Widget toolkits

Focused children aren't remembered, you must implement that yourself in WM_ACTIVATE/WM_SETFOCUS.

Add a handler using g_signal_connect(), override the corresponding virtual method in a subclass, or implement an interface.

Some events may need enabling via GtkWidget.add_events(), and some require adding event controllers.

Handle messages sent to the top-level/parent window, or to the control itself, through subclassing.

Notifications from ‘common controls’ are usually wrapped in WM_NOTIFY or WM_COMMAND messages.

Edit and Rich Edit controls may need EM_SETEVENTMASK.

Win32 has no ‘window server’ and can synthesize events on-demand, thus there's no need to enable WM_MOUSEMOVE.

One or more of:

• Assign a ‘delegate’ or ‘dataSource’ object implementing the corresponding protocol.
• Set ‘target’ and ‘action’ or ‘doubleAction’ properties of classes such as NSControl.
• Override methods in a subclass, e.g., NSView.draw(_:).
• Use key-value observing.
• Use Cocoa bindings.
• Use NSNotificationCenter.
• Add NSGestureRecognizers.
• Directly pass a callback block.

Additionally, you may need to enable acceptsMouseMovedEvents in NSWindow.

GMainLoop, entered by Gtk.main() or various *_run() functions

Explicit {Get,Peek}Message() loops, possibly wrapped in MsgWaitForMultipleObjects()

Implicitly entered by DialogBox(), Get{Open,Save}FileName(), etc.

Message dispatch is summarized in The Old New Thing, chapter 15.

NSApplication/{NS,CF}RunLoop,
combined with the Grand Central Dispatch library

manual reference counting with ‘floating’ references

optional automatic reference counting

Manually managed code can use autorelease pools.

signals

Action signals complete the similitude.

messages, callback blocks

KVO and NSNotificationCenter finish the picture.

GObject::notify

addObserver(_:forKeyPath:options:context:)
(from NSObject/NSKeyValueObserving)

Key paths are actually a query language.

g_object_bind_property{,_full}()

bind(_:to:withKeyPath:options:)
(from NSObject/NSKeyValueBindingCreation)

This forms an entirely separate namespace in Cocoa, documented separately from properties.