std.signals

Signals and Slots are an implementation of the Observer Pattern. Essentially, when a Signal is emitted, a list of connected Observers (called slots) are called.

There have been several D implementations of Signals and Slots. This version makes use of several new features in D, which make using it simpler and less error prone. In particular, it is no longer necessary to instrument the slots.

References: A Deeper Look at Signals and Slots

Observer pattern
Wikipedia
Boost Signals
Qt

There has been a great deal of discussion in the D newsgroups over this, and several implementations:

signal slots library
Signals and Slots in D
Dynamic binding -- Qt's Signals and Slots vs Objective-C
Dissecting the SS
about harmonia
Another event handling module
Suggestion: signal/slot mechanism
Signals and slots?
Signals and slots ready for evaluation
Signals & Slots for Walter
Signal/Slot mechanism?
Modern Features?
Delegates vs interfaces
The importance of component programming (properties, signals and slots, etc)
signals and slots

Bugs

Slots can only be delegates referring directly to

class or interface member functions. If a delegate to something else is passed to connect(), such as a struct member function, a nested function, a COM interface, a closure, undefined behavior will result.

Not safe for multiple threads operating on the same signals or slots.