecuteTypeInfo.FieldObservable;
+
+                return true;
+            }
 
         Failure:
             canExecuteTypeInfo = null;