Iceshrimp.NET/Iceshrimp.Backend/Components/Generic/NavLink.cs

using System.Diagnostics;
using System.Globalization;
using Microsoft.AspNetCore.Components;
using Microsoft.AspNetCore.Components.Rendering;
using Microsoft.AspNetCore.Components.Routing;

namespace Iceshrimp.Backend.Components.Generic;

/// <summary>
/// A component that renders an anchor tag, automatically toggling its 'active'
/// class based on whether its 'href' matches the current URI.
/// </summary>
public class NavLink : ComponentBase, IDisposable
{
	private const string DefaultActiveClass = "active";

	private bool    _isActive;
	private string? _hrefAbsolute;
	private string? _class;

	/// <summary>
	/// Gets or sets the CSS class name applied to the NavLink when the
	/// current route matches the NavLink href.
	/// </summary>
	[Parameter]
	public string? ActiveClass { get; set; }

	/// <summary>
	/// Gets or sets a collection of additional attributes that will be added to the generated
	/// <c>a</c> element.
	/// </summary>
	[Parameter(CaptureUnmatchedValues = true)]
	public IReadOnlyDictionary<string, object>? AdditionalAttributes { get; set; }

	/// <summary>
	/// Gets or sets the computed CSS class based on whether or not the link is active.
	/// </summary>
	protected string? CssClass { get; set; }

	/// <summary>
	/// Gets or sets the child content of the component.
	/// </summary>
	[Parameter]
	public RenderFragment? ChildContent { get; set; }

	/// <summary>
	/// Gets or sets a value representing the URL matching behavior.
	/// </summary>
	[Parameter]
	public NavLinkMatch Match { get; set; }

	[Inject] private NavigationManager NavigationManager { get; set; } = default!;

	/// <inheritdoc />
	protected override void OnInitialized()
	{
		// We'll consider re-rendering on each location change
		NavigationManager.LocationChanged += OnLocationChanged;
	}

	/// <inheritdoc />
	protected override void OnParametersSet()
	{
		// Update computed state
		string? href = null;
		if (AdditionalAttributes != null && AdditionalAttributes.TryGetValue("href", out var obj))
		{
			href = Convert.ToString(obj, CultureInfo.InvariantCulture);
		}

		_hrefAbsolute = href == null ? null : NavigationManager.ToAbsoluteUri(href).AbsoluteUri;
		_isActive     = ShouldMatch(NavigationManager.Uri);

		_class = null;
		if (AdditionalAttributes != null && AdditionalAttributes.TryGetValue("class", out obj))
		{
			_class = Convert.ToString(obj, CultureInfo.InvariantCulture);
		}

		UpdateCssClass();
	}

	/// <inheritdoc />
	public void Dispose()
	{
		// To avoid leaking memory, it's important to detach any event handlers in Dispose()
		NavigationManager.LocationChanged -= OnLocationChanged;
	}

	private void UpdateCssClass()
	{
		CssClass = _isActive ? CombineWithSpace(_class, ActiveClass ?? DefaultActiveClass) : _class;
	}

	private void OnLocationChanged(object? sender, LocationChangedEventArgs args)
	{
		// We could just re-render always, but for this component we know the
		// only relevant state change is to the _isActive property.
		var shouldBeActiveNow = ShouldMatch(args.Location);
		if (shouldBeActiveNow != _isActive)
		{
			_isActive = shouldBeActiveNow;
			UpdateCssClass();
			StateHasChanged();
		}
	}

	private bool ShouldMatch(string currentUriAbsolute)
	{
		if (_hrefAbsolute == null)
			return false;
		if (EqualsHrefExactlyOrIfTrailingSlashAdded(currentUriAbsolute))
			return true;
		if (Match == NavLinkMatch.AllExcludingQuery && EqualsHrefExcludingQuery(currentUriAbsolute))
			return true;

		return Match == NavLinkMatch.Prefix && IsStrictlyPrefixWithSeparator(currentUriAbsolute, _hrefAbsolute);
	}

	private bool EqualsHrefExactlyOrIfTrailingSlashAdded(string currentUriAbsolute)
	{
		if (string.Equals(currentUriAbsolute, _hrefAbsolute, StringComparison.OrdinalIgnoreCase))
		{
			return true;
		}

