Add NavigationManager.OnLocationChanging (#14962)

src/Components/Components/src/NavigationManager.cs

@@ -11,6 +11,25 @@ namespace Microsoft.AspNetCore.Components
     /// </summary>
     public abstract class NavigationManager
     {
+        /// <summary>
+        /// An event that fires when the navigation location is about to change.
+        /// </summary>
+        public event EventHandler<LocationChangingEventArgs> LocationChanging
+        {
+            add
+            {
+                AssertInitialized();
+                _locationChanging += value;
+            }
+            remove
+            {
+                AssertInitialized();
+                _locationChanging -= value;
+            }
+        }
+
+        private EventHandler<LocationChangingEventArgs> _locationChanging;
+
         /// <summary>
         /// An event that fires when the navigation location has changed.
         /// </summary>
@@ -93,7 +112,10 @@ protected set
         public void NavigateTo(string uri, bool forceLoad = false)
         {
             AssertInitialized();
-            NavigateToCore(uri, forceLoad);
+            if (CanNavigateTo(uri, forceLoad))
+            {
+                NavigateToCore(uri, forceLoad);
+            }
         }
 
         /// <summary>
@@ -224,6 +246,20 @@ private void AssertInitialized()
             }
         }
 
+        private bool CanNavigateTo(string uri, bool forceLoad)
+        {
+            try
+            {
+                var args = new LocationChangingEventArgs(uri, forceLoad);
+                _locationChanging?.Invoke(this, args);
+                return !args.IsNavigationPrevented;
+            }
+            catch (Exception ex)
+            {
+                throw new LocationChangeException("An exception occurred while dispatching a location changed event.", ex);
+            }
+        }
+
         private static bool TryGetLengthOfBaseUriPrefix(Uri baseUri, string uri, out int length)
         {
             if (uri.StartsWith(baseUri.OriginalString, StringComparison.Ordinal))

src/Components/Components/src/Routing/LocationChangingEventArgs.cs

@@ -0,0 +1,48 @@
+// 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;
+
+namespace Microsoft.AspNetCore.Components.Routing
+{
+    /// <summary>
+    /// <see cref="EventArgs" /> for <see cref="NavigationManager.LocationChanging" />.
+    /// </summary>
+    public class LocationChangingEventArgs : EventArgs
+    {
+        /// <summary>
+        /// Initializes a new instance of <see cref="LocationChangingEventArgs" />.
+        /// </summary>
+        /// <param name="location">The location of the navigation request.</param>
+        /// <param name="forceLoad">If true, the requested navigation will bypass client-side routing and force the browser to load the new page from the server, whether or not the URI would normally be handled by the client-side router.</param>
+        public LocationChangingEventArgs(string location, bool forceLoad)
+        {
+            Location = location;
+            ForceLoad = forceLoad;
+        }
+
+        /// <summary>
+        /// Gets the changed location.
+        /// </summary>
+        public string Location { get; }
+
+        /// <summary>
+        /// Gets a value that determines if navigation should be prevented.
+        /// </summary>
+        public bool IsNavigationPrevented { get; private set; }
+
+        /// <summary>
+        /// If true, the requested navigation will bypass client-side routing and force the browser to load the new page from the server, whether or not the URI would normally be handled by the client-side router.
+        /// </summary>
+        public bool ForceLoad { get; }
+
+        /// <summary>
+        /// Indicates to the <see cref="Microsoft.AspNetCore.Components.NavigationManager"/>
+        /// that the navigation should be prevented.
+        /// </summary>
+        public void PreventNavigation()
+        {
+            IsNavigationPrevented = true;
+        }
+    }
+}

src/Components/Components/test/NavigationManagerTest.cs

@@ -92,10 +92,36 @@ public void ToBaseRelativePath_ThrowsForInvalidBaseRelativePaths(string baseUri,
                 ex.Message);
         }
 
+        [Theory]
+        [InlineData("scheme://host/allowed", true)]
+        [InlineData("scheme://host/prevented", false)]
+        public void OnLocationChanging_CanPreventNavigation(string uri, bool allowNavigation)
+        {
+            bool hasNavigated = false;
+            var navigationManager = new TestNavigationManager(() => hasNavigated = true);
+            navigationManager.Initialize(uri, uri);
+            navigationManager.LocationChanging += (sender, args) =>
+            {
+                if (!allowNavigation)
+                    args.PreventNavigation();
+            };
+            navigationManager.NavigateTo(uri);
+
+            Assert.Equal(allowNavigation, hasNavigated);
+        }
+
         private class TestNavigationManager : NavigationManager
         {
+            private Action NavigateAction;
+
             public TestNavigationManager()
             {
+                NavigateAction = () => throw new NotImplementedException();
+            }
+
+            public TestNavigationManager(Action navigateAction)
+            {
+                NavigateAction = navigateAction;
             }
 
             public TestNavigationManager(string baseUri = null, string uri = null)
@@ -110,7 +136,7 @@ public TestNavigationManager(string baseUri = null, string uri = null)
 
             protected override void NavigateToCore(string uri, bool forceLoad)
             {
-                throw new System.NotImplementedException();
+                NavigateAction();
             }
         }
     }