RegionBinder.cs

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using System;
using System.Collections.Generic;
using System.Diagnostics.CodeAnalysis;
using System.Windows;
using System.Windows.Controls;
 
namespace Dreamine.MVVM.Locators.Wpf
{
    public static class RegionBinder
    {
        private static readonly object SyncRoot = new();
        private static readonly Dictionary<string, WeakReference<ContentControl>> RegionMap = new();

        public static readonly DependencyProperty RegionNameProperty =
            DependencyProperty.RegisterAttached(
                "RegionName",
                typeof(string),
                typeof(RegionBinder),
                new PropertyMetadata(null, OnRegionNameChanged));

        public static void SetRegionName(DependencyObject obj, string? value)
        {
            obj.SetValue(RegionNameProperty, value);
        }

        public static string? GetRegionName(DependencyObject obj)
        {
            return (string?)obj.GetValue(RegionNameProperty);
        }

        public static void Navigate(string regionName, object viewModel)
        {
            if (string.IsNullOrWhiteSpace(regionName))
            {
                throw new ArgumentException("Region name must not be empty.", nameof(regionName));
            }
 
            if (!TryGetRegion(regionName, out ContentControl? control))
            {
                throw new InvalidOperationException($"Region \"{regionName}\" is not registered.");
            }
 
            var view = ViewModelBinder.ResolveView(viewModel);
            control.Content = view;
        }

        public static void Clear()
        {
            lock (SyncRoot)
            {
                RegionMap.Clear();
            }
        }

        private static void OnRegionNameChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            if (d is not ContentControl control)
            {
                return;
            }
 
            if (e.OldValue is string oldName)
            {
                Unregister(oldName, control);
            }
 
            control.Loaded -= OnRegionLoaded;
            control.Unloaded -= OnRegionUnloaded;
 
            if (e.NewValue is string name && !string.IsNullOrWhiteSpace(name))
            {
                Register(name, control);
                control.Loaded += OnRegionLoaded;
                control.Unloaded += OnRegionUnloaded;
            }
        }

        private static void OnRegionLoaded(object sender, RoutedEventArgs e)
        {
            if (sender is ContentControl control)
            {
                string? regionName = GetRegionName(control);
                if (!string.IsNullOrWhiteSpace(regionName))
                {
                    Register(regionName, control);
                }
            }
        }

        private static void OnRegionUnloaded(object sender, RoutedEventArgs e)
        {
            if (sender is ContentControl control)
            {
                string? regionName = GetRegionName(control);
                if (!string.IsNullOrWhiteSpace(regionName))
                {
                    Unregister(regionName, control);
                }
            }
        }

        private static void Register(string regionName, ContentControl control)
        {
            lock (SyncRoot)
            {
                RegionMap[regionName] = new WeakReference<ContentControl>(control);
            }
        }

        private static void Unregister(string regionName, ContentControl control)
        {
            lock (SyncRoot)
            {
                if (RegionMap.TryGetValue(regionName, out WeakReference<ContentControl>? reference) &&
                    reference.TryGetTarget(out ContentControl? registeredControl) &&
                    ReferenceEquals(registeredControl, control))
                {
                    RegionMap.Remove(regionName);
                }
            }
        }

        private static bool TryGetRegion(string regionName, [NotNullWhen(true)] out ContentControl? control)
        {
            lock (SyncRoot)
            {
                if (!RegionMap.TryGetValue(regionName, out WeakReference<ContentControl>? reference))
                {
                    control = null;
                    return false;
                }
 
                if (reference.TryGetTarget(out control))
                {
                    return true;
                }
 
                RegionMap.Remove(regionName);
                control = null;
                return false;
            }
        }
    }
}