Undo the IsFixed change

Author: javiercn

src/Components/Components/src/Binding/CascadingModelBinder.cs

@@ -23,6 +23,14 @@ public sealed class CascadingModelBinder : IComponent, ICascadingValueComponent,
     /// </summary>
     [Parameter] public string Name { get; set; } = "";
 
+    /// <summary>
+    /// If true, indicates that <see cref="ModelBindingContext.BindingContextId"/> will not change.
+    /// This is a performance optimization that allows the framework to skip setting up
+    /// change notifications. Set this flag only if you will not change
+    /// <see cref="Name"/> of this context or its parents' context during the component's lifetime.
+    /// </summary>
+    [Parameter] public bool IsFixed { get; set; }
+
     /// <summary>
     /// Specifies the content to be rendered inside this <see cref="CascadingModelBinder"/>.
     /// </summary>
@@ -70,7 +78,7 @@ private void Render()
         {
             _hasPendingQueuedRender = false;
             builder.OpenComponent<CascadingValue<ModelBindingContext>>(0);
-            builder.AddComponentParameter(1, nameof(CascadingValue<ModelBindingContext>.IsFixed), true);
+            builder.AddComponentParameter(1, nameof(CascadingValue<ModelBindingContext>.IsFixed), IsFixed);
             builder.AddComponentParameter(2, nameof(CascadingValue<ModelBindingContext>.Value), _bindingContext);
             builder.AddComponentParameter(3, nameof(CascadingValue<ModelBindingContext>.ChildContent), ChildContent?.Invoke(_bindingContext!));
             builder.CloseComponent();
@@ -109,7 +117,7 @@ internal void UpdateBindingInformation(string url)
             _bindingContext : new ModelBindingContext(name, bindingId);
 
         // It doesn't matter that we don't check IsFixed, since the CascadingValue we are setting up will throw if the app changes.
-        if (_bindingContext != null && _bindingContext != bindingContext)
+        if (IsFixed && _bindingContext != null && _bindingContext != bindingContext)
         {
             // Throw an exception if either the Name or the BindingContextId changed. Once a CascadingModelBinder has been initialized
             // as fixed, it can't change it's name nor its BindingContextId. This can happen in several situations:

src/Components/Components/src/PublicAPI.Unshipped.txt

@@ -7,6 +7,8 @@ Microsoft.AspNetCore.Components.CascadingModelBinder
 Microsoft.AspNetCore.Components.CascadingModelBinder.CascadingModelBinder() -> void
 Microsoft.AspNetCore.Components.CascadingModelBinder.ChildContent.get -> Microsoft.AspNetCore.Components.RenderFragment<Microsoft.AspNetCore.Components.ModelBindingContext!>!
 Microsoft.AspNetCore.Components.CascadingModelBinder.ChildContent.set -> void
+Microsoft.AspNetCore.Components.CascadingModelBinder.IsFixed.get -> bool
+Microsoft.AspNetCore.Components.CascadingModelBinder.IsFixed.set -> void
 Microsoft.AspNetCore.Components.CascadingModelBinder.Name.get -> string!
 Microsoft.AspNetCore.Components.CascadingModelBinder.Name.set -> void
 Microsoft.AspNetCore.Components.ComponentBase.DispatchExceptionAsync(System.Exception! exception) -> System.Threading.Tasks.Task!

src/Components/Components/test/CascadingModelBinderTest.cs

@@ -200,6 +200,7 @@ public void Throws_WhenIsFixedAndNameChanges()
         {
             builder.OpenComponent<CascadingModelBinder>(0);
             builder.AddAttribute(1, nameof(CascadingModelBinder.Name), contextName);
+            builder.AddAttribute(2, nameof(CascadingModelBinder.IsFixed), true);
             builder.AddAttribute(3, nameof(CascadingModelBinder.ChildContent), contents);
             builder.CloseComponent();
         });
@@ -213,6 +214,85 @@ public void Throws_WhenIsFixedAndNameChanges()
         Assert.Equal("'CascadingModelBinder' 'Name' can't change after initialized.", exception.Message);
     }
 
