Flyweight
Intent
Use sharing to support large numbers of fine-grained objects
efficiently.
Problem
Designing objects down to the lowest levels of system "granularity"
provides optimal flexibility, but can be unacceptably expensive in
terms of performance and memory usage.
Discussion
The Flyweight pattern describes how to share objects to allow their use
at fine granularities without prohibitive cost. Each "flyweight"
object is divided into two pieces: the state-dependent (extrinsic)
part, and the state-independent (intrinsic) part. Intrinsic state is
stored (shared) in the Flyweight object. Extrinsic state is stored or
computed by client objects, and passed to the Flyweight when its
operations are invoked.
An illustration of this approach would be Motif widgets that have been
re-engineered as light-weight gadgets. Whereas widgets are
"intelligent" enough to stand on their own; gadgets exist in a
dependent relationship with their parent layout manager widget. Each
layout manager provides context-dependent event handling, real estate
management, and resource services to its flyweight gadgets, and each
gadget is only responsible for context-independent state and behavior.
Structure
Note that Row and Column are playing the role of Composite
in the Composite pattern. The Context parameters are providing the
requisite extrinsic state to each method. No "factory" is modeled here for
serving up flyweight objects.
Example
The Flyweight uses sharing to support large numbers of objects
efficiently. The public switched telephone network is an example of a
Flyweight. There are several resources such as dial tone generators,
ringing generators, and digit receivers that must be shared between all
subscribers. A subscriber is unaware of how many resources are in the
pool when he or she lifts the handset to make a call. All that matters
to subscribers is that a dial tone is provided, digits are received, and
the call is completed. [Michael Duell, "Non-software examples of
software design patterns", Object Magazine, Jul 97, p54]
Non-software example
Rules of thumb
Whereas Flyweight shows how to make lots of little objects, Facade
shows how to make a single object represent an entire subsystem. [GOF,
p138]
Flyweight is often combined with Composite to implement shared leaf
nodes. [GOF, p206]
Terminal symbols within Interpreter's abstract syntax tree can be
shared with Flyweight. [GOF. p255]
Flyweight explains when and how State objects can be shared. [GOF,
p313]
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