Behavioral Design Patterns

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Behavioral Design Patterns Lidan Hiﬁ Sagi Avasker Amit Bedarshi Foundations of software engineering, Fall 2015

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Agenda • What is Behavioral Design Pattern? • Observer • Mediator • State • Strategy • Iterator • Visitor

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Behavioral Patterns • Deﬁnes the communication between objects. • Dynamic behavior (changeable in runtime) using polymorphism. • Objects are able to talk each other, and still loosely coupled!

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Last Summer

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Motivation Deﬁne a one-to-many dependency between objects, so that when one object changes its state, all its dependents are notiﬁed and   updated automatically.

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Solution #1: Busy-wait

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Solution #1: Busy-wait

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Solution #2: Notiﬁcation through composition

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Solution #2: Notiﬁcation through composition for (Recipient x : recipients) { if (x instanceof Android) { sendAndroid(x, msg); } else if (x instanceof IOS) { sendiOS(x, msg); } ... }

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Solution #2: Notiﬁcation through composition for (Recipient x : recipients) { if (x instanceof Android) { sendAndroid(x, msg); } else if (x instanceof IOS) { sendiOS(x, msg); } ... }

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Solution using Observer Pattern

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Observer

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Examples • Event Listeners • C# Delegates (register a function, and invoke it later)

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Observer Demo

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Oref (HFC) Devices

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Monitoring- response time, errors App servers Autoscaling Logging

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Logging Monitoring- response time, errors App servers Autoscaling

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Logging Monitoring- response time, errors Application Servers (Mediator)

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Mediator Pattern • Deﬁne an object that encapsulates how a set of objects interact. • Mediator promotes   loose coupling by   keeping objects from   referring to each other   explicitly.

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Mediator- Structure

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Relationship One-to-many vs. Many-to-many

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Reusability

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Responsibility

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Possible Solution • Use enum that represents the current state / screen (watching, VOD menu, EPG, etc.) • Switch-case statement each time the user clicks on a multi-state button.

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enum State { CHANNEL_INFO, TV, VOD, EPG, MENU, GAMES } class DigitalTVRemote { State m_state; public void menuButton() { ... } public void infoButton() { ... } public void exitButton() { switch (m_state) { case MENU: m_state = TV; showChannel(); break; case VOD: backButton(); break; case CHANNEL_INFO: m_state = TV; hideChannelInfo(); break; } } }

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Solution using State Pattern

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State Pattern- Motivation • An object-oriented State Machine. • Allow an object to alter its behavior at runtime when its internal state changes. The object will appear to change its class

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State Pattern- Structure

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State- Pros & Cons Pros: • Provides an easy way to change the behavior of a given object in runtime, based on its current state. • Adding a new state is very easy. Cons: • Many classes which are not part of the system design are added.

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Strategy- Motivation • Deﬁnes a family of algorithms, encapsulate each one and make then interchangeable. • Lets the algorithm vary independently from the client that use it. • Choose the preferred algorithm to solving the problem, according to the current situation.

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Strategy- Structure

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Strategy- Pros & Cons Pros: • Provides an easy way to change the behavior of a given object in runtime, based on the current situation. • Adding a new algorithm is very easy. Cons: • Client must be aware of a different strategies. • Creates many classes which are not part of the system design directly.

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State Changing who changes the state?

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Encapsulation

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When to use

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Problem

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Problem collection.sort(); collection.merge(c2); collection.find(x);

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Iterator- Motivation • Provide a way to access the elements of an aggregate object sequentially without exposing its underlying implementation. • Provides a uniform interface for traversing different kinds of collections.

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Iterator- Structure

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Java Iterator public interface Iterator { boolean hasNext(); E next(); void remove(); }

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Java Iterator List arr = new ArrayList(); Iterator it = arr.iterator(); Integer i = it.next(); // for loop using iterator for (Iterator it = arr.iterator();   it.hasNext(); it.next()) { ... } // Syntactic suger (foreach loop) for (Integer i : arr) { ... }

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Internal vs. External Iterator • Internal- iteration controlled by the iterator itself. • External- client controls iteration by requesting the next element.

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Complex data structures

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Elements in the collection may be removed

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Visitor

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Problem

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Solution using Visitors

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Motivation • Represent an operation to be performed on the elements of an object structure. • Visitor lets you deﬁne a new operation, without changing the classes of the elements on which it operates.

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Structure

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Visitor- Pros & Cons Pros: • Easy to add more services: just add a visitor class. Cons: • Hard to add a new class to the original hierarchy- need to change all the visitors!

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Summary • Observer • Mediator • State • Strategy • Iterator • Visitor

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