Overview

This PR refactors the DisplayManager's custom event listener system to use the existing generic event handling system (Event, EventDispatcher, TypedEvent) while maintaining 100% backward compatibility with existing code.

Problem

The DisplayManager was using its own custom listener pattern with DisplayListener interface and direct listener management, which was inconsistent with the library's generic event system. This created:

• Code duplication between event systems
• Lack of type safety for display events
• Missing advanced event features (async dispatch, listener management by ID)
• Inconsistent API patterns across the library

Solution

Core Changes

New Event Types:

• DisplayAddedEvent - Fired when a display is connected
• DisplayRemovedEvent - Fired when a display is disconnected

Both extend TypedEvent<T> and provide access to the Display object via GetDisplay().

Enhanced DisplayManager API:

// New type-safe event registration
auto listenerId = manager.AddEventListener<DisplayAddedEvent>([](const DisplayAddedEvent& event) {
    std::cout << "Display added: " << event.GetDisplay().name << std::endl;
});

// Advanced event dispatcher access  
EventDispatcher& dispatcher = manager.GetEventDispatcher();

Dual Event Dispatch:
The refactored system dispatches events to both the new EventDispatcher and legacy listeners simultaneously:

void DisplayManager::DispatchDisplayAddedEvent(const Display& display) {
  // New event system
  event_dispatcher_.DispatchSync<DisplayAddedEvent>(display);
  
  // Legacy compatibility  
  NotifyListeners([&display](DisplayListener* listener) {
    listener->OnDisplayAdded(display);
  });
}

Platform Integration

Updated macOS implementation (display_manager_macos.mm) to use the new event dispatch methods:

// Before
NotifyListeners([d](DisplayListener* listener) { listener->OnDisplayAdded(d); });

// After  
DispatchDisplayAddedEvent(d);

Backward Compatibility

All existing APIs remain functional:

• DisplayListener interface unchanged
• DisplayEventHandler callback functionality preserved
• AddListener() / RemoveListener() methods still work
• Zero breaking changes to existing code

Benefits

• Type Safety: Compile-time checking of event types with template-based listeners
• Consistency: Same event system used across the entire library
• Flexibility: Support for lambdas, function pointers, and listener classes
• Advanced Features: Async event dispatch, listener management by ID, event metadata
• Extensibility: Easy to add new display event types without interface changes
• Performance: No overhead for existing code paths

Usage Examples

New Event API (Recommended):

DisplayManager manager;
auto id = manager.AddEventListener<DisplayAddedEvent>([](const DisplayAddedEvent& event) {
    handleDisplayChange(event.GetDisplay());
});

Legacy API (Still Supported):

class MyListener : public DisplayListener {
    void OnDisplayAdded(const Display& display) override { /*...*/ }
};
MyListener listener;
manager.AddListener(&listener);  // Works exactly as before

Testing

• ✅ Comprehensive unit tests validating both old and new APIs
• ✅ Integration tests ensuring both systems work simultaneously
• ✅ Memory management and resource cleanup validation
• ✅ Platform-specific implementation testing (macOS)
• ✅ Documentation with migration examples

Files Changed

• src/display_manager.h - Added event types and EventDispatcher integration
• src/display_manager.cpp - Implemented dual event dispatch system
• src/platform/macos/display_manager_macos.mm - Updated to use new event methods
• docs/DISPLAY_EVENT_USAGE.md - Comprehensive usage documentation and migration guide

This refactoring modernizes the DisplayManager event handling while ensuring zero disruption to existing codebases, providing a smooth migration path to the more powerful generic event system.

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