		if (currentUriAbsolute.Length == _hrefAbsolute!.Length - 1)
		{
			// Special case: highlight links to http://host/path/ even if you're
			// at http://host/path (with no trailing slash)
			//
			// This is because the router accepts an absolute URI value of "same
			// as base URI but without trailing slash" as equivalent to "base URI",
			// which in turn is because it's common for servers to return the same page
			// for http://host/vdir as they do for host://host/vdir/ as it's no
			// good to display a blank page in that case.
			if (_hrefAbsolute[^1] == '/'
			    && _hrefAbsolute.StartsWith(currentUriAbsolute, StringComparison.OrdinalIgnoreCase))
			{
				return true;
			}
		}

		return false;
	}

	private bool EqualsHrefExcludingQuery(string currentUriAbsolute)
	{
		Debug.Assert(_hrefAbsolute != null);

		currentUriAbsolute = currentUriAbsolute.Split('?')[0];

		if (string.Equals(currentUriAbsolute, _hrefAbsolute, StringComparison.OrdinalIgnoreCase))
		{
			return true;
		}

		if (currentUriAbsolute.Length == _hrefAbsolute.Length - 1)
		{
			// Special case: highlight links to http://host/path/ even if you're
			// at http://host/path (with no trailing slash)
			//
			// This is because the router accepts an absolute URI value of "same
			// as base URI but without trailing slash" as equivalent to "base URI",
			// which in turn is because it's common for servers to return the same page
			// for http://host/vdir as they do for host://host/vdir/ as it's no
			// good to display a blank page in that case.
			if (_hrefAbsolute[^1] == '/'
			    && _hrefAbsolute.StartsWith(currentUriAbsolute, StringComparison.OrdinalIgnoreCase))
			{
				return true;
			}
		}

		return false;
	}

	/// <inheritdoc/>
	protected override void BuildRenderTree(RenderTreeBuilder builder)
	{
		builder.OpenElement(0, "a");

		builder.AddMultipleAttributes(1, AdditionalAttributes);
		builder.AddAttribute(2, "class", CssClass);
		if (_isActive)
		{
			builder.AddAttribute(3, "aria-current", "page");
		}

		builder.AddContent(4, ChildContent);

		builder.CloseElement();
	}

	private static string CombineWithSpace(string? str1, string str2) => str1 == null ? str2 : $"{str1} {str2}";

	private static bool IsStrictlyPrefixWithSeparator(string value, string prefix)
	{
		var prefixLength = prefix.Length;
		if (value.Length > prefixLength)
		{
			return value.StartsWith(prefix, StringComparison.OrdinalIgnoreCase)
			       && (
				       // Only match when there's a separator character either at the end of the
				       // prefix or right after it.
				       // Example: "/abc" is treated as a prefix of "/abc/def" but not "/abcdef"
				       // Example: "/abc/" is treated as a prefix of "/abc/def" but not "/abcdef"
				       prefixLength == 0
				       || !IsUnreservedCharacter(prefix[prefixLength - 1])
				       || !IsUnreservedCharacter(value[prefixLength])
			       );
		}

		return false;
	}

	private static bool IsUnreservedCharacter(char c)
	{
		// Checks whether it is an unreserved character according to 
		// https://datatracker.ietf.org/doc/html/rfc3986#section-2.3
		// Those are characters that are allowed in a URI but do not have a reserved
		// purpose (e.g. they do not separate the components of the URI)
		return char.IsLetterOrDigit(c) || c == '-' || c == '.' || c == '_' || c == '~';
	}
}

/// <summary>
/// Modifies the URL matching behavior for a <see cref="T:Microsoft.AspNetCore.Components.Routing.NavLink" />.
/// </summary>
public enum NavLinkMatch
{
	/// <summary>
	/// Specifies that the <see cref="T:Microsoft.AspNetCore.Components.Routing.NavLink" /> should be active when it matches any prefix
	/// of the current URL.
	/// </summary>
	Prefix,

	/// <summary>
	/// Specifies that the <see cref="T:Microsoft.AspNetCore.Components.Routing.NavLink" /> should be active when it matches the entire
	/// current URL.
	/// </summary>
	All,
	AllExcludingQuery
}