Ability to cancel a Navigation event

src/Components/Components/src/NavigationManager.cs

@@ -2,6 +2,9 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
 using Microsoft.AspNetCore.Components.Routing;
 
 namespace Microsoft.AspNetCore.Components
@@ -30,6 +33,11 @@ public event EventHandler<LocationChangedEventArgs> LocationChanged
 
         private EventHandler<LocationChangedEventArgs>? _locationChanged;
 
+        private readonly IList<IHandleLocationChanging> _locationChangingListeners = new List<IHandleLocationChanging>();
+
+
+        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;
@@ -211,6 +219,90 @@ protected void NotifyLocationChanged(bool isInterceptedLink)
             }
         }
 
+        /// <summary>
+        /// Calls all registered <see cref="IHandleLocationChanging"/> handlers, to see if navigation should be canceled.
+        /// Returns true if navigation should be canceled.
+        /// </summary>
+        protected async ValueTask<bool> NotifyLocationChanging(string uri, bool isInterceptedLink, bool forceLoad)
+        {
+            try
+            {
+                if (_locationChangingListeners.Count > 0)
+                {
+                    var context = new LocationChangingContext(uri, isInterceptedLink, forceLoad);
+                    //Copy List to local array, to prevent exception when user removes handler during execution
+                    foreach (var listener in _locationChangingListeners.ToArray())
+                    {
+                        if (await listener.OnLocationChanging(context))
+                        {
+                            return true;
+                        }
+                    }
+                }
+                return false;
+            }
+            catch (Exception ex)
+            {
+                throw new LocationChangeException("An exception occurred while dispatching a location changing event.", ex);
+            }
+        }
+
+        /// <summary>
+        /// Called when <see cref="IHandleLocationChanging"/> 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 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;
+        }
+
+        /// <summary>
+        /// Calls <see cref="SetHasLocationChangingEventHandlers"/> when needed
+        /// This function is normally called when event handlers are added or removed from <see cref="NavigationManager"/>
+        /// </summary>
+        protected void UpdateHasLocationChangingEventHandlers()
+        {
+            var value = _locationChangingListeners.Any();
+            if (_hasLocationChangingEventHandlers != 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;
+                }
+            }
+        }
+
+        /// <summary>
+        /// Will add a <see cref="IHandleLocationChanging"/> handler to be called during a navigation Location change
+        /// </summary>
+        /// <param name="locationChangingHandler"> <see cref="IHandleLocationChanging"/> handler to be added.</param>
+        public void AddLocationChangingHandler(IHandleLocationChanging locationChangingHandler)
+        {
+            AssertInitialized();
+            _locationChangingListeners.Add(locationChangingHandler);
+            UpdateHasLocationChangingEventHandlers();
+        }
+
+        /// <summary>
+        /// Will remove a <see cref="IHandleLocationChanging"/> handler that was previously added by <see cref="AddLocationChangingHandler"/>.
+        /// </summary>
+        /// <param name="locationChangingHandler"> <see cref="IHandleLocationChanging"/> handler to be removed.</param>
+        /// <returns></returns>
+        public bool RemoveLocationChangingHandler(IHandleLocationChanging locationChangingHandler)
+        {
+            AssertInitialized();
+            var isRemoved = _locationChangingListeners.Remove(locationChangingHandler);
+            if (isRemoved)
+            {
+                UpdateHasLocationChangingEventHandlers();
+            }
+            return isRemoved;
+        }
+
         private void AssertInitialized()
         {
             if (!_isInitialized)

