// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
 
namespace System.Windows.Forms.Design;
 
/// <summary>
///  The event handler service provides a unified way to handle
///  the various events that our form designer must process. What
///  we want to be able to do is to write code in one place that
///  handles events of a certain type. We also may need to globally
///  change the behavior of these events for modal functions like
///  the tab order UI. Our designer, however, is in many pieces
///  so we must somehow funnel these events to a common place.
///  This service implements an "event stack" that contains the
///  current set of event handlers. There can be different
///  types of handlers on the stack. For example, we may push
///  a keyboard handler and a mouse handler. When you request
///  a handler, we will find the topmost handler on the stack
///  that fits the class you requested. This way the service
///  can be extended to any eventing scheme, and it also allows
///  sections of a handler to be replaced (eg, you can replace
///  mouse handling without effecting menus or the keyboard).
/// </summary>
internal interface IEventHandlerService
{
    /// <summary>
    ///  Gets the control that handles focus changes
    ///  for this event handler service.
    /// </summary>
    Control? FocusWindow { get; }
 
    event EventHandler? EventHandlerChanged;
 
    /// <summary>
    ///  Gets the currently active event handler of the specified type.
    /// </summary>
    object? GetHandler(Type handlerType);
 
    /// <summary>
    ///  Pops
    ///  the given handler off of the stack.
    /// </summary>
    void PopHandler(object handler);
 
    /// <summary>
    ///  Pushes a new event handler on the stack.
    /// </summary>
    void PushHandler(object handler);
}