Reinhard Poetz wrote: <snip/>
As promised, I've tried to come up with some pseudo-code that shows how "single-inheritance + composition" looks like in a particular example (portal + skinning).
That is excelent.
- o -
We have following block interfaces: =================================== - http://cocoon.apache.org/interfaces/portal/1.0 - http://cocoon.apache.org/interfaces/portal-skin/1.0 - http://mycompany.com/interfaces/skin/1.0
Here the block implementations: ===============================
---------------------------------------------------------------------------
"Portal"
---------------------------------------------------------------------------
BLOCK.XML
<block xmlns="http://apache.org/cocoon/blocks/cob/1.0";
id="http://cocoon.apache.org/blocks/portal/1.0.0";>
<name>portal</name>
<requirements>
<requires interface="http://cocoon.apache.org/interface/portal-skin/1.0";
name="portal"
default="http://cocoon.apache.org/blocks/portal-skin/1.0.0"/>
</requirements>
<implements>
<interface id="http://cocoon.apache.org/interfaces/portal/1.0"/>
</implements>
</block>
---------------------------------------------------------------------------
"Portal-Skin"
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BLOCK.XML <block xmlns="http://apache.org/cocoon/blocks/cob/1.0"; id="http://cocoon.apache.org/blocks/portal-skin/1.0.0";> <name>portal-skin</name> <implements> <interface id="http://cocoon.apache.org/interfaces/portal-skin/1.0"/> </implements> </block>
SITEMAP.XMAP <map:match pattern="*.css"> <map:read src="styles/css/{1}.css"/> </map:match> <map:match pattern="styles/portal-page.xsl"> <map:read src="styles/portal-page.xsl"/> </map:match>
---------------------------------------------------------------------------
"MyPortal"
---------------------------------------------------------------------------
Is MyPortal an actual application or is it a block that you are going to use for building applications? Let's call them application block and reusable block respectively. We must take application blocks into account to get the whole picture, so I assume that MyPortal is an application block that will contain real user profiles etc. Seeing that it neither implements nor extends but rather use the portal block enforces that view.
BLOCK.XML <block xmlns="http://apache.org/cocoon/blocks/cob/1.0"; id="http://mycompany.com/blocks/my-Portal/1.0.0";> <name>MyPortal</name> <requirements> <requires interface="http://mycompany.com/interfaces/skin/1.0"; name="skin" default="http://mycompany.com/blocks/myskin/1.0.0"/> <requires interface="http://cocoon.apache.org/interfaces/portal/1.0"; name="portal" default="http://cocoon.apache.org/blocks/portal/1.0.0"/> </requirements> </block>
SITEMAP.XMAP <map:match pattern="portal"> <map:act type="portal:auth-protect"> <map:parameter name="handler" value="portal-handler"/> <map:parameter name="application" value="portal"/>
<map:generate type="portal:portal"/> <map:transform src="blocks://skin/styles/portal-page.xsl"> <map:parameter name="user" value="{ID}"/> </map:transform> <map:transform type="core:cinclude"/> <map:transform type="portal:portal-coplet"/> <map:transform type="portal:portal-new-eventlink"/> <map:transform type="core:encodeURL"/> <map:serialize type="portal:html-include"/> </map:act> </map:match>
Shouldn't this sitemap be part of Portal rather than MyPortal, AFAICS it is a "blockified" version of the "portal" rule from the demo portal, so there seem no to be no reason to reimplement it in MyBlock.
(Note: Most of the used components come from the portal block, the rest from Cocon core; the stylesheet is provided by the "skin" block.)
---------------------------------------------------------------------------
"MySkin"
---------------------------------------------------------------------------
BLOCK.XML <block xmlns="http://apache.org/cocoon/blocks/cob/1.0"; id="http://mycompany.com/blocks/my-Portal/1.0.0";> <name>MySkin</name> <implements> <interface id="http://cocoon.apache.org/interfaces/portal/1.0"/> </implements> <extends>http://cocoon.apache.org/blocks/portal-skin/1.0.0</extends> </block>
What does extends actually buy us here, couldn't we just use "requrires" for the relation to portal-skin/1.0.0.
IIUC using "extends" was the original idea here.
>
<requires block="http://cocoon.apache.org/blocks/portal-skin/1.0.0"; name="skin"/>
and be explicit about what we expose from portal-skin/1.0.0?
SITEMAP.XMAP <map:match pattern="one-special.css"> <map:read src="styles/css/one-special.css"/> </map:match>
Being explicit means that we end the sitemap with:
<map:mount uri-prefix="" src="blocks://skin"/>
- o -
The project that wants to use the Portal is in the block "MyPortal". It needs several components from "Portal" (generator, several transformers) and it needs a block that provides the skin, or more precisly, it needs a block that implements http://cocoon.apache.org/interfaces/portal-skin/1.0. This can either be "portal