Allow cancellation of navigation events in Blazor

src/Components/Components/src/NavigationManager.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Buffers;
 using Microsoft.AspNetCore.Components.Routing;
 
 namespace Microsoft.AspNetCore.Components;
@@ -31,6 +32,10 @@ public event EventHandler<LocationChangedEventArgs> LocationChanged
 
     private EventHandler<LocationChangedEventArgs>? _locationChanged;
 
+    private readonly List<Func<LocationChangingContext, ValueTask>> _locationChangingHandlers = new();
+
+    private CancellationTokenSource? _locationChangingCts;
+
     // 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;
@@ -274,6 +279,178 @@ protected void NotifyLocationChanged(bool isInterceptedLink)
         }
     }
 
+    /// <summary>
+    /// Notifies the registered handlers of the current location change.
+    /// </summary>
+    /// <param name="uri">The destination URI. This can be absolute, or relative to the base URI.</param>
+    /// <param name="state">The state associated with the target history entry.</param>
+    /// <param name="isNavigationIntercepted">Whether this navigation was intercepted from a link.</param>
+    /// <returns>A <see cref="ValueTask{TResult}"/> representing the completion of the operation. If the result is <see langword="true"/>, the navigation should continue.</returns>
+    protected async ValueTask<bool> NotifyLocationChangingAsync(string uri, string? state, bool isNavigationIntercepted)
+    {
+        _locationChangingCts?.Cancel();
+        _locationChangingCts = null;
+
+        var handlerCount = _locationChangingHandlers.Count;
+
+        if (handlerCount == 0)
+        {
+            return true;
+        }
+
+        var cts = new CancellationTokenSource();
+
+        _locationChangingCts = cts;
+
+        var cancellationToken = cts.Token;
+        var context = new LocationChangingContext(uri, state, isNavigationIntercepted, cancellationToken);
+
+        try
+        {
+            if (handlerCount == 1)
+            {
+                var handlerTask = InvokeLocationChangingHandlerAsync(_locationChangingHandlers[0], context);
+
+                if (context.DidPreventNavigation)
+                {
+                    return false;
+                }
+
+                if (!handlerTask.IsCompletedSuccessfully)
+                {
+                    await handlerTask.AsTask().WaitAsync(cancellationToken);
+                }
+            }
+            else
+            {
+                var locationChangingHandlersCopy = ArrayPool<Func<LocationChangingContext, ValueTask>>.Shared.Rent(handlerCount);
+
+                try
+                {
+                    _locationChangingHandlers.CopyTo(locationChangingHandlersCopy);
+
+                    var locationChangingTasks = new HashSet<Task>();
+
+                    for (var i = 0; i < handlerCount; i++)
+                    {
+                        var handlerTask = InvokeLocationChangingHandlerAsync(locationChangingHandlersCopy[i], context);
+
+                        if (handlerTask.IsFaulted)
+                        {
+                            await handlerTask;
+                            return false; // Unreachable because the previous line will throw.
+                        }
+
+                        if (context.DidPreventNavigation)
+                        {
+                            return false;
+                        }
+
+                        locationChangingTasks.Add(handlerTask.AsTask());
+                    }
+
+                    while (locationChangingTasks.Count != 0)
+                    {
+                        var completedHandlerTask = await Task.WhenAny(locationChangingTasks).WaitAsync(cancellationToken);
+
+                        if (completedHandlerTask.IsFaulted)
+                        {
+                            await completedHandlerTask;