+    [Theory]
+    [InlineData(true)]
+    [InlineData(false)]
+    public void Throws_WhenIsFixed_Changes(bool isFixed)
+    {
+        ModelBindingContext capturedContext = null;
+        RenderFragment<ModelBindingContext> contents = (ctx) => b => { capturedContext = ctx; };
+        var testComponent = new TestComponent(builder =>
+        {
+            builder.OpenComponent<CascadingModelBinder>(0);
+            builder.AddAttribute(1, nameof(CascadingModelBinder.IsFixed), isFixed);
+            builder.AddAttribute(2, nameof(CascadingModelBinder.ChildContent), contents);
+            builder.CloseComponent();
+        });
+        var id = _renderer.AssignRootComponentId(testComponent);
+        _renderer.RenderRootComponent(id);
+
+        // Act
+        isFixed = !isFixed;
+        var exception = Assert.Throws<InvalidOperationException>(testComponent.TriggerRender);
+
+        Assert.Equal("The value of IsFixed cannot be changed dynamically.", exception.Message);
+    }
+
+    [Fact]
+    public void CanChange_Name_WhenNotFixed()
+    {
+        ModelBindingContext capturedContext = null;
+        ModelBindingContext originalContext = null;
+        RenderFragment<ModelBindingContext> contents = (ctx) => b => { capturedContext = ctx; };
+        var contextName = "parent-context";
+
+        var testComponent = new TestComponent(builder =>
+        {
+            builder.OpenComponent<CascadingModelBinder>(0);
+            builder.AddAttribute(1, nameof(CascadingModelBinder.Name), contextName);
+            builder.AddAttribute(3, nameof(CascadingModelBinder.ChildContent), contents);
+            builder.CloseComponent();
+        });
+        var id = _renderer.AssignRootComponentId(testComponent);
+        _renderer.RenderRootComponent(id);
+
+        originalContext = capturedContext;
+        contextName = "changed";
+
+        // Act
+        testComponent.TriggerRender();
+
+        Assert.NotSame(capturedContext, originalContext);
+        Assert.Equal("changed", capturedContext.Name);
+    }
+
+    [Fact]
+    public void CanChange_BindingContextId_WhenNotFixed()
+    {
+        ModelBindingContext capturedContext = null;
+        ModelBindingContext originalContext = null;
+        RenderFragment<ModelBindingContext> contents = (ctx) => b => { capturedContext = ctx; };
+
+        var testComponent = new TestComponent(builder =>
+        {
+            builder.OpenComponent<CascadingModelBinder>(0);
+            builder.AddComponentParameter(1, nameof(CascadingModelBinder.Name), "context-name");
+            builder.AddComponentParameter(2, nameof(CascadingModelBinder.ChildContent), contents);
+            builder.CloseComponent();
+        });
+        var id = _renderer.AssignRootComponentId(testComponent);
+        _renderer.RenderRootComponent(id);
+
+        originalContext = capturedContext;
+
+        // Act
+        _navigationManager.NavigateTo(_navigationManager.ToAbsoluteUri("fetch-data/6").ToString());
+        testComponent.TriggerRender();
+
+        Assert.NotSame(capturedContext, originalContext);
+        Assert.Equal("fetch-data/6?handler=context-name", capturedContext.BindingContextId);
+    }
+
     private class RouteViewTestNavigationManager : NavigationManager
     {
         public RouteViewTestNavigationManager() =>

src/Components/Components/test/RouteViewTest.cs

@@ -89,7 +89,7 @@ public void RendersPageInsideLayoutView()
         var cascadingModelBinderFrames = _renderer.GetCurrentRenderTreeFrames(cascadingModelBinderComponentId).AsEnumerable();
         Assert.Collection(cascadingModelBinderFrames,
             frame => AssertFrame.Component<CascadingValue<ModelBindingContext>>(frame, sequence: 0, subtreeLength: 4),
-            frame => AssertFrame.Attribute(frame, nameof(CascadingValue<ModelBindingContext>.IsFixed), true, sequence: 1),
+            frame => AssertFrame.Attribute(frame, nameof(CascadingValue<ModelBindingContext>.IsFixed), false, sequence: 1),
             frame => AssertFrame.Attribute(frame, nameof(CascadingValue<ModelBindingContext>.Value), typeof(ModelBindingContext), sequence: 2),
             frame => AssertFrame.Attribute(frame, nameof(CascadingValue<ModelBindingContext>.ChildContent), typeof(RenderFragment), sequence: 3));