Restore SEBPatch

SafeExamBrowser.WindowsApi/Hooks/SystemHook.cs

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+/*
+ * Copyright (c) 2024 ETH Zürich, IT Services
+ * 
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+using System;
+using System.Threading;
+using SafeExamBrowser.WindowsApi.Constants;
+using SafeExamBrowser.WindowsApi.Contracts.Events;
+using SafeExamBrowser.WindowsApi.Delegates;
+
+namespace SafeExamBrowser.WindowsApi.Hooks
+{
+	internal class SystemHook
+	{
+		private SystemEventCallback callback;
+		private AutoResetEvent detachEvent, detachResultAvailableEvent;
+		private bool detachSuccess;
+		private EventDelegate eventDelegate;
+		private uint eventId;
+		private IntPtr handle;
+
+		internal Guid Id { get; private set; }
+
+		public SystemHook(SystemEventCallback callback, uint eventId)
+		{
+			this.callback = callback;
+			this.detachEvent = new AutoResetEvent(false);
+			this.detachResultAvailableEvent = new AutoResetEvent(false);
+			this.eventId = eventId;
+			this.Id = Guid.NewGuid();
+		}
+
+		internal void Attach()
+		{
+			// IMPORTANT:
+			// Ensures that the hook delegate does not get garbage collected prematurely, as it will be passed to unmanaged code.
+			// Not doing so will result in a <c>CallbackOnCollectedDelegate</c> error and subsequent application crash!
+			eventDelegate = new EventDelegate(LowLevelSystemProc);
+			handle = User32.SetWinEventHook(eventId, eventId, IntPtr.Zero, eventDelegate, 0, 0, Constant.WINEVENT_OUTOFCONTEXT);
+		}
+
+		internal void AwaitDetach()
+		{
+			detachEvent.WaitOne();
+			detachSuccess = User32.UnhookWinEvent(handle);
+			detachResultAvailableEvent.Set();
+		}
+
+		internal bool Detach()
+		{
+			detachEvent.Set();
+			detachResultAvailableEvent.WaitOne();
+
+			return detachSuccess;
+		}
+
+		private void LowLevelSystemProc(IntPtr hWinEventHook, uint eventType, IntPtr hwnd, int idObject, int idChild, uint dwEventThread, uint dwmsEventTime)
+		{
+			callback(hwnd);
+		}
+	}
+}