Design PatternsPart 2

Prepared By: Ahmed EL-Harouny

AgendaWhat is design patterns?Strategy Pattern

DemoWhat is architecture patterns?Model-View-Controller Pattern

DemoRepository Pattern

DemoQuestions?

What are design patterns?

Design Patterns in life

Design Patterns In Programming

•Experience reuse.•Don’t reinvent the wheel.•Patterns allows you to stand on the shoulder of giants.•We can focus on creative solutions.•Mix patterns to come up with new combinations.•Common Language, patterns allows us to communicate effectively.

Why?

•Determine the need for a pattern.•Not all situations need patterns.•Ask yourself; will the pattern will add value and simplify the solution?•Find the right pattern. (The proper selection of patterns comes with the experience of selection wrong patterns sometimes)•Don’t use the pattern unless you are sure that pattern adds value!!!

How?

1. Get some free code.2. Play with it in a simple test project to understand the pattern

well. (get a sense of the gained value)3. Implement the pattern in your code.

Apply the pattern

Strategy PatternBehavioural Design Pattern

What is Strategy Pattern?

•The strategy pattern is useful for situations where it is necessary to dynamically swap the algorithms used in an application. The strategy pattern is intended to provide a means to define a family of algorithms, encapsulate each one as an object, and make them interchangeable.

Benefits•Allows us to divide up the behaviors•Used to provide a model for quickly updating and changing behaviors as needed.•Allows for a wide variety of behaviors without having to maintain the same behavior in multiple classes.

When to use it?In a scenario where you have objects that share some basic functionality but have a lot of different behaviors; without strategy pattern you have two options each one have a drawback:1. Classic Inheritance (you have to add behaviors to objects

that doesn’t need . Or reduce that amount of shared functionality in base classes which will lead to maintainability problems)

2. Interfacing (you have to re-implement behaviors in all classes which will lead to maintainability problems)

Demo!

What is Architectural patterns?

Architectural Patterns Architectural pattern are software patterns that offer well-

established solutions to architectural problems in software engineering. It gives description of the elements and relation type together with a set of constraints on how they may be used. An architectural pattern expresses a fundamental structural organization schema for a software system, which consists of subsystems, their responsibilities and interrelations. In comparison to design patterns, architectural patterns are larger in scale.Some architectural patterns are implemented in software

frameworks which can be used to more quickly build specific programs.

MVC Architectural Pattern

Model–view–controllerModel–View–Controller (MVC) is an architectural pattern used in software engineering. It isolates business logic from input and presentation, permitting independent development, testing and maintenance of each.It is common to split an application into separate layers that can be analyzed, and sometimes implemented, separately.MVC is often seen in web applications, where the view is the actual HTML or XHTML page, and the controller is the code that gathers dynamic data and generates the content within the HTML or XHTML. Finally, the model is represented by the actual content, which is often stored in a database or in XML nodes, and the business rules that transform that content based on user actions.

Example1- The user interacts with the user interface (Provided by the view) in some way (for example, presses a mouse button).2-The controller handles the input event from the user interface, often via a registered handler or callback.3- The controller notifies the model of the user action, possibly resulting in a change in the model's state. (For example, the controller updates the user's shopping cart.)[5]4- A view uses the model indirectly to generate an appropriate user interface (for example, the view lists the shopping cart's contents). The view gets its own data from the model. The model and controller have no direct knowledge of the view.

Demo!

Repository Architectural Pattern

RepositoryMediates between the domain and data mapping layers using a collection-like interface for accessing domain objects.

A Repository mediates between the domain and data mapping layers, acting like an in-memory domain object collection. Client objects construct query specifications declaratively and submit them to Repository for satisfaction. Objects can be added to and removed from the Repository, as they can from a simple collection of objects, and the mapping code encapsulated by the Repository will carry out the appropriate operations behind the scenes. Conceptually, a Repository encapsulates the set of objects persisted in a data store and the operations performed over them, providing a more object-oriented view of the persistence layer. Repository also supports the objective of achieving a clean separation and one-way dependency between the domain and data mapping layers.

BenefitsA system with a complex domain model often benefits from a layer, such as the one provided by Data Mapper , that isolates domain objects from details of the database access code. In such systems it can be worthwhile to build another layer of abstraction over the mapping layer where query construction code is concentrated. This becomes more important when there are a large number of domain classes or heavy querying. In these cases particularly, adding this layer helps minimize duplicate query logic.

Diagram

Demo!