// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System.ComponentModel;

namespace System.Windows.Forms;

/// <summary>
///  ApplicationContext provides contextual information about an application
///  thread. Specifically this allows an application author to redefine what
///  circumstances cause a message loop to exit. By default the application
///  context listens to the close event on the mainForm, then exits the
///  thread's message loop.
/// </summary>
public class ApplicationContext : IDisposable
{
    private Form? _mainForm;

    /// <summary>
    ///  Creates a new ApplicationContext with no mainForm.
    /// </summary>
    public ApplicationContext() : this(null)
    {
    }

    /// <summary>
    ///  Creates a new ApplicationContext with the specified mainForm.
    ///  If OnMainFormClosed is not overridden, the thread's message
    ///  loop will be terminated when mainForm is closed.
    /// </summary>
    public ApplicationContext(Form? mainForm)
    {
        // These are subclasses of the ApplicationContext for which we don't need to call the finalizer,
        //  because it's empty. See https://github.com/dotnet/winforms/issues/6858.
        if (GetType() == typeof(ApplicationContext) || GetType() == Application.s_typeOfModalApplicationContext)
            GC.SuppressFinalize(this);

        MainForm = mainForm;
    }

    // NOTE: currently this finalizer is unneeded (empty). See https://github.com/dotnet/winforms/issues/6858.
    // All classes that are not need to be finalized must be checked in ApplicationContext(Form? mainForm) constructor.
    // Consider to modify it if needed.
    ~ApplicationContext() => Dispose(false);

    /// <summary>
    ///  Determines the mainForm for this context. This may be changed
    ///  at anytime.
    ///  If OnMainFormClosed is not overridden, the thread's message
    ///  loop will be terminated when mainForm is closed.
    /// </summary>
    public Form? MainForm
    {
        get => _mainForm;
        set
        {
            EventHandler onClose = OnMainFormDestroy;
            if (_mainForm is not null)
            {
                _mainForm.HandleDestroyed -= onClose;
            }

            _mainForm = value;

            if (_mainForm is not null)
            {
                _mainForm.HandleDestroyed += onClose;
            }
        }
    }

    [SRCategory(nameof(SR.CatData))]
    [Localizable(false)]
    [Bindable(true)]
    [SRDescription(nameof(SR.ControlTagDescr))]
    [DefaultValue(null)]
    [TypeConverter(typeof(StringConverter))]
    public object? Tag { get; set; }

    /// <summary>
    ///  Is raised when the thread's message loop should be terminated.
    ///  This is raised by calling ExitThread.
    /// </summary>
    public event EventHandler? ThreadExit;

    /// <summary>
    ///  Disposes the context. This should dispose the mainForm. This is
    ///  called immediately after the thread's message loop is terminated.
    ///  Application will dispose all forms on this thread by default.
    /// </summary>
    public void Dispose()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }

    protected virtual void Dispose(bool disposing)
    {
        if (disposing)
        {
            if (_mainForm is not null)
            {
                if (!_mainForm.IsDisposed)
                {
                    _mainForm.Dispose();
                }

                _mainForm = null;
            }
        }

        // If you are adding releasing unmanaged resources code here (disposing == false), you need to:
        // 1. remove GC.SuppressFinalize from constructor of this class and from all of its subclasses
        // 2. remove ApplicationContext_Subclasses_SuppressFinalizeCall test
        // 3. modify ~ApplicationContext() description.
    }

    /// <summary>
    ///  Causes the thread's message loop to be terminated. This will call ExitThreadCore.
    /// </summary>
    public void ExitThread() => ExitThreadCore();

    /// <summary>
    ///  Causes the thread's message loop to be terminated.
    /// </summary>
    protected virtual void ExitThreadCore() => ThreadExit?.Invoke(this, EventArgs.Empty);

    /// <summary>
    ///  Called when the mainForm is closed. The default implementation
    ///  of this will call ExitThreadCore.
    /// </summary>
    protected virtual void OnMainFormClosed(object? sender, EventArgs e) => ExitThreadCore();

    /// <summary>
    ///  Called when the mainForm is closed. The default implementation
    ///  of this will call ExitThreadCore.
    /// </summary>
    private void OnMainFormDestroy(object? sender, EventArgs e)
    {
        Debug.Assert(sender is Form);
        Form form = (Form)sender;
        if (!form.RecreatingHandle)
        {
            form.HandleDestroyed -= OnMainFormDestroy;
            OnMainFormClosed(sender, e);
        }
    }
}
