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09s-proxy-adapter-flyweight.html
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---
layout: presentation
title: Proxy, Adapter and Flyweight
permalink: /09s-proxy-adapter-flyweight/
---
layout: true
<footer>
<span class="icon github">
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<a href="https://github.com/sikoried"><span class="username">sikoried</span></a>
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---
# Design Patterns, pt. 3
## Proxy, Adapter and Flyweight
Korbinian Riedhammer
---
# Proxy
---
# Proxy
## Variants
**Remote** proxy (aka. _Ambassador_): Provides local proxy to remote object (different process or physical location)
**Virtual** proxy: Creates expensive objects on demand; not to be confused with singleton (unique instance)
**Protection** Proxy: controls access to the original object, e.g. read-only access that simulates write.
---
# Proxy
## Examples
Caching for network requests.
Log output routing.
Lazy initialization of expensive objects.
Related: security facade; behaves like proxy, but hides error handling or authentication.
---
# Proxy, Decorator and Composite
## Decorator
Adds functionality without subclassing: one enclosed instance plus extra logic.
## Composite
Models a recursive structure, such as user interface widgets: arbitrary number of enclosed instances, logic typically restricted to traversing the structure or specific to leaf classe.
## Proxy
Mimics the original object (!) while adding access control or caching.
---
# Adapter
## Class Adapter
.skip[
]
---
# Adapter
## Object Adapter
.skip[
]
Best choice if implementation of Adaptee unknown.
---
# Flyweight
Example HTML to render:
```html
<ul>
<li><img alt="Exhibit 1" src="picasso.png"></li>
<li><img alt="Exhibit 2" src="vangogh.png"></li>
<li><img alt="Exhibit 3" src="munch.png"></li>
<li><img alt="Exhibit 4" src="monet.png"></li>
</ul>
```
---
# Flyweight
A different example:
```html
<ul>
<li><img alt="Exhibit 1" src="picasso.png"></li>
<li><img alt="Also Picasso" src="picasso.png"></li>
<li><img alt="Picasso, too" src="picasso.png"></li>
<li><img alt="Oh look, Picasso" src="picasso.png"></li>
</ul>
```
---
# Flyweight
**Intrinsic state** becomes the _flyweight_.
**Extrinsic state** managed by client; extrinsic state injected.
---
# Flyweight
## Recipe
0. Do you create a lot of objects?
1. Identify what's _intrinsic_ and _extrinsic_ to your class.
2. Move intrinsic parts to _flyweight_, create factory.
3. Reduce original class to extrinsic parts
## Notes
- The term _flyweight_ is misleading: it is _light_ in a sense of _less and static parts_, but often contains the "heavy" objects.
- Often there is no `operation()`, but just a reference to a shared object.
- The flyweight if often used in combination with the composite pattern (hence `operation()`)
---
# Flyweight
## Examples
Glyph (letter) rendering for text fields; intrinsic state: true-type fonts (often several MB), extrinsic state: position on screen, scale (size).
Browser rendering the same media multiple times; intrinsic state: actual media (image, video, audio), extrinsic state: location on screen
Android `RecyclerView`; intrinsic state: inflated layout of `RecycleView`, extrinsic state: actual contents to be displayed (often nested with further Flyweight).
Video games rendering/tiling engines; intrinsic state: actual texture or tile, extrinsic state: 3D location and orientation
---
# Summary of Design Patterns
There is a total of 23 design patterns described by Gamma _et al._
## Creational Patterns
- Factory and factory method
- Singleton
---
# Summary of Design Patterns
## Structural Patterns
- Adapter
- Composite
- Decorator
- Flyweight
- Proxy
---
# Summary of Design Patterns
## Behavioral Patterns
- Command
- Iterator
- Observer
- State
- Strategy
- Template method