Flyweight Design Pattern

When will we Need the Flyweight Design Patterns?

Consider we are creating an open-world game where we need to place a lot of trees for decorative purposes.

The trees are either in green or brown color with a height of6ft. A Tree object has the below three attributes.

• color - The color of the tree. Either green or brown.
• height - The heights of the tree is always $6ft$.
• position - The coordinate $(x, y)$ of the tree in the game plane.

Let's say we want to place 10,000 trees (green - 5000 and brown - 5000) in the game and find the total memory required to store those Tree objects in the memory.

We need a total of $17 bytes$ to store a Tree object in memory. So we need $170,000$ bytes ($~170$ KB) to store $10,000$ Tree objects in the memory. But if we look closely, we are storing the same colors and height in all the objects; we are wasting a total of 90 KB (color - 50 KB and height - 40 KB) to store duplicate values. The only data varies across the Tree objects in position.

The flyweight design patterns help to eliminate this unwanted space by storing the shared states (color and height) within the Tree object and the unshared states (position) outside the Tree object. The Tree object is called the Flyweight object.

How does Flyweight Design Pattern Work?

Before working on the flyweight design pattern, we must understand the intrinsic and extrinsic states.

• Intrinsic state refers to the state that belongs to the flyweight object. The intrinsic state is immutable. E.g.: color and height.
• Extrinsic state varies for each object and is stored outside the object. The extrinsic state is mutable. E.g., position

We understand the working on the flyweight design pattern by implementing the example of the above-mentioned trees. We will store the intrinsic states color and height in the Tree object and the extrinsic state position in the TreePosition object.

Structure

The flyweight design pattern has core participants such as Flyweight, FlyweightFactory, Context, and Client.

Flyweight Flyweight class contains the intrinsic states of the objects shared across multiple objects. The intrinsic states are immutable. E.g.: color and height.

FlyweightFactory FlyweightFactory class is responsible for returning the Flyweight object. It caches the flyweight object the first time it is created and returns it for all successive requests. E.g.: TreeFactory.

Context The Context class is responsible for maintaining all the flyweight objects with their extrinsic states.

Client The Client is the main class that uses the Context class to create and store the flyweight objects with their extrinsic states.

Implementation

1. Create the flyweight class Tree with the intrinsic states color and height.
2. Create the TreePosition class to store the flyweight object's extrinsic state position.
3. Create the TreeFactory class with a single method, getTree(String color) that creates the Tree object with the given color. The TreeFactory class maintains an internal cache to store the created flyweight objects.
4. Create the Game context class that maintains the flyweight objects and their extrinsic state.
5. Create the Client class responsible for creating the Tree objects by passing the extrinsic state position.

Pseudocode

Tree

The Tree is the flyweight class that contains the intrinsic states color and height. The intrinsic states are immutable, so we only have getter methods for color and height.

TreePosition

The TreePosition class maintains the extrinsic state position. The extrinsic state is mutable, so we have both getter and setter methods for position.

TreeFactory

The TreeFactory class creates the Tree object with the given color. It maintains an internal map to cache the created flyweight objects.

Game

The Game class stores and maintains the flyweight objects and their extrinsic states.

Client

The Client class uses the Game instance to add trees with the given position and color.

Output

Explanation

We created two classes, Tree and TreePosition, that store both the intrinsic and extrinsic states. The Game class stores and maintains the Tree flyweight objects and its extrinsic state TreePosition. We used the FlyweightFactory class to create and cache the flyweight objects.

Language Specific Code

Java

Tree

TreePosition

TreeFactory

Game

Client

Output

C++

Tree

TreePosition

TreeFactory

Game

Client

Output

Python

Tree

TreePosition

TreeFactory

Game

Client

Output

Pros and Cons of Flyweight Design Pattern

Pros

• Flyweight design pattern reduces memory usage and increases performance by reducing the number of objects.

Cons

• Flyweight design pattern can increase the time complexity as we store the extrinsic states separately, and it takes additional time to query those states.
• Flyweight design pattern can increase the complexity of the code as we need to add additional logic to store and retrieve the extrinsic states.

Difference between Flyweight Design Pattern and Singleton Design Pattern