The document discusses the factory pattern and provides an example of how it can be used to create map rendering objects for either Google Maps or Apple Maps depending on the context. It also discusses how mutable arrays are implemented internally using a circular buffer data structure, which allows efficient addition and removal of elements at any index through minimal shifting of elements in memory compared to traditional C arrays. The factory method pattern creates object instances by having a class act as a factory based on context, with one interface class and subclasses that provide templates for the different object types that can be created.

1. Design
Patterns
Factory Pattern in detail

2. 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.

3. Guesses ???

4. GoF
 Eric Gamma, Richard Helm, Ralph
Johnson and John Vlissides.
 First work 15 years before.
 Included 23 design patterns categorised
into 3 types.

5. Nirvana
What ?
- How well we understand them
Why ?
- How well we know their need
Where ?
- Which design pattern where

6. 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.

7. 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

8. Architectural Patterns
 MVC
 MVP
 MVVM
 N-Tier
 Repository
….

10. 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.

11. Analogy – Building
construction
 How many rooms
 How many doors
 How many exits
 Entrance security mechanisms
What should fall next to what
… (Architectural)

12. 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)

14. 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

16. Case Study – Factory Pattern

17. 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 !!!!

18. 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.

19. 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

20. 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.

21. Inserting A into 0th index

22. Removing A from 0th index

23. 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 ?)

24. 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

25. C code for objectAtIndex

26. Memory Layout
_size > fetchOffset

27. Memory Layout
_size <= fetchOffset

28. 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

29. 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.

30. Case of deletion

31. Case of Addition

32. Deletion from Middle

33. Tries to minimize the amount of memory moved. Thus at most half
Of the elements will be moved

34. 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)

35. Other Implementations
 Padded buffers with zeroes from both
ends to make enumeration faster from
both ends.

36. 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

37. 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

39. Thanks !!