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Visitor 1 Visitor Intent Represent an operation to be performed on - - PowerPoint PPT Presentation
Visitor 1 Visitor Intent Represent an operation to be performed on - - PowerPoint PPT Presentation
Visitor 1 Visitor Intent 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. (Behavioral) 2 Some claim
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- Performing an operation on elements in an
- bject structure is the primary purpose for
Visitor.
- If you think of performing the operation on a
single element the Visitor name may seem unusual.
- The operation is typically performed on the
entire object structure by having a concrete visitor “visit” each element to perform the
- peration on each element.
Some claim that Visitor is a misleading name for this design pattern.
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A file system has many element types, and many
- perations that you might like to perform on it.
- Basic Types
- Directories
- Text files
- Image files
- Video files
- Sound files
- Operations
- Number of files
- Total size
- Newest text file
- Resize images
mydir subdir1 subdir2 image1 video1 subsubdir text1 sound2 image2
directory directory directory directory mp4 gif jpeg text mp3
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There are a number of considerations to do a good OO design of this subsystem.
- Operations
- Number of files
- Total size
- Newest text file
- Resize images
mydir subdir1 subdir2 image1 video1 subsubdir text1 sound2 image2
directory directory directory directory mp4 gif jpeg text mp3
What processing steps will these
- perations have in common?
How should the operations and information be encapsulated? What if separation of concerns is most important?
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The Visitor pattern separates concerns for structure maintenance and doing operations.
Visitor visitCEA(CEA a) or visit(CEA a) visitCEB(CEB b) or visit(CEB b) ConcreteVisitorA
visitCEA(CEA a) visitCEB(CEB b)
Element accept(Visitor v) ConcreteElementB
- peration()
accept(Visitor v) v.visitConcreteElementA(this)
- r v.visit(this)
v.visitConcreteElementA(this)
- r v.visit(this)
The Operations
ObjectStructure ConcreteVisitorB
visitCEA(CEA a) visitCEB(CEB b)
ConcreteElementA
- peration()
accept(Visitor v)
Structure Maintenance
Client
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Here is the object structure for the file system example with just two element types.
FileVisitor visit(Dir d) visit(Text t) NewestVisitor visit(Dir d) visit(Text t) getNewest() TotalLinesVisitor visit(Dir d) visit(Text t) getLines() File name() size() Dir
size() elements()
Text
size() lineCount()
v.visit(this) v.visit(this)
The Object Structure The Operations
FileSystem accept(FileVisitor v)
accept(FileVisitor v) accept(FileVisitor v)
TotalTimeVisitor visit(Dir d) visit(Text t) getTime()
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