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

@@ -1,4 +1,4 @@
-#nullable enable
+#nullable enable
 Microsoft.AspNetCore.Components.BindConverter
 Microsoft.AspNetCore.Components.BindElementAttribute
 Microsoft.AspNetCore.Components.BindElementAttribute.BindElementAttribute(string! element, string? suffix, string! valueAttribute, string! changeAttribute) -> void
@@ -115,15 +115,19 @@ Microsoft.AspNetCore.Components.NavigationException
 Microsoft.AspNetCore.Components.NavigationException.Location.get -> string!
 Microsoft.AspNetCore.Components.NavigationException.NavigationException(string! uri) -> void
 Microsoft.AspNetCore.Components.NavigationManager
+Microsoft.AspNetCore.Components.NavigationManager.AddLocationChangingHandler(Microsoft.AspNetCore.Components.Routing.IHandleLocationChanging! locationChangingHandler) -> void
 Microsoft.AspNetCore.Components.NavigationManager.BaseUri.get -> string!
 Microsoft.AspNetCore.Components.NavigationManager.BaseUri.set -> void
 Microsoft.AspNetCore.Components.NavigationManager.Initialize(string! baseUri, string! uri) -> void
 Microsoft.AspNetCore.Components.NavigationManager.LocationChanged -> System.EventHandler<Microsoft.AspNetCore.Components.Routing.LocationChangedEventArgs!>!
 Microsoft.AspNetCore.Components.NavigationManager.NavigateTo(string! uri, bool forceLoad = false) -> void
 Microsoft.AspNetCore.Components.NavigationManager.NavigationManager() -> void
 Microsoft.AspNetCore.Components.NavigationManager.NotifyLocationChanged(bool isInterceptedLink) -> void
+Microsoft.AspNetCore.Components.NavigationManager.NotifyLocationChanging(string! uri, bool isInterceptedLink, bool forceLoad) -> System.Threading.Tasks.ValueTask<bool>
+Microsoft.AspNetCore.Components.NavigationManager.RemoveLocationChangingHandler(Microsoft.AspNetCore.Components.Routing.IHandleLocationChanging! locationChangingHandler) -> bool
 Microsoft.AspNetCore.Components.NavigationManager.ToAbsoluteUri(string! relativeUri) -> System.Uri!
 Microsoft.AspNetCore.Components.NavigationManager.ToBaseRelativePath(string! uri) -> string!
+Microsoft.AspNetCore.Components.NavigationManager.UpdateHasLocationChangingEventHandlers() -> void
 Microsoft.AspNetCore.Components.NavigationManager.Uri.get -> string!
 Microsoft.AspNetCore.Components.NavigationManager.Uri.set -> void
 Microsoft.AspNetCore.Components.OwningComponentBase
@@ -266,6 +270,8 @@ Microsoft.AspNetCore.Components.RouteView.RouteData.get -> Microsoft.AspNetCore.
 Microsoft.AspNetCore.Components.RouteView.RouteData.set -> void
 Microsoft.AspNetCore.Components.RouteView.RouteView() -> void
 Microsoft.AspNetCore.Components.RouteView.SetParametersAsync(Microsoft.AspNetCore.Components.ParameterView parameters) -> System.Threading.Tasks.Task!
+Microsoft.AspNetCore.Components.Routing.IHandleLocationChanging
+Microsoft.AspNetCore.Components.Routing.IHandleLocationChanging.OnLocationChanging(Microsoft.AspNetCore.Components.Routing.LocationChangingContext! context) -> System.Threading.Tasks.ValueTask<bool>
 Microsoft.AspNetCore.Components.Routing.IHostEnvironmentNavigationManager
 Microsoft.AspNetCore.Components.Routing.IHostEnvironmentNavigationManager.Initialize(string! baseUri, string! uri) -> void
 Microsoft.AspNetCore.Components.Routing.INavigationInterception
@@ -274,6 +280,11 @@ Microsoft.AspNetCore.Components.Routing.LocationChangedEventArgs
 Microsoft.AspNetCore.Components.Routing.LocationChangedEventArgs.IsNavigationIntercepted.get -> bool
 Microsoft.AspNetCore.Components.Routing.LocationChangedEventArgs.Location.get -> string!
 Microsoft.AspNetCore.Components.Routing.LocationChangedEventArgs.LocationChangedEventArgs(string! location, bool isNavigationIntercepted) -> void
+Microsoft.AspNetCore.Components.Routing.LocationChangingContext
+Microsoft.AspNetCore.Components.Routing.LocationChangingContext.ForceLoad.get -> bool
+Microsoft.AspNetCore.Components.Routing.LocationChangingContext.IsNavigationIntercepted.get -> bool
+Microsoft.AspNetCore.Components.Routing.LocationChangingContext.Location.get -> string!
+Microsoft.AspNetCore.Components.Routing.LocationChangingContext.LocationChangingContext(string! location, bool isNavigationIntercepted, bool forceLoad) -> void
 Microsoft.AspNetCore.Components.Routing.NavigationContext
 Microsoft.AspNetCore.Components.Routing.NavigationContext.CancellationToken.get -> System.Threading.CancellationToken
 Microsoft.AspNetCore.Components.Routing.NavigationContext.Path.get -> string!
@@ -410,6 +421,7 @@ virtual Microsoft.AspNetCore.Components.ComponentBase.OnParametersSetAsync() ->
 virtual Microsoft.AspNetCore.Components.ComponentBase.SetParametersAsync(Microsoft.AspNetCore.Components.ParameterView parameters) -> System.Threading.Tasks.Task!
 virtual Microsoft.AspNetCore.Components.ComponentBase.ShouldRender() -> bool
 virtual Microsoft.AspNetCore.Components.NavigationManager.EnsureInitialized() -> void
+virtual Microsoft.AspNetCore.Components.NavigationManager.SetHasLocationChangingEventHandlers(bool value) -> bool
 virtual Microsoft.AspNetCore.Components.OwningComponentBase.Dispose(bool disposing) -> void
 virtual Microsoft.AspNetCore.Components.RenderTree.Renderer.DispatchEventAsync(ulong eventHandlerId, Microsoft.AspNetCore.Components.RenderTree.EventFieldInfo! fieldInfo, System.EventArgs! eventArgs) -> System.Threading.Tasks.Task!
 virtual Microsoft.AspNetCore.Components.RenderTree.Renderer.Dispose(bool disposing) -> void

