Add confirmation dialog when about to discard a playlist

The confirmation will only show if items have been added to the
playlist.

Closes https://github.com/ppy/osu/issues/19444.

osu.Game/Screens/Menu/ConfirmDiscardChangesDialog.cs

@@ -0,0 +1,39 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using osu.Framework.Graphics.Sprites;
+using osu.Game.Overlays.Dialog;
+
+namespace osu.Game.Screens.Menu
+{
+    public class ConfirmDiscardChangesDialog : PopupDialog
+    {
+        /// <summary>
+        /// Construct a new discard changes confirmation dialog.
+        /// </summary>
+        /// <param name="onConfirm">An action to perform on confirmation.</param>
+        /// <param name="onCancel">An optional action to perform on cancel.</param>
+        public ConfirmDiscardChangesDialog(Action onConfirm, Action? onCancel = null)
+        {
+            HeaderText = "Are you sure you want to go back?";
+            BodyText = "This will discard any unsaved changes";
+
+            Icon = FontAwesome.Solid.ExclamationTriangle;
+
+            Buttons = new PopupDialogButton[]
+            {
+                new PopupDialogDangerousButton
+                {
+                    Text = @"Yes",
+                    Action = onConfirm
+                },
+                new PopupDialogCancelButton
+                {
+                    Text = @"No I didn't mean to",
+                    Action = onCancel
+                },
+            };
+        }
+    }
+}

osu.Game/Screens/Menu/ConfirmExitDialog.cs

@@ -1,8 +1,6 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
-#nullable disable
-
 using System;
 using osu.Framework.Graphics.Sprites;
 using osu.Game.Overlays.Dialog;
@@ -16,7 +14,7 @@ namespace osu.Game.Screens.Menu
         /// </summary>
         /// <param name="onConfirm">An action to perform on confirmation.</param>
         /// <param name="onCancel">An optional action to perform on cancel.</param>
-        public ConfirmExitDialog(Action onConfirm, Action onCancel = null)
+        public ConfirmExitDialog(Action onConfirm, Action? onCancel = null)
         {
             HeaderText = "Are you sure you want to exit osu!?";
             BodyText = "Last chance to turn back";

osu.Game/Screens/OnlinePlay/Match/RoomSubScreen.cs

@@ -28,6 +28,7 @@ using osu.Game.Overlays;
 using osu.Game.Overlays.Mods;
 using osu.Game.Rulesets;
 using osu.Game.Rulesets.Mods;
+using osu.Game.Screens.Menu;
 using osu.Game.Screens.OnlinePlay.Match.Components;
 using osu.Game.Screens.OnlinePlay.Multiplayer;
 
@@ -280,13 +281,30 @@ namespace osu.Game.Screens.OnlinePlay.Match
             };
         }
 
+        [Resolved(canBeNull: true)]
+        private IDialogOverlay dialogOverlay { get; set; }
+
         public override bool OnBackButton()
         {
             if (Room.RoomID.Value == null)
             {
-                // room has not been created yet; exit immediately.
+                if (dialogOverlay == null || Room.Playlist.Count == 0)
-                settingsOverlay.Hide();
+                {
-                return base.OnBackButton();
+                    settingsOverlay.Hide();
+                    return base.OnBackButton();
+                }
+
+                // if the dialog is already displayed, block exiting until the user explicitly makes a decision.
+                if (dialogOverlay.CurrentDialog is ConfirmDiscardChangesDialog)
+                    return true;
+
+                dialogOverlay?.Push(new ConfirmDiscardChangesDialog(() =>
+                {
+                    settingsOverlay.Hide();
+                    this.Exit();
+                }));
+
+                return true;
             }
 
             if (UserModsSelectOverlay.State.Value == Visibility.Visible)