Patterns

Design comes from Modeling (requirements, analysis, problem) Mending (patch, refactoring) Memory (patterns, recall)

Patterns Things that repeat. Solutions to a problem in a context. Patterns for coding/low-level design / architecture/user interface/problem domain

Coding patterns Smalltalk Best Practice Patterns Kent Beck Prentice-Hall, 1997

OOD patterns Design Patterns: Elements of Reusable Object-Oriented Software Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides Addison Wesley, 1995.

Architectural Patterns Pattern-Oriented Software Architecture F. Buschmann, R. Meunier, H. Rohnert, P.Sommerlad, M. Stal Wiley,

Business Software Patterns of Enterprise Application Architecture Martin Fowler et. al. Addison-Wesley,

Domain Driven Design Eric Evans Addison-Wesley,

Security patterns Munawar Hafiz /patterns/index.htm

Replication Keep a copy of data on several computers. Every time one computer makes a change, make a change on the replicas, as well.

Peer-to-peer games GUI Model GUI Model events broker

Peer-to-peer games Event-oriented, not object-oriented Easy to convert from single-cpu to distributed Easy to debug Easy if only one person can make a move at once Must synchronize simultanious moves Hard to make secure

Fault tolerance Primary and backup Messages go to everyone, only primary responds If primary doesn’t respond, backup takes over

Croquet 3D virtual world Rendering takes place on local machine Events must be synchronized, then all copies of the world are changed Opencroquet.org

Persistence Keep everything in memory Replicate for backup Synchronize with a single lock Very fast No SQL Must fit in memory

Replication Advantages: Speed – local copy is faster to access Persistence – copy on disk Fault tolerance – backup can respond immediately Disadvantages?

Replication Disadvantages: Complexity Can be hard to detect changes If changes can occur in more than one place, must synchronize. Must communicate changes – can reduce response time

How to document patterns Motivating example Definition (problem, solution) Worked out examples

How to document Get an idea. Look for good example. Write draft. Flesh out with more examples. Get feedback. Improve. Repeat as needed.

Pattern Language Patterns that a person or a group uses Christopher Alexander “A Timeless Way of Building” “A Pattern Language” A genetic code for human acts of building

Pattern Language Describes set of patterns One pattern leads to another High-level come first, then lower-level Complete set of patterns for a particular problem domain

Patterns Help you gain experience, but not a substitute for experience Can be misused - useful for experts to direct Should describe pitfalls, costs

Where to find patterns Books Conferences (PLoP, EuroPLoP) Web sites (

My research in patterns Security patterns with Munawar Hafiz Safety patterns with Lui Sha Parallel programming patterns upcrc.illinois.edu/patterns

Summary Design is hard Design is about tradeoffs There is no perfect solution, just a solution whose defects don’t bother you. Patterns are no panacea, but fit people.

Smalltalk Coding Patterns mostly from Smalltalk Best Practice Patterns Kent Beck Prentice-Hall, 1997

The Book 1. Introduction 2. Patterns 3. Behavior(Today 4. State(Next time 5. Collections 6. Classes 7. Formatting

Coding Standards Standards improve communication let code be the design make code more habitable change

Coding Standards for Smalltalk Variables have no types Names can be any length Operations named with keywords Pretty printer

Names Names should mean something. Standard protocols Object (printOn:, =) Collection (do:, add:, at:put:, size) Standard naming conventions

Intention Revealing Selector Readability of message send is more important than readability of method. Name should specify what method does, not how.

Method Names If there is already a standard name, use it instead of following these rules. Three kinds of methods change state of receiver change state of argument return value from receiver

Change state of receiver method name is verb phrase - translateBy: - add:

Change state of argument Verb phrase ending with preposition like on or to. - displayOn: - addTo:

Return value from receiver method name is noun phrase or adjective, a description rather than a command - translatedBy: - size - topLeft

Method Names Specialized names for specialized purposes. Double-dispatching methods Accessing methods Query methods Boolean property setting Converter methods

Accessing Methods Many instance variables have accessing methods, methods for reading and writing them. Accessing methods come in pairs. name, name: width, width: x, x:

When to use Accessing Methods Two opinions: Always, including an object’s own instance variable lazy initialization, subclassing is easier Only when you need to use it. better information hiding

Query Method Methods that return a value often describe the type of the value because they are noun phrases. Query methods are not noun phrases, but are predicates. How can we make the return type clear?

Query Method Prefix every query method with "is". isNil isControlWanted isEmpty Put query methods in protocol “testing”

Boolean Property Setting Don't make accessing methods whose only argument is a boolean. Create two methods beginning with ”be". (or “make”) Add "toggle" if necessary. beVisible / beInvisible / toggleVisible beDirty / beClean

Converter Method Often you want to return the receiver in a new format. Prepend "as" to the name of the class of object returned. asSet (in Collection) asFloat (in Number) asComposedText (in Text)

Converter Constructor Method A class can get cluttered with converter methods that have no relationship with its main responsibility. Make a Constructor Method in the target class that takes the object to be converted as an argument. Date fromString: ‘09/10/01’

Constructor Method Class methods that create instances are in category "instance creation methods". Creation followed by initialization is the most flexible. - Point new x: 0; y: 0 Important to preserve invariants and avoid ill-formed objects.

Constructor Method Instance creation methods should create well- formed instances. Pass all required parameters to them. - Point x: 0 y: 0 - SortedCollection sortBlock: aBlock - Set new

Constructor Parameter Method How should a Constructor Method initialize new object? Separate setters are most flexible x: aNumber y: anotherNumber ^self new x: aNumber; y: anotherNumber Must maintain invarients.

Creation Parameter Method Provide a single method that sets all the variables. Preface its name with "set", then the names of the variables. x: aNumber y: anotherNumber ^self new setX: aNumber y: anotherNumber

Shortcut Constructor Method It is verbose to keep mentioning class names, and they are duplicated. Point x: width y: height height