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<A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></a>
<UL CLASS="ChildLinks">
<LI><A href="node12.html#SECTION0012100000000000000000">11.1 Introduction</a>
<LI><A href="node12.html#SECTION0012200000000000000000">11.2 Basic example</a>
<LI><A href="node12.html#SECTION0012300000000000000000">11.3 How is it used in CookieAuthenticate and HttpAuthenticate</a>
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<H1><A NAME="SECTION0012000000000000000000">
11. How to use AOP (Aspect Oriented Programing) with CherryPy</A>
</H1>
<P>
<H1><A NAME="SECTION0012100000000000000000">
11.1 Introduction</A>
</H1>
This HowTo is intended to people who are already familiar with OOP, but who are not necessarily familiar with AOP.
If this is your case, check out <a class="url" href="http://aosd.net">http://aosd.net</a> for an introduction to AOP.
<P>
CherryPy only implements a few of AOP concepts. I don't have a PHD in AOP and it is not my goal to make
CherryPy an AOP reference :-)
<P>
Instead, I implemented a few features that I thought might be useful (and they already are, since both
HttpAuthenticate and CookieAuthenticate modules use AOP features).
<P>
<H1><A NAME="SECTION0012200000000000000000">
11.2 Basic example</A>
</H1>
On most websites a common header and footer is used for all (or most) pages of the site. In CherryPy, you
can easily implement that using a "classic" object oriented approach: first you define a class that contains
the header and the footer that you want to use for all your pages.
<P>
<div class="verbatim"><pre>
CherryClass NormalHeaderAndFooter:
mask:
def header(self):
<html><body>I'm the header<br><br>
def footer(self):
<br><br>I'm the footer</body></html>
</pre></div>
<P>
Then, all you have to do is derive your CherryClasses from <var>NormalHeaderAndFooter</var> and call <var>header</var>
and <var>footer</var> in each of your masks or views:
<P>
<div class="verbatim"><pre>
CherryClass Root(NormalHeaderAndFooter):
mask:
def index(self):
<py-eval="self.header()">
Hello, world !
<py-eval="self.footer()">
def otherPage(self):
<py-eval="self.header()">
I love cherry pie !
<py-eval="self.footer()">
</pre></div>
<P>
That's the "classic" object oriented approach. It's pretty good, but if we look at it, we'll realize that we
have to repeat <var><py-eval="self.header()"></var> and <var><py-eval="self.footer()"></var> for each mask and view, and this
has several drawbacks:
<P>
<UL>
<LI>First of all, I <b>never</b> like to repeat code. Whenever I do it, I feel like there must be something
I'm doing wrong and my code could certainly be improved !
</LI>
<LI>For a real life website, we probably would have many CherryClasses that inherit from <var>NormalHeaderAndFooter</var>, and all these
CherryClasses would have many masks that all make the same call to <var>header</var> and <var>footer</var>. What if we want to change
the name of the <var>header</var> method ? In that case, we would have to go through all the masks of all the CherryClasses to
change the line !
</LI>
<LI>In this trivial example, we only have 2 lines of code to repeat, but what if we had other things to do for each page
(like log the request in a database or check that the user is authenticated) ? We could end up repeating tens of lines
of code for each method !
</LI>
</UL>
<P>
That's where AOP comes in ... Instead of just declaring "normal" <var>header</var> and <var>footer</var> methods, we'll declare
"aspect" methods. This basically adds another information which is: "include my code at the beginning or at the end
of each of the methods of all derived classes".
<P>
Therefore, the implementation of our simple example using AOP would be:
<P>
<div class="verbatim"><pre>
CherryClass NormalHeaderAndFooter:
aspect:
(1) start:
_page.append("<html><body>I'm the header<br><br>")
(1) end:
_page.append("<br><br>I'm the footer</body></html>")
CherryClass Root(NormalHeaderAndFooter):
mask:
def index(self):
Hello, world !
def otherPage(self):
I love cherry pie !
