Code shared from run.dlang.io.

main.d

import std.stdio;

enum EventType
{
    WindowResize,
    WindowTitleChange
}

struct WindowResize
{
    uint newWidth, newHeight;
}

struct WindowTitle
{
    string newTitle;
}

alias EventHandler = void function(Event);


class EventEngine
{
    EventHandler[EventType] handlers;
    
    void addHandler(EventType type, EventHandler handler)
    {
        handlers[type] = handler;
    }
    
    void emit(Event evt)
    {
        handlers[evt.type](evt);
    }              
}

struct Event
{
    EventType type;
    
    union
    {
        WindowResize windowResize;
        WindowTitle windowTitle;
    }
}

void onResize(Event evt)
{
    writefln("Resized: %d %d", evt.windowResize.newWidth, evt.windowResize.newHeight);
}
 
void onWindowTitle(Event evt)
{
    writefln("Window title changed: %s", evt.windowTitle.newTitle);
}

void main()
{
    EventEngine engine = new EventEngine();
    engine.addHandler(EventType.WindowResize, &onResize);
    engine.addHandler(EventType.WindowTitleChange, &onWindowTitle);
    
    Event resizeEvent;
    resizeEvent.type = EventType.WindowResize;
    resizeEvent.windowResize.newWidth = 1024;
    resizeEvent.windowResize.newHeight = 768;
    engine.emit(resizeEvent);

    Event titleEvent;
    titleEvent.windowTitle.newTitle = "new title";
    titleEvent.type = EventType.WindowTitleChange;
    engine.emit(titleEvent);
}