+                            return false; // Unreachable because the previous line will throw.
+                        }
+
+                        if (context.DidPreventNavigation)
+                        {
+                            return false;
+                        }
+
+                        locationChangingTasks.Remove(completedHandlerTask);
+                    }
+                }
+                finally
+                {
+                    ArrayPool<Func<LocationChangingContext, ValueTask>>.Shared.Return(locationChangingHandlersCopy);
+                }
+            }
+
+            return !context.DidPreventNavigation;
+        }
+        catch (TaskCanceledException ex)
+        {
+            if (ex.CancellationToken == cancellationToken)
+            {
+                // This navigation was in progress when a successive navigation occurred.
+                // We treat this as a canceled navigation.
+                return false;
+            }
+
+            throw;
+        }
+        finally
+        {
+            cts.Cancel();
+            cts.Dispose();
+
+            if (_locationChangingCts == cts)
+            {
+                _locationChangingCts = null;
+            }
+        }
+    }
+
+    /// <summary>
+    /// Invokes the provided <paramref name="handler"/>, passing it the given <paramref name="context"/>.
+    /// This method can be overridden to analyze the state of the handler task even after
+    /// <see cref="NotifyLocationChangingAsync(string, string?, bool)"/> completes. For example, this can be useful for
+    /// processing exceptions thrown from handlers that continue running after the navigation ends.
+    /// </summary>
+    /// <param name="handler">The handler to invoke.</param>
+    /// <param name="context">The context to pass to the handler.</param>
+    /// <returns></returns>
+    protected virtual ValueTask InvokeLocationChangingHandlerAsync(Func<LocationChangingContext, ValueTask> handler, LocationChangingContext context)
+        => handler(context);
+
+    /// <summary>
+    /// Sets whether navigation is currently locked. If it is, then implementations should not update <see cref="Uri"/> and call
+    /// <see cref="NotifyLocationChanged(bool)"/> until they have first confirmed the navigation by calling
+    /// <see cref="NotifyLocationChangingAsync(string, string?, bool)"/>.
+    /// </summary>
+    /// <param name="value">Whether navigation is currently locked.</param>
+    protected virtual void SetNavigationLockState(bool value)
+        => throw new NotSupportedException($"To support navigation locks, {GetType().Name} must override {nameof(SetNavigationLockState)}");
+
+    /// <summary>
+    /// Adds a handler to process incoming navigation events.
+    /// </summary>
+    /// <param name="locationChangingHandler">The handler to process incoming navigation events.</param>
+    public void AddLocationChangingHandler(Func<LocationChangingContext, ValueTask> locationChangingHandler)
+    {
+        AssertInitialized();
+
+        var isFirstHandler = _locationChangingHandlers.Count == 0;
+
+        _locationChangingHandlers.Add(locationChangingHandler);
+
+        if (isFirstHandler)
+        {
+            SetNavigationLockState(true);
+        }
+    }
+
+    /// <summary>
+    /// Removes a previously-added location changing handler.
+    /// </summary>
+    /// <param name="locationChangingHandler">The handler to remove.</param>
+    public void RemoveLocationChangingHandler(Func<LocationChangingContext, ValueTask> locationChangingHandler)
+    {
+        AssertInitialized();
+
+        if (_locationChangingHandlers.Remove(locationChangingHandler) && _locationChangingHandlers.Count == 0)
+        {
+            SetNavigationLockState(false);
+        }
+    }
+
     private void AssertInitialized()
     {
         if (!_isInitialized)

