Flyweight

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Facade Computer Science Design Patterns
Flyweight
Proxy

It is a mechanism by which you can avoid creating a large number of object instances to represent the entire system. To decide if some part of a program is a candidate for using Flyweights, consider whether it is possible to remove some data from the class and make it extrinsic.

Examples

Libreoffice writer nice pic of ui.png

A classic example usage of the flyweight pattern is the data structures for graphical representation of characters in a word processor. It might be desirable to have, for each character in a document, a glyph object containing its font outline, font metrics, and other formatting data, but this would amount to hundreds or thousands of bytes for each character. Instead, for every character there might be a reference to a flyweight glyph object shared by every instance of the same character in the document; only the position of each character (in the document and/or the page) would need to be stored internally. Another example is string interning.

Game Cloud Screenshot.png

In video games, it is usual that you have to display the same sprite (i.e. an image of an item of the game) several times. It would highly use the CPU and the memory if each sprite was a different object. So the sprite is created once and then is rendered at different locations in the screen. This problem can be solved using the flyweight pattern. The object that renders the sprite is a flyweight.

Cost

There are several implementations for this pattern. So it's up to you to find a cheap implementation. Only implement this pattern if you have or will have CPU or memory issues.

Creation

This pattern is quite easy to create.

Maintenance

This pattern is quite easy to maintain.

Removal

This pattern is quite easy to remove too.

Advises

  • Use pre-existing tools from the language like the sets in Java.

Implementations

Implementation in Java

The following programs illustrate the document example given above: the flyweights are called FontData in the Java example. The examples illustrate the flyweight pattern used to reduce memory by loading only the data necessary to perform some immediate task from a large Font object into a much smaller FontData (flyweight) object.

import java.lang.ref.WeakReference;
import java.util.WeakHashMap;
import java.util.Collections;
import java.util.EnumSet;
import java.util.Set;
import java.awt.Color;
public final class FontData {
    enum FontEffect {
        BOLD, ITALIC, SUPERSCRIPT, SUBSCRIPT, STRIKETHROUGH
    }
    /**
     * A weak hash map will drop unused references to FontData.
     * Values have to be wrapped in WeakReferences, 
     * because value objects in weak hash map are held by strong references.
     */
    private static final WeakHashMap<FontData, WeakReference<FontData>> FLY_WEIGHT_DATA =
        new WeakHashMap<FontData, WeakReference<FontData>>();
    private final int pointSize;
    private final String fontFace;
    private final Color color;
    private final Set<FontEffect> effects;
    private FontData(int pointSize, String fontFace, Color color, EnumSet<FontEffect> effects) {
        this.pointSize = pointSize;
        this.fontFace = fontFace;
        this.color = color;
        this.effects = Collections.unmodifiableSet(effects);
    }
    public static FontData create(int pointSize, String fontFace, Color color,
        FontEffect... effects) {
        EnumSet<FontEffect> effectsSet = EnumSet.noneOf(FontEffect.class);
        for (FontEffect fontEffect : effects) {
            effectsSet.add(fontEffect);
        }
        // We are unconcerned with object creation cost, we are reducing overall memory consumption
        FontData data = new FontData(pointSize, fontFace, color, effectsSet);
        FontData result = null;
        // Retrieve previously created instance with the given values if it (still) exists
        WeakReference<FontData> ref = FLY_WEIGHT_DATA.get(data);
        if (ref != null) {
            result = ref.get();
        }
        // Store new font data instance if no matching instance exists
        if(result == null){      	
            FLY_WEIGHT_DATA.put(data, new WeakReference<FontData> (data));
            result = data;
        }
        // return the single immutable copy with the given values
        return result;
    }
    @Override
    public boolean equals(Object obj) {
        if (obj instanceof FontData) {
            if (obj == this) {
                return true;
            }
            FontData other = (FontData) obj;
            return other.pointSize == pointSize && other.fontFace.equals(fontFace)
                && other.color.equals(color) && other.effects.equals(effects);
        }
        return false;
    }
    @Override
    public int hashCode() {
        return (pointSize * 37 + effects.hashCode() * 13) * fontFace.hashCode();
    }
    // Getters for the font data, but no setters. FontData is immutable.
}


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Facade Computer Science Design Patterns
Flyweight
Proxy


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