From 7bf4c15e506b0a768da775204f6ea25d60fff2c5 Mon Sep 17 00:00:00 2001
From: David Hall <dahall@byu.net>
Date: Mon, 16 Jan 2023 16:41:23 -0700
Subject: [PATCH] Added MessagePump and ExaminedMessagePump as means to process
 Get/PeekMessage->DispatchMessage loops.

---
 PInvoke/User32/MessagePump.cs | 105 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 105 insertions(+)
 create mode 100644 PInvoke/User32/MessagePump.cs

diff --git a/PInvoke/User32/MessagePump.cs b/PInvoke/User32/MessagePump.cs
new file mode 100644
index 00000000..e4b49ed7
--- /dev/null
+++ b/PInvoke/User32/MessagePump.cs
@@ -0,0 +1,105 @@
+using static Vanara.PInvoke.User32;
+
+namespace Vanara.PInvoke;
+
+/// <summary>Delegate for a method that processes a <see cref="MSG"/> structure.</summary>
+/// <param name="msg">The <see cref="MSG"/> structure to process.</param>
+public delegate void MsgPumpDelegate(ref MSG msg);
+
+/// <summary>
+/// Delegate for a method that processes a <see cref="MSG"/> structure and returns a value determining if the next step should be processed.
+/// </summary>
+/// <param name="msg">The <see cref="MSG"/> structure to process.</param>
+/// <returns><see langword="true"/> if the pump should proceed; otherwise <see langword="false"/>.</returns>
+public delegate bool MsgPumpPredicateDelegate(ref MSG msg);
+
+/// <summary>Interface defining a message pump.</summary>
+public interface IMessagePump
+{
+	/// <summary>Runs the message pump on the optionally specified window.</summary>
+	/// <param name="mainWindow">The window instance.</param>
+	/// <returns>
+	/// The result of <see cref="PeekMessage(out MSG, HWND, uint, uint, PM)"/> or <see cref="GetMessage(out MSG, HWND, uint, uint)"/>.
+	/// </returns>
+	int Run(IWindowCore mainWindow = null);
+}
+
+/// <summary>A basic message pump to use independently or with a window instance.</summary>
+/// <example>
+/// Simple example of a window creation and message pump.
+/// <code>using (var win = new BasicMessageWindow() { Text = "Title", Visible = true })
+///     return new MessagePump().Run(win);</code></example>
+/// <seealso cref="IMessagePump" />
+public class MessagePump : IMessagePump
+{
+	/// <summary>Easy access to WM_QUIT value.</summary>
+	protected const ushort quitMsg = (ushort)WindowMessage.WM_QUIT;
+
+	/// <inhertdoc/>
+	public int Run(IWindowCore mainWindow = null)
+	{
+		if (mainWindow is not null)
+			mainWindow.Destroyed += onDestroy;
+
+		try
+		{
+			return RunLoop();
+		}
+		finally
+		{
+			if (mainWindow is not null)
+				mainWindow.Destroyed -= onDestroy;
+		}
+		static void onDestroy() => PostQuitMessage(0);
+	}
+
+	/// <summary>Defines and executes the message pump.</summary>
+	/// <returns>
+	/// The result of <see cref="PeekMessage(out MSG, HWND, uint, uint, PM)"/> or <see cref="GetMessage(out MSG, HWND, uint, uint)"/>.
+	/// </returns>
+	protected virtual int RunLoop()
+	{
+		int bRet;
+		while ((bRet = GetMessage(out MSG msg)) != 0)
+		{
+			if (bRet == -1)
+				Win32Error.ThrowLastError();
+			TranslateMessage(msg);
+			DispatchMessage(msg);
+		}
+		return bRet;
+	}
+}
+
+/// <summary>A message pump with events to process each step to use independently or with a window instance.</summary>
+/// <seealso cref="MessagePump"/>
+public class ExaminedMessagePump : MessagePump
+{
+	/// <summary>Occurs after <see cref="DispatchMessage(in MSG)"/>.</summary>
+	public event MsgPumpDelegate PostProcess;
+
+	/// <summary>Occurs after <see cref="TranslateMessage(in MSG)"/> and determines if message should be dispatched.</summary>
+	public event MsgPumpPredicateDelegate PostTranslate;
+
+	/// <summary>
+	/// Occurs after <see cref="PeekMessage(out MSG, HWND, uint, uint, PM)"/> and determines if message should be translated or dispatched.
+	/// </summary>
+	public event MsgPumpPredicateDelegate PreProcess;
+
+	/// <inhertdoc/>
+	protected override int RunLoop()
+	{
+		MSG msg;
+		do
+		{
+			if (PeekMessage(out msg, default, 0, 0, PM.PM_REMOVE) && (PreProcess?.Invoke(ref msg) ?? true))
+			{
+				TranslateMessage(msg);
+				if (PostTranslate?.Invoke(ref msg) ?? true)
+					DispatchMessage(msg);
+				PostProcess?.Invoke(ref msg);
+			}
+		} while (Macros.LOWORD(msg.message) != quitMsg);
+		return 0;
+	}
+}
\ No newline at end of file