src/Components/Components/src/Routing/IHandleLocationChanging.cs

@@ -0,0 +1,20 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.Threading.Tasks;
+
+namespace Microsoft.AspNetCore.Components.Routing
+{
+    /// <summary>
+    /// Interface implemented by objects that want to react to a Location change in <see cref="NavigationManager"/>
+    /// </summary>
+    public interface IHandleLocationChanging
+    {
+        /// <summary>
+        /// This function is called whenever the <see cref="NavigationManager"/> wants to change it's Location
+        /// </summary>
+        /// <param name="context"></param>
+        /// <returns>A <see cref="ValueTask"/> whose result is a boolean, which if true will cancel the current Location change</returns>
+        ValueTask<bool> OnLocationChanging(LocationChangingContext context);
+    }
+}

src/Components/Components/src/Routing/LocationChangingContext.cs

@@ -0,0 +1,40 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+namespace Microsoft.AspNetCore.Components.Routing
+{
+
+    /// <summary>
+    /// context used by <see cref="IHandleLocationChanging" /> to see what kind of navigation is occuring.
+    /// </summary>
+    public class LocationChangingContext
+    {
+        /// <summary>
+        /// Initializes a new instance of <see cref="LocationChangingContext" />.
+        /// </summary>
+        /// <param name="location">The location.</param>
+        /// <param name="isNavigationIntercepted">A value that determines if navigation for the link was intercepted.</param>
+        /// <param name="forceLoad">A value that shows if the forceLoad flag was set during a call to <see cref="NavigationManager.NavigateTo" /> </param>
+        public LocationChangingContext(string location, bool isNavigationIntercepted, bool forceLoad)
+        {
+            Location = location;
+            IsNavigationIntercepted = isNavigationIntercepted;
+            ForceLoad = forceLoad;
+        }
+
+        /// <summary>
+        /// Gets the location we are about to change to.
+        /// </summary>
+        public string Location { get; }
+
+        /// <summary>
+        /// Gets a value that determines if navigation for the link was intercepted.
+        /// </summary>
+        public bool IsNavigationIntercepted { get; }
+
+        /// <summary>
+        /// Gets a value if the Forceload flag was set during a call to <see cref="NavigationManager.NavigateTo" /> 
+        /// </summary>
+        public bool ForceLoad { get; }
+    }
+}

src/Components/Server/src/Circuits/CircuitHost.cs

@@ -475,6 +475,48 @@ await Renderer.Dispatcher.InvokeAsync(() =>
             }
         }
 
