前言

最近讀書會在讀深入淺出設計模式，趁這個機會複習一下設計模式，試著舉出簡單的例子並且用非模式與模式的方式來實作，比較它們的差異與優缺點。

範例問題

我們要實作一個繪圖軟體，其中支援的圖形有點(Point)、線(Line)與圓(Circle)，而這三個圖形都實作了 Drawable 這個 module，也就是每個圖形都會支援移動(move)與繪圖(draw)的功能。

module Drawable
  def draw
    fail 'You should implement "draw" method.'
  end

  def move(delta_x, delta_y)
    fail 'You should implement "move" method.'
  end
end

class Point
  include Drawable

  def initialize(x, y)
    @x = x
    @y = y
  end

  def draw
    "A point on (#{@x}, #{@y})."
  end

  def move(delta_x, delta_y)
    @x = @x + delta_x
    @y = @y + delta_y
  end
end

class Line
  include Drawable

  def initialize(x1, y1, x2, y2)
    @x1 = x1
    @y1 = y1
    @x2 = x2
    @y2 = y2
  end

  def draw
    "A line from (#{@x1}, #{@y1}) to (#{@x2}, #{@y2})."
  end

  def move(delta_x, delta_y)
    @x1 = @x1 + delta_x
    @y1 = @y1 + delta_y
    @x2 = @x2 + delta_x
    @y2 = @y2 + delta_y
  end
end

class Circle
  include Drawable

  def initialize(x, y, r)
    @x = x
    @y = y
    @r = r
  end

  def draw
    "A circle with the center at (#{@x}, #{@y}) and #{@r} redius."
  end

  def move(delta_x, delta_y)
    @x = @x + delta_x
    @y = @y + delta_y
  end
end

p1 = Point.new(1, 1)
l1 = Line.new(1, 2, 3, 4)
c1 = Circle.new(5, 5, 3)

p c1.draw # A circle with the center at (5, 5) and 3 redius.
c1.move(5,5)
p c1.draw # A circle with the center at (10, 10) and 3 redius.

現在要實作群組(Group)的功能，一個群組可以包含多的圖形，同時也可以包含其它的群組。當群組移動/繪圖時，它所包含的圖形與群組也必須跟著移動/繪圖。

沒有使用模式的實作

class Group
  def initialize(shapes)
    @shapes = shapes
  end

  def draw_all
    draws = []
    @shapes.each do |shape|
      if shape.is_a?(Drawable)
        draws << shape.draw
      end
      if shape.is_a?(Group)
        draws << shape.draw_all
      end
    end
    draws.join(' ')
  end

  def move_all(delta_x, delta_y)
    @shapes.each do |shape|
      if shape.is_a?(Drawable)
        shape.move(delta_x, delta_y)
      end
      if shape.is_a?(Group)
        shape.move_all(delta_x, delta_y)
      end
    end
  end
end

p1 = Point.new(1,1)
l1 = Line.new(1,2,3,4)
c1 = Circle.new(5,5,3)

g1 = Group.new([p1, l1, c1])

p g1.draw_all # A point on (1, 1). A line from (1, 2) to (3, 4). A circle with the center at (5, 5) and 3 redius.

g1.move_all(5,5)

p g1.draw_all # A point on (6, 6). A line from (6, 7) to (8, 9). A circle with the center at (10, 10) and 3 redius.

p2 = Point.new(100,100)
g2 = Group.new([p2, g1])

p g2.draw_all # A point on (100, 100). A point on (6, 6). A line from (6, 7) to (8, 9). A circle with the center at (10, 10) and 3 redius.

g2.move_all(10,20)

p g2.draw_all # A point on (110, 120). A point on (16, 26). A line from (16, 27) to (18, 29). A circle with the center at (20, 30) and 3 redius.

上面實作的缺點

在處理移動與繪圖的動作都要區分是一般的Drawable還是Group。只要Drawable被修改了，例如多了一個刪除(delete)的功能，則必須記得Group也要加上相同的修改。

使用模式的實作

其實從上面的例子很明顯可以看的出來，刻意區分一般的Drawable與Group並不是一個好的做法，相反的因為Group也有move與draw的method，所以可以把Group視做一個Drawable，這時候處理Group就像處理一般的Drawable一樣，行為可以一致。另外Group的move與draw method的實作，則是直接呼叫它所包含的Drawable對應的move與draw，也就是委派(delegate)method給它底下的Drawable。

class Group
  include Drawable

  def initialize(shapes)
    @shapes = shapes
  end

  def draw
    @shapes.map(&:draw).join(' ')
  end

  def move(delta_x, delta_y)
    @shapes.each { |shape| shape.move(delta_x, delta_y) }
  end
end

p1 = Point.new(1,1)
l1 = Line.new(1,2,3,4)
c1 = Circle.new(5,5,3)

g1 = Group.new([p1, l1, c1])

p g1.draw # A point on (1, 1). A line from (1, 2) to (3, 4). A circle with the center at (5, 5) and 3 redius.

g1.move(5,5)

p g1.draw # A point on (6, 6). A line from (6, 7) to (8, 9). A circle with the center at (10, 10) and 3 redius.

p2 = Point.new(100,100)
g2 = Group.new([p2, g1])

p g2.draw # A point on (100, 100). A point on (6, 6). A line from (6, 7) to (8, 9). A circle with the center at (10, 10) and 3 redius.

g2.move(10,20)

p g2.draw # A point on (110, 120). A point on (16, 26). A line from (16, 27) to (18, 29). A circle with the center at (20, 30) and 3 redius.