Add default JsRuntime

src/Assembly.cs

@@ -1 +1 @@
-[assembly: System.Runtime.CompilerServices.InternalsVisibleTo("Egil.RazorComponents.Testing.Library.Tests")]
+[assembly: System.Runtime.CompilerServices.InternalsVisibleTo("Egil.RazorComponents.Testing.Library.Tests")]

src/Mocking/JSInterop/MissingMockJsRuntimeException.cs

@@ -0,0 +1,41 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using System.Diagnostics.CodeAnalysis;
+
+namespace Egil.RazorComponents.Testing
+{
+    /// <summary>
+    /// Exception use to indicate that a MockJsRuntime is required by a test
+    /// but was not provided.
+    /// </summary>
+    [SuppressMessage("Design", "CA1032:Implement standard exception constructors", Justification = "<Pending>")]
+    public class MissingMockJsRuntimeException : Exception
+    {
+        /// <summary>
+        /// Identifer string used in the JSInvoke method.
+        /// </summary>
+        public string Identifier { get; }
+
+        /// <summary>
+        /// Arguments passed to the JSInvoke method.
+        /// </summary>
+        public IReadOnlyList<object> Arguments { get; }
+
+        /// <summary>
+        /// Creates a new instance of the <see cref="MissingMockJsRuntimeException"/>
+        /// with the arguments used in the invocation.
+        /// </summary>
+        /// <param name="identifier">The identifer used in the invocation.</param>
+        /// <param name="arguments">The args used in the invocation, if any</param>
+        public MissingMockJsRuntimeException(string identifier, object[] arguments)
+            : base($"This test requires a IJsRuntime to be supplied, because the component under test invokes the IJsRuntime during the test. The invoked method is '{identifier}' and the invocation arguments are stored in the {nameof(Arguments)} property of this exception. Guidance on mocking the IJsRuntime is available in the testing library's Wiki.") 
+        {
+            Identifier = identifier;
+            Arguments = arguments;
+            HelpLink = "https://github.com/egil/razor-components-testing-library/wiki/Mocking-JsRuntime";
+        }
+    }
+}

src/Mocking/JSInterop/PlaceholderJsRuntime.cs

@@ -0,0 +1,26 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.JSInterop;
+
+namespace Egil.RazorComponents.Testing.Mocking.JSInterop
+{
+    /// <summary>
+    /// This JsRuntime is used to provide users with helpful exceptions if they fail to provide a mock when required. 
+    /// </summary>
+    internal class PlaceholderJsRuntime : IJSRuntime
+    {
+        public ValueTask<TValue> InvokeAsync<TValue>(string identifier, object[] args)
+        {
+            throw new MissingMockJsRuntimeException(identifier, args);
+        }
+
+        public ValueTask<TValue> InvokeAsync<TValue>(string identifier, CancellationToken cancellationToken, object[] args)
+        {
+            throw new MissingMockJsRuntimeException(identifier, args);
+        }
+    }
+}

src/TestServiceProvider.cs

@@ -1,4 +1,6 @@
-using Microsoft.Extensions.DependencyInjection;
+using Egil.RazorComponents.Testing.Mocking.JSInterop;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.JSInterop;
 using System;
 using System.Diagnostics.CodeAnalysis;
 
@@ -13,6 +15,11 @@ public sealed class TestServiceProvider : IServiceProvider, IDisposable
         private readonly ServiceCollection _serviceCollection = new ServiceCollection();
         private ServiceProvider? _serviceProvider;
 
+        public TestServiceProvider()
+        {
+            _serviceCollection.AddSingleton<IJSRuntime, PlaceholderJsRuntime>();
+        }
+
         /// <summary>
         /// Gets whether this <see cref="TestServiceProvider"/> has been initialized, and 
         /// no longer will accept calls to the <c>AddService</c>'s methods.

tests/TestServiceProviderTest.cs

@@ -0,0 +1,33 @@
+using Egil.RazorComponents.Testing.EventDispatchExtensions;
+using Egil.RazorComponents.Testing.Extensions;
+using Egil.RazorComponents.Testing.Mocking.JSInterop;
+using Egil.RazorComponents.Testing.SampleComponents;
+using Egil.RazorComponents.Testing.SampleComponents.Data;
+using Shouldly;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using Xunit;
+
+namespace Egil.RazorComponents.Testing
+{
+    public class TestServiceProviderTest : ComponentTestFixture
+    {
+        [Fact(DisplayName = "The test service provider should register a placeholder IJSRuntime " +
+             "which throws exceptions")]
+        public void Test001()
+        {
+            var ex = Assert.Throws<AggregateException>(() => RenderComponent<SimpleWithJsRuntimeDep>());
+            Assert.True(ex?.InnerException is MissingMockJsRuntimeException);
+        }
+
+        [Fact(DisplayName = "The placeholder IJSRuntime is overriden by a supplied mock and does not throw")]
+        public void Test002()
+        {
+            Services.AddMockJsRuntime();
+
+            RenderComponent<SimpleWithJsRuntimeDep>();
+        }
+    }
+}