+        public async Task<bool> OnLocationChangingAsync(string uri, bool intercepted)
+        {
+            AssertInitialized();
+            AssertNotDisposed();
+
+            var cancel = false;
+
+            try
+            {
+                cancel = await Renderer.Dispatcher.InvokeAsync( async () =>
+                {
+                    Log.LocationChanging(_logger, uri, CircuitId);
+                    var navigationManager = (RemoteNavigationManager)Services.GetRequiredService<NavigationManager>();
+                    return await navigationManager.HandleLocationChanging(uri, intercepted, false);
+                });
+            }
+
+            // In either case, a well-behaved client will not send invalid URIs, and we don't really
+            // want to continue processing with the circuit if setting the URI failed inside application
+            // code. The safest thing to do is consider it a critical failure since URI is global state,
+            // and a failure means that an update to global state was partially applied.
+            catch (LocationChangeException nex)
+            {
+                // LocationChangeException means that it failed in user-code. Treat this like an unhandled
+                // exception in user-code.
+                Log.LocationChangeFailedInCircuit(_logger, uri, CircuitId, nex);
+                await TryNotifyClientErrorAsync(Client, GetClientErrorMessage(nex, "Location changing failed."));
+                UnhandledException?.Invoke(this, new UnhandledExceptionEventArgs(nex, isTerminating: false));
+            }
+            catch (Exception ex)
+            {
+                // Any other exception means that it failed inside the NavigationManager. Treat
+                // this like bad data.
+                Log.LocationChangeFailed(_logger, uri, CircuitId, ex);
+                await TryNotifyClientErrorAsync(Client, GetClientErrorMessage(ex, $"Location changing to '{uri}' failed."));
+                UnhandledException?.Invoke(this, new UnhandledExceptionEventArgs(ex, isTerminating: false));
+            }
+            return cancel;
+        }
+
+
+
         public void SetCircuitUser(ClaimsPrincipal user)
         {
             // This can be called before the circuit is initialized.
@@ -610,6 +652,7 @@ private static class Log
             private static readonly Action<ILogger, string, CircuitId, Exception> _locationChangeFailed;
             private static readonly Action<ILogger, string, CircuitId, Exception> _locationChangeFailedInCircuit;
             private static readonly Action<ILogger, long, CircuitId, Exception> _onRenderCompletedFailed;
+            private static readonly Action<ILogger, string, CircuitId, Exception> _locationChanging;
 
             private static class EventIds
             {
@@ -645,6 +688,7 @@ private static class EventIds
                 public static readonly EventId LocationChangeFailed = new EventId(210, "LocationChangeFailed");
                 public static readonly EventId LocationChangeFailedInCircuit = new EventId(211, "LocationChangeFailedInCircuit");
                 public static readonly EventId OnRenderCompletedFailed = new EventId(212, "OnRenderCompletedFailed");
+                public static readonly EventId LocationChanging = new EventId(213, "LocationChanging");
             }
 
             static Log()
@@ -798,6 +842,11 @@ static Log()
                     LogLevel.Debug,
                     EventIds.OnRenderCompletedFailed,
                     "Failed to complete render batch '{RenderId}' in circuit host '{CircuitId}'.");
+
+                _locationChanging = LoggerMessage.Define<string, CircuitId>(
+                    LogLevel.Debug,
+                    EventIds.LocationChanging,
+                    "Location is about to change to {URI} in circuit '{CircuitId}'.");
             }
 
             public static void InitializationStarted(ILogger logger) => _initializationStarted(logger, null);
@@ -853,6 +902,7 @@ public static void BeginInvokeDotNetFailed(ILogger logger, string callId, string
                 }
             }
 
+            public static void LocationChanging(ILogger logger, string uri, CircuitId circuitId) => _locationChanging(logger, uri, circuitId, null);
             public static void LocationChange(ILogger logger, string uri, CircuitId circuitId) => _locationChange(logger, uri, circuitId, null);
             public static void LocationChangeSucceeded(ILogger logger, string uri, CircuitId circuitId) => _locationChangeSucceeded(logger, uri, circuitId, null);
             public static void LocationChangeFailed(ILogger logger, string uri, CircuitId circuitId, Exception exception) => _locationChangeFailed(logger, uri, circuitId, exception);