decorator是一個滿常使用的方式，只要透過裝飾，就能很容易新增一個功能，然而被裝飾者，完全不必知道裝飾者的存在，減少耦合性。
例如今天有一個Canvas，要將文字畫在canvas上，並且加上邊框，文字並不需要知道邊框存不存在，這時就可以使用decorator附加上去。
Example：
首先簡單定義一個canvas，draw會先將內容寫在變數裡，然後在由show將結果顯示在terminal上面。
class Canvas( object ):
"""
Define a sample canvas for terminal.
"""
def __init__( self, width=15, height=5 ):
"""
Init a params.
"""
self.__data = []
self.__width = width
self.__height = height
self.__setup( width, height )
def __setup( self, width, height ):
"""
Create a data block.
"""
for hi in range( height ):
self.__data.append( [] )
for wi in range( width ):
self.__data[ hi ].append( " " )
def draw( self, content, x, y ):
"""
Draw content to data block.
"""
width = self.__width
for char in content:
if x >= width:
break
self.__data[y][x] = char
x = x + 1
def show( self ):
"""
Print to terminal.
"""
for hi in range( self.__height ):
print "".join( self.__data[ hi ] )
接著View定義了基本的位置，和width計算，在由drawTo繪製到canvas。
class View( object ):
"""
Define a sample view.
"""
def setY( self, y ):
"""
Set a position of y.
"""
self._y = y
def getY( self ):
"""
Get a position of y.
"""
return self._y
def setX( self, x ):
"""
Set a position of x.
"""
self._x = x
def getX( self ):
"""
Get a position of x.
"""
return self._x
def getWidth( self ):
"""
Get the view width.
"""
return 0
def drawTo( self, canvas, parent = None ):
"""
Draw something to canvas.
"""
pass
定義一個decorator，讓border裝飾。
class ViewDecorator( object ):
"""
Decorator of View.
"""
def __init__( self, *views ):
self.__views = views
def __call__( self, drawTo ):
views = self.__views;
def draw_chain( base_view, canvas ):
drawTo( base_view, canvas )
for view in views:
view.drawTo( canvas, base_view )
return draw_chain
Text和border分別繼承於View，text會將文字畫到canvas，而border則是畫框([])，此時可以看到text已經裝飾了border "@ViewDecorator``( Border() )"。
class Border( View ):
"""
Extend a View.
"""
def drawTo( self, canvas, parent = None ):
"""
Draw a border to canvas by parent position.
"""
if parent:
width = parent.getWidth()
x = parent.getX()
y = parent.getY()
canvas.draw( "[", x - 1 , y )
canvas.draw( "]", width + x , y )
class Text( View ):
"""
Extend a view.
"""
def __init__( self, content ):
"""
Set a text.
"""
self.__content = content
def getWidth( self ):
"""
Get a width of text.
"""
return len( self.__content )
@ViewDecorator( Border() )
def drawTo( self, canvas, parent = None ):
"""
Draw a text to canvas.
"""
canvas.draw( self.__content, self.getX(), self.getY() )
執行python decorator.py
canvas = Canvas() text = Text( "Sparrow" ) text.setX( 3 ) text.setY( 2 ) text.drawTo( canvas ) canvas.show()
最後就會顯示下列資訊在螢幕上。
[Sparrow]