</pre></div>
<A NAME="tex2html1"
HREF="#foot239"><SUP>11.1</SUP></A>
<P>
See ... we got rid of the <var><py-eval="self.header()"></var> and <var><py-eval="self.footer()"></var> lines ...
<P>
Let's see what we can notice from looking at the code:
<UL>
<LI>In CherryClass <var>NormalHeaderAndFooter</var>, the "mask" section has disappeared and has been replaced by a new "aspect" section.
</LI>
<LI>In this "aspect" section, the line where we normally declare methods (using the "def" keyword) has been replaced by a special
line that contains "start" or "end" (we'll explain the syntax later).
</LI>
<LI>The lines that normally contain the body of the function are still there, and they're written using regular Python.
</LI>
<LI><var><py-eval="self.header()"></var> and <var><py-eval="self.footer()"></var> have disappeared from the methods of <var>Root</var>.
<var>Root</var> is still derived from <var>NormalHeaderAndFooter</var>.
</LI>
</UL>
<P>
Here is how it works:
<P>
The header for an aspect method has two parts:
<div class="verbatim"><pre>
(condition) startOrEnd:
</pre></div>
<var>startOrEnd</var> can be either the "start" or the "end" keyword. This indicates whether the code should be appended at
the start or at the end of the methods.
<P>
<var>condition</var> is a python expression used to indicated which method the aspect should apply to. If the condition
is always true (as in "1"), then it means that the aspect will apply to all methods of the derived CherryClasses. The
python expression can use a special variable called <var>method</var> that contains the following member variables:
<UL>
<LI><var>name</var>: contains the name of the method
</LI>
<LI><var>type</var>: contains the type of the method ("mask", "view", "variable", ...)
</LI>
<LI><var>isHidden</var>: is true if the mask or view is hidden (false otherwise)
</LI>
<LI><var>className</var>: name of the CherryClass the method belongs to
</LI>
</UL>
<P>
Let's go back to our example: let's imagine that we want to use the regular header and footer for both
pages <var>index</var> and <var>otherPage</var>, but we want a third page called <var>yetAnotherPage</var> that has its own header
and footer.
Here is what the code would look like:
<div class="verbatim"><pre>
CherryClass NormalHeaderAndFooter:
aspect:
(method.type=='mask' and method.name!='yetAnotherPage') start:
_page.append("<html><body>I'm the header<br><br>")
(method.type=='mask' and method.name!='yetAnotherPage') end:
_page.append("<br><br>I'm the footer</body></html>")
CherryClass Root(NormalHeaderAndFooter):
mask:
def index(self):
Hello, world !
def otherPage(self):
I love cherry pie !
def yetAnotherPage(self):
<html><body bgcolor=red>
I love cherry pie !
</body></html>
</pre></div>
As we can see, we use the aspect condition to indicate which method the aspect will apply to. Note that
the "method.type='mask'" condition is useless, since all methods of <var>Root</var> are masks. So it is
only there to show you an example of an aspect condition.
<P>
<H1><A NAME="SECTION0012300000000000000000">
11.3 How is it used in CookieAuthenticate and HttpAuthenticate</A>
</H1>
Now that you know how it works, you can look at the source code for these two modules and you should be able to
understand it.
<P>
Basically, when you declare a CherryClass that inherits from CookieAuthenticate or HttpAuthenticate, some code is
automatically added at the beginning of each of your masks and views. This code checks if the user is authenticated
or not. If not, it returns the login page instead of the regular page ...
<P>
<BR><HR><H4>Footnotes</H4>
<DL>
<DT><A NAME="foot239">...
</A><A
HREF="node12.html#tex2html1"><SUP>11.1</SUP></A></DT>
<DD>This sample code requires CherryPy-0.7 or later to work
</DD>
</DL>
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<span class="release-info">Release 0.10, documentation updated on 19 March 2004.</span>
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