組合模式（Composite）

目的：

將物件組織成樹狀結構、『部分－全體』層級關係，讓外界以一致性的方式對待個別物件和整體物件。

結構：

程式碼範例：

class Component
  # @return [Component]
  def parent
    @parent
  end

  def parent=(parent)
    @parent = parent
  end

  def add(component)
    raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'"
  end

  def remove(component)
    raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'"
  end

  def composite?
    false
  end

  def operation
    raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'"
  end
end


class Leaf < Component
  def operation
    'Leaf'
  end
end


class Composite < Component
  def initialize
    @children = []
  end

  def add(component)
    @children.append(component)
    component.parent = self
  end

  def remove(component)
    @children.remove(component)
    component.parent = nil
  end

  def composite?
    true
  end

  def operation
    results = []
    @children.each { |child| results.append(child.operation) }
    "Branch(#{results.join('+')})"
  end
end

def client_code(component)
  puts "RESULT: #{component.operation}"
end

def client_code2(component1, component2)
  component1.add(component2) if component1.composite?

  print "RESULT: #{component1.operation}"
end


simple = Leaf.new
puts 'Client: I\'ve got a simple component:'
client_code(simple)
puts "\n"


tree = Composite.new

branch1 = Composite.new
branch1.add(Leaf.new)
branch1.add(Leaf.new)

branch2 = Composite.new
branch2.add(Leaf.new)

tree.add(branch1)
tree.add(branch2)

puts 'Client: Now I\'ve got a composite tree:'
client_code(tree)
puts "\n"

puts 'Client: I don\'t need to check the components classes even when managing the tree:'
client_code2(tree, simple)

結果

Client: I've got a simple component:
RESULT: Leaf

Client: Now I've got a composite tree:
RESULT: Branch(Branch(Leaf+Leaf)+Branch(Leaf))

Client: I don't need to check the components classes even when managing the tree:
RESULT: Branch(Branch(Leaf+Leaf)+Branch(Leaf)+Leaf)

解釋

可以將整個設計概念想成是節點(Composite)與端點(Leaf)，節點可以包涵端點或者節點，的是端點就不包含其他東西。而多個節點(Composite)組合，可以變成一個tree
當然也可以想成盒子(Composite)，內容物則可能包含更多盒子(Composite)與產品(Leaf)，而外部最大的盒子，就是一個tree

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