visitor pattern by steve hayes

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VisitorSteve Hayes

Cogent Consulting

http://www.cogentconsulting.com.au



The Problem




Show me everything in a consistent manner...





... and send it to my friend via email






oh, and send it to her phone as well...







and print it.....




Some analysis music please...




Commonality is in the eye of the beholder




With a little casual application of polymorphism...




Does this look bad to you?




The solution




Visitors




“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”

-- GOF




A solution to a problem in a context




Headings, Details, ...




and the Iterator




Let’s show it in code!




Time for visitors...




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Let's talk about tests, baby...




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Let's talk about tests, baby...Some long babbling about test naming heuristics




Cool!




When?




“an object structure contains many classes of objects with differing interfaces, and

you want to perform operations on these objects that depend on their concrete

classes”




“many distinct and unrelated operations need to be performed on objects in an object structure, and you want to avoid

‘polluting’ their classes with these operations. Visitor lets you keep unrelated operations together by defining them in one class. When the object structure is

shared by many applications, use Visitor to put operations in just those applications

that need them”







and print it.....




“the classes defining the object structure rarely change, but you often want to

define new operations over the structure. Changing the object structure classes requires redefining the interface to all

visitors, which is potentially costly. If the object structure classes change often, then

it’s probably better to define the operations in those classes”




Consequences




Visitor makes adding new operations easy




A visitor gathers related operations and separates unrelated ones




Adding new ConcreteElement classes is hard




Visitors can cross object hierarchies




Visitors can accumulate state




Visitors may compromise encapsulation




Discussion



visitor pattern by steve hayes

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