Decorator Structural Pattern

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Decorator - Structural Pattern

• Sreeparvathy Pillai

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Outline

• Structural patterns deals with the composition of
  classes and objects
• Decorator (also known as wrapper) is a structural
  pattern that composes objects recursively to add
  responsibilities.
• It attaches responsibilities dynamically to
  individual objects not to the entire class

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More on Decorator...

• The decorator object encloses a component and
  adds additional functionality to that component.
• Decorator extends an object transparently
• You can add any number of layers on an object at
  run time.

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Structure
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Example

• Say we have a TextView object and we want to add
  borders and scroll bars to it.
• 3 Types of Borders Plain, 3D, Fancy
• 2 Types of Scrollbars Horizontal, Vertical

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Structure(Example)
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Example(Contd..)
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Applicability

• To add/withdraw responsibilities to individual
  objects dynamically without affecting other
  object.
• When extension by subclassing is impractical.

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Consequences

• Benefits
• More flexibility than static inheritance
• Avoids overwhelming higher level classes with too
  many features
• Liabilities
• Too many objects
• A decorator and its component are not identical