An Event_loop is responsible for waiting on some event to happen, and then
posting that event to the global Event_queue. Each event loop should be run in
its own thread.
The main User_input_event_loop waits on user input from the mouse, keyboard,
or a terminal resize, and then posts that event to the correct Widget. This is
run on the main thread.
New Event Loop types can be created, these are useful if there is an async operation in your program. The new loop can wait on whatever async activity and post a (typically custom) event to the Event Queue to be processed safely in the main thread. Posting events to the Event Queue is thread-safe. Events processed from the Event Queue are handled one at a time and make it simple to have thread-safe handling of each event, even if it is posted from another thread.
A new Event Loop action can be created by calling Event_loop::run or
Event_loop::run_async with an invokable object with signature void(). This
function will be called on each iteration of the loop, and should probably be
posting an Event to the Event_queue.
The new Event Loop can be owned by a Widget, the void Event_loop::run_async()
method should be used to launch the loop in a new thread.
All Event Loops post their events to a single, global queue.