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

@@ -1,10 +1,19 @@
 #nullable enable
+Microsoft.AspNetCore.Components.NavigationManager.AddLocationChangingHandler(System.Func<Microsoft.AspNetCore.Components.Routing.LocationChangingContext!, System.Threading.Tasks.ValueTask>! locationChangingHandler) -> void
 Microsoft.AspNetCore.Components.NavigationManager.HistoryEntryState.get -> string?
 Microsoft.AspNetCore.Components.NavigationManager.HistoryEntryState.set -> void
+Microsoft.AspNetCore.Components.NavigationManager.NotifyLocationChangingAsync(string! uri, string? state, bool isNavigationIntercepted) -> System.Threading.Tasks.ValueTask<bool>
+Microsoft.AspNetCore.Components.NavigationManager.RemoveLocationChangingHandler(System.Func<Microsoft.AspNetCore.Components.Routing.LocationChangingContext!, System.Threading.Tasks.ValueTask>! locationChangingHandler) -> void
 Microsoft.AspNetCore.Components.NavigationOptions.HistoryEntryState.get -> string?
 Microsoft.AspNetCore.Components.NavigationOptions.HistoryEntryState.init -> void
 Microsoft.AspNetCore.Components.Rendering.RenderTreeBuilder.AddContent(int sequence, Microsoft.AspNetCore.Components.MarkupString? markupContent) -> void
 Microsoft.AspNetCore.Components.Routing.LocationChangedEventArgs.HistoryEntryState.get -> string?
+Microsoft.AspNetCore.Components.Routing.LocationChangingContext
+Microsoft.AspNetCore.Components.Routing.LocationChangingContext.CancellationToken.get -> System.Threading.CancellationToken
+Microsoft.AspNetCore.Components.Routing.LocationChangingContext.HistoryEntryState.get -> string?
+Microsoft.AspNetCore.Components.Routing.LocationChangingContext.IsNavigationIntercepted.get -> bool
+Microsoft.AspNetCore.Components.Routing.LocationChangingContext.PreventNavigation() -> void
+Microsoft.AspNetCore.Components.Routing.LocationChangingContext.TargetLocation.get -> string!
 static Microsoft.AspNetCore.Components.CompilerServices.RuntimeHelpers.CreateInferredEventCallback<T>(object! receiver, Microsoft.AspNetCore.Components.EventCallback<T> callback, T value) -> Microsoft.AspNetCore.Components.EventCallback<T>
 static Microsoft.AspNetCore.Components.CompilerServices.RuntimeHelpers.InvokeAsynchronousDelegate(System.Action! callback) -> System.Threading.Tasks.Task!
 static Microsoft.AspNetCore.Components.CompilerServices.RuntimeHelpers.InvokeAsynchronousDelegate(System.Func<System.Threading.Tasks.Task!>! callback) -> System.Threading.Tasks.Task!
@@ -41,3 +50,5 @@ static Microsoft.AspNetCore.Components.EventCallbackFactoryBinderExtensions.Crea
 static Microsoft.AspNetCore.Components.EventCallbackFactoryBinderExtensions.CreateBinder(this Microsoft.AspNetCore.Components.EventCallbackFactory! factory, object! receiver, Microsoft.AspNetCore.Components.EventCallback<short?> setter, short? existingValue, System.Globalization.CultureInfo? culture = null) -> Microsoft.AspNetCore.Components.EventCallback<Microsoft.AspNetCore.Components.ChangeEventArgs!>
 static Microsoft.AspNetCore.Components.EventCallbackFactoryBinderExtensions.CreateBinder(this Microsoft.AspNetCore.Components.EventCallbackFactory! factory, object! receiver, Microsoft.AspNetCore.Components.EventCallback<string?> setter, string! existingValue, System.Globalization.CultureInfo? culture = null) -> Microsoft.AspNetCore.Components.EventCallback<Microsoft.AspNetCore.Components.ChangeEventArgs!>
 static Microsoft.AspNetCore.Components.EventCallbackFactoryBinderExtensions.CreateBinder<T>(this Microsoft.AspNetCore.Components.EventCallbackFactory! factory, object! receiver, Microsoft.AspNetCore.Components.EventCallback<T> setter, T existingValue, System.Globalization.CultureInfo? culture = null) -> Microsoft.AspNetCore.Components.EventCallback<Microsoft.AspNetCore.Components.ChangeEventArgs!>
+virtual Microsoft.AspNetCore.Components.NavigationManager.InvokeLocationChangingHandlerAsync(System.Func<Microsoft.AspNetCore.Components.Routing.LocationChangingContext!, System.Threading.Tasks.ValueTask>! handler, Microsoft.AspNetCore.Components.Routing.LocationChangingContext! context) -> System.Threading.Tasks.ValueTask
+virtual Microsoft.AspNetCore.Components.NavigationManager.SetNavigationLockState(bool value) -> void

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

@@ -0,0 +1,49 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace Microsoft.AspNetCore.Components.Routing;
+
+/// <summary>
+/// Contains context for a change to the browser's current location.
+/// </summary>
+public class LocationChangingContext
+{
+    internal LocationChangingContext(string targetLocation, string? historyEntryState, bool isNavigationIntercepted, CancellationToken cancellationToken)
+    {
+        TargetLocation = targetLocation;
+        HistoryEntryState = historyEntryState;
+        IsNavigationIntercepted = isNavigationIntercepted;
+        CancellationToken = cancellationToken;
+    }
+
+    internal bool DidPreventNavigation { get; private set; }
+
+    /// <summary>
+    /// Gets the target location.
+    /// </summary>
+    public string TargetLocation { get; }
+
+    /// <summary>
+    /// Gets the state associated with the target history entry.
+    /// </summary>
+    public string? HistoryEntryState { get; }
+
+    /// <summary>
+    /// Gets whether this navigation was intercepted from a link.
+    /// </summary>
+    public bool IsNavigationIntercepted { get; }
+
+    /// <summary>
+    /// Gets a <see cref="System.Threading.CancellationToken"/> that can be used to determine if this navigation was canceled
+    /// (for example, because the user has triggered a different navigation).
+    /// </summary>
+    public CancellationToken CancellationToken { get; }
+
+    /// <summary>
+    /// Prevents this navigation from continuing.
+    /// </summary>
+    public void PreventNavigation()
+    {
+        DidPreventNavigation = true;
+    }
+}