Fix for Early initialzation problem.

When we add LocationChanging eventhandlers before the JSRuntime was initialized, the RemoteNavigationManager would crash.
Also the navigation subsystem would not be informed we had Eventhandlers unless that state changed.

Author: MariovanZeist

src/Components/Components/src/NavigationManager.cs

@@ -11,8 +11,6 @@ namespace Microsoft.AspNetCore.Components
     /// </summary>
     public abstract class NavigationManager
     {
-        private bool _hasLocationChangingListeners;
-
         /// <summary>
         /// An event that fires when the navigation location has changed.
         /// </summary>
@@ -41,18 +39,20 @@ public event EventHandler<LocationChangingEventArgs> LocationChanging
             {
                 AssertInitialized();
                 _locationChanging += value;
-                UpdateHasLocationChangingListeners(true);
+                UpdateHasLocationChangingEventHandlers();
             }
             remove
             {
                 AssertInitialized();
                 _locationChanging -= value;
-                UpdateHasLocationChangingListeners(_locationChanging != null);
+                UpdateHasLocationChangingEventHandlers();
             }
         }
 
         private EventHandler<LocationChangingEventArgs>? _locationChanging;
 
+        private bool _hasLocationChangingEventHandlers;
+
         // For the baseUri it's worth storing as a System.Uri so we can do operations
         // on that type. System.Uri gives us access to the original string anyway.
         private Uri? _baseUri;
@@ -255,17 +255,28 @@ protected bool NotifyLocationChanging(string uri, bool isInterceptedLink)
         /// Called when <see cref="LocationChanging"/> the fact that any event handlers are present or not changes.
         /// this can be used by descendants to inform the JSRuntime that there are locationchanging event handlers
         /// </summary>
-        /// <param name="value">true if there are eventhandlers</param>
-        protected virtual void SetHasLocationChangingListeners(bool value)
+        /// <param name="value">true if there are event handlers</param>
+        /// <returns>true when the navigation subsystem could be informed that we have event handlers</returns>
+        protected virtual bool SetHasLocationChangingEventHandlers(bool value)
         {
+            return true;
         }
 
-        private void UpdateHasLocationChangingListeners(bool value)
+        /// <summary>
+        /// Calls <see cref="SetHasLocationChangingEventHandlers"/> when needed
+        /// This function is normally called when event handlers are added or removed from <see cref="LocationChanging"/>
+        /// </summary>
+        protected void UpdateHasLocationChangingEventHandlers()
         {
-            if (_hasLocationChangingListeners != value)
+            var value = _locationChanging != null;
+            if (_hasLocationChangingEventHandlers != value)
             {
-                _hasLocationChangingListeners = value;
-                SetHasLocationChangingListeners(value);
+                //If SetHasLocationChangingEventHandlers returns false, we won't update the _hasLocationChangingEventHandlers.
+                //This way we can call this function again at a later time (for example when JSRuntime is initialized, See RemoteNavigationManager)
+                if (SetHasLocationChangingEventHandlers(value))
+                {
+                    _hasLocationChangingEventHandlers = value;
+                }
             }
         }
 

src/Components/Server/src/Circuits/RemoteNavigationManager.cs

@@ -52,8 +52,8 @@ public void AttachJsRuntime(IJSRuntime jsRuntime)
             {
                 throw new InvalidOperationException("JavaScript runtime already initialized.");
             }
-
             _jsRuntime = jsRuntime;
+            UpdateHasLocationChangingEventHandlers();
         }
 
         public void NotifyLocationChanged(string uri, bool intercepted)
@@ -98,9 +98,10 @@ protected override void NavigateToCore(string uri, bool forceLoad)
         }
 
         /// <inheritdoc />
-        protected override void SetHasLocationChangingListeners(bool value)
+        protected override bool SetHasLocationChangingEventHandlers(bool value)
         {
             _jsRuntime?.InvokeAsync<object>(Interop.SetHasLocationChangingListeners, value);
+            return _jsRuntime != null;
         }
 
         private static class Log

src/Components/WebAssembly/WebAssembly/src/Services/WebAssemblyNavigationManager.cs

@@ -47,9 +47,10 @@ protected override void NavigateToCore(string uri, bool forceLoad)
         }
 
         /// <inheritdoc />
-        protected override void SetHasLocationChangingListeners(bool value)
+        protected override bool SetHasLocationChangingEventHandlers(bool value)
         {
             DefaultWebAssemblyJSRuntime.Instance.Invoke<object>(Interop.SetHasLocationChangingListeners, value);
+            return true;
         }
     }
 }