Design patterns - How much we understand and know ??

Design

PatternsFactory Pattern in detail

A bit of History First work by Christopher Alexander a Civil

Engineer. Documented how time and

again the same architecture for buildings

when used was liked and produced the

desired output.

About 15 years ago Software

Proffessionals inspired by Alexander’s work

started incorporating it into SW

development practices.

Guesses ???

GoF

Eric Gamma, Richard Helm, Ralph

Johnson and John Vlissides.

First work 15 years before.

Included 23 design patterns categorised

into 3 types.

Nirvana

What ?

- How well we understand them

Why ?

- How well we know their need

Where ?

- Which design pattern where

What are Design Patterns

Time tested solutions to recurring problems

Template that has to be implemented in

correct situation

Relationship between classes and objects

with defined responsibilities that act in

concert to carry out the solution.

Language independent. Not just a class

or library. Much more than that.

Types Structural

- relationship between entities making it easier for entities to work together.

Creational

- instantiation mechanisms making it easier to create objects suiting the situation

Behavioral

- communication between entities and make it more easier and comfortable for entities to communicate

Architectural Patterns

MVC

MVP

MVVM

N-Tier

Repository

….

GoF and Architectural

Architectural is the pattern used overall

the project. Bigger Impact. Used at a

higher level of problem solving.

Design Pattern (GoF) can be occuring all

over the place in Architectural.

Conentrates on solving a particular

situational problem.

Analogy – Building

construction

How many rooms

How many doors

How many exits

Entrance security mechanisms

What should fall next to what

… (Architectural)

Analogy - Continued

Templates for a bay area (cubicles, open

bays, adjacent bays..)

Out of the box solution for the selected

authentication type (Strategy Design

Pattern being the most famous)

Design for exits (fire exits, lifts etc)

… (Analogy for GoF design patterns)

Structure Of A Design Pattern

Design Patterns are highly structured

Documented from a template that

identifies the information needed to

understand the problem and the context

Relationship between classes and objects

needed to implement the solution

Case Study – Factory Pattern

The Factory Method Pattern is a creation pattern.

It does exactly as it sounds ; A class that acts as a factory of

object instances

Based on the context the type of object that has to be created

is decided in the logic of the class

Generally has one interface class with many sub classes providing

other templates of object that can be created

Example : I had to once create a generic class for rendering

maps in a mobile app. Based on selection the class was

expected to either render a google map(google sdk for iOS)

or native Apple maps.

First time I got introduced to Factory Pattern

Yaaaaaayyyyyy !!!!

When can I use this

When you have multiple variations of a

single entity.

Define an interface for creating an object

but let sub classes decide which class to

instantiate

Let a class defer instantiation to sub

classes.

Case Study – Mutable Array

Wondered on how mutable arrays work

internally

Immutable arrays provide enormous

benefits. Thread safe as well as copying is

free.

They are quite boring – contents cant be

modified

Problem of Plain old C Arrays

Continuous segment of memory that can

be easily read and written to.

Malloc-ed block of memory for dynamic

array.

If an item has to be added at index 0 ?

If an item at index 0 has to be deleted ?

Memmove is to be used.

Inserting A into 0th index

Removing A from 0th index

So the real questions ?

How arrays are so powerful at indexing

even when mutable ?

How deletion at a particular index ensures

minimal shifting and minimal traversing ?

How addition at a particular index ensures

minimal shifting and minimal traversing ?

How is it internally implemented ? (Linked

list ? Hash table ? Hash of linked lists ?)

iVars in Mutable Array

_used which indicates the count of

elements

_list which is a pointer to the buffer

_size is the size of the buffer

_offset is the index of the first element in

buffer

C code for objectAtIndex

Memory Layout_size > fetchOffset

Memory Layout

_size <= fetchOffset

Data Structure used for buffer

Circular buffer

Contents wrap around when either end is

reached

Unless buffer is full, insertion/deletion

doesn’t need any memory to be moved

How its superior to C array

Addition and deletion at any index

doesn’t need as many relocations in

memory as a C array needs

Not much memory operations needed as

that in case of C arrays.

Case of deletion

Case of Addition

Deletion from Middle

Tries to minimize the amount of memory moved. Thus at most half

Of the elements will be moved

Some points about Mutable

Arrays

Non integral growth factor – buffer size

grows by 1.625 times once it is full

Once grown it doesn’t shrink.

Initial capacity doesn’t matter (its max 16

till all the blocks in the buffer are filled)

Other Implementations

Padded buffers with zeroes from both

ends to make enumeration faster from

both ends.

Class Clustering

All such different implementations are in

different classes which all are sub classes

of public interface NSMutableArray in

Objective C

Hence when we place an order to create

an object of type Mutable Array, the

order is received by the public interface

and based on the situation an instance of

one of its sub class type is returned

Benefits

Even if the underlying implementation in

the class cluster changes the code will

have no impact.

Can keep adding as many sub classes as

needed in the under lying class clusters

Thanks !!