Flyweight pattern

1. Prepared By : Hasnaeen Rizvi Rahman
Astha School of Advanced Computing

2.  Use sharing to support large number of
fine-grained objects efficiently.

3.  An application uses many objects.
 A flyweight is a shared object that can be
used in multiple contexts simultaneously.
 It acts as an individual object in each
context.
 The key concept is the distinction
between intrinsic and extrinsic state.

8.  An application uses a large number of
objects.
 Storage costs are high because of the
sheer quantity of objects.
 Most objects state can be made extrinsic.
 Many groups of objects may be replaced
by relatively few shared objects once
extrinsic state is removed.
 The application doesn’t depends on
object identity.

11.  Flyweight (Glyph)
• Declare an interface through which flyweights can receive
and act on extrinsic state.
 ConcreteFlyweight (Character)
• Implements the Flyweight interface and adds storage for
intrinsic state, if any.
 UnsharedConcreteFlyweight (Row,Column)
• Not all flyweight subclasses need to be shared.
 Flyweight Factory
• Creates and manages flyweight objects.
• Ensures that flyweights are shared properly.
 Client
• Maintains a reference to flyweights
• Computes or stores the extrinsic state of flyweight.

12.  Client passes extrinsic state to flyweight.
 Client invokes FlyweightFactory to create
Flyweight object.

13.  Flyweight may introduce run-time cost.
 Storage savings are a function of several
factors:
• The reduction in the total number of instances
that comes from sharing.
• The amount of intrinsic state per object.
• Whether extrinsic state is computed or stored.

14.  Removing extrinsic state.
 Managing shared objects.

15.  Design an application which stores list of
entities.
• Entity: It has an ID, Name and an attribute.
• Attribute: Attributes are composite data where each
attribute is either a leaf or a composite. It has all
child management operation suggested in
composite pattern. A leaf attribute have a name and
value. A Composite attribute have a name but no
value. Attribute returns an XML having Name and
Value node. The composite attribute returns the list
of child attribute XML.

16.  Entity XML could be something like…
<Entity>
<ID>1</ID>
<Name>Entity 1</Name>
<Attribute>
<Name>Entity Attribute</Name>
<Children>
<Attribute>
<Name>OrgChart</Name>
<Children>
<Attribute>
<Name>Type</Name>
<Value>Corporation</Value>
</Attribute>
<Attribute>
<Name>Country</Name>
<Value>Bangladesh</Value>
</Attribute>
</Children>
</Attribute>
</Children>
</Attribute>
</Entity>

17.  Use Flyweight pattern to reduce memory usage for
attributes.