using System;
using static Vanara.PInvoke.Kernel32;
using static Vanara.PInvoke.User32;

namespace Vanara.PInvoke
{
	/// <summary>
	/// <para>
	/// This class encapsulates the management of a message loop for an application. It supports queuing a callback to the application via
	/// the message loop to enable the app to return from a call and continue processing that call later. This behavior is needed when
	/// implementing a shell verb as verbs must not block the caller.
	/// </para>
	/// <note type="note">The ComObject derived class should call QueueNonBlockingCallback in its invoke function, for example
	/// IExecuteCommand::Execute() or IDropTarget::Drop() passing a method that will complete the initialization work.</note>
	/// </summary>
	public class MessageLoop
	{
		private readonly uint curThreadId;
		private Action<object> appCallback;
		private uint callbackMsg;
		private object callbackObj;
		private IntPtr timeoutTimerId;   // timer id used to exit the app if the app is not called back within a certain time

		/// <summary>Initializes a new instance of the <see cref="MessageLoop"/> class.</summary>
		public MessageLoop() => curThreadId = GetCurrentThreadId();

		/// <summary>Occurs when a new message is available.</summary>
		public event EventHandler<MessageEventArgs> ProcessMessage;

		/// <summary>Gets a value indicating whether this <see cref="MessageLoop"/> is running.</summary>
		/// <value><see langword="true"/> if running; otherwise, <see langword="false"/>.</value>
		public virtual bool Running { get; private set; } = false;

		/// <summary>
		/// Cancel the timeout timer. This should be called when the application knows that it wants to keep running, for example when it
		/// receives the incoming call to invoke the verb.
		/// </summary>
		public virtual void CancelTimeout()
		{
			if (timeoutTimerId == default) return;
			KillTimer(default, timeoutTimerId);
			timeoutTimerId = default;
		}

		/// <summary>
		/// Queues a one-time callback function via the message loop. This method is not intended to be used simultaneously by multiple callers.
		/// </summary>
		/// <param name="callback">The callback delegate method.</param>
		/// <param name="tag">An optional argument that will be passed to the callback.</param>
		/// <exception cref="InvalidOperationException">Another callback is currently queued.</exception>
		/// <exception cref="ArgumentNullException"><paramref name="callback"/> cannot be <see langword="null"/>.</exception>
		public virtual void QueueCallback(Action<object> callback, object tag = null)
		{
			if (appCallback != null) throw new InvalidOperationException("Another callback is currently queued.");
			appCallback = callback ?? throw new ArgumentNullException(nameof(callback));
			callbackObj = tag;
			if (callbackMsg == 0)
				callbackMsg = RegisterWindowMessage(GetType().FullName + "+Callback");
			PostThreadMessage(curThreadId, callbackMsg);
		}

		/// <summary>Quits the running message loop by calling <see cref="PostQuitMessage"/>.</summary>
		/// <param name="exitCode">An optional exit code.</param>
		public virtual void Quit(int exitCode = 0)
		{
			CancelTimeout();
			if (Running)
				PostQuitMessage(exitCode);
		}

		/// <summary>Runs the message loop.</summary>
		/// <param name="timeout">
		/// The time span after which the message loop will be terminated. If this value equals TimeSpan.Zero or is not specified, the
		/// message loop will run until the <see cref="Quit"/> method is called or the message loop receives a quit message.
		/// </param>
		public virtual void Run(TimeSpan timeout = default)
		{
			const uint WM_TIMER = 0x0113;

			if (timeout != TimeSpan.Zero)
				timeoutTimerId = SetTimer(uElapse: (uint)timeout.TotalMilliseconds);

			Running = true;
			while (GetMessage(out var msg) != 0)
			{
				System.Diagnostics.Debug.WriteLine($"Message loop: message={msg.message}");
				try { ProcessMessage?.Invoke(this, new MessageEventArgs(msg)); } catch { }
				if (msg.message == WM_TIMER)
				{
					KillTimer(default, msg.wParam);
					if (msg.wParam == timeoutTimerId)
					{
						timeoutTimerId = IntPtr.Zero;
						PostQuitMessage(0);
					}
				}
				else if (msg.message == callbackMsg)
				{
					try { appCallback?.Invoke(callbackObj); } catch { }
					appCallback = null;
					callbackObj = null;
				}
				TranslateMessage(msg);
				DispatchMessage(msg);
			}
			Running = false;
		}

		/// <summary>Holds a copy of the MSG instance retrieved by GetMessage.</summary>
		/// <seealso cref="System.EventArgs"/>
		public class MessageEventArgs : EventArgs
		{
			internal MessageEventArgs(MSG msg) => MSG = msg;

			/// <summary>Gets or sets the MSG.</summary>
			/// <value>The MSG.</value>
			public MSG MSG { get; set; }
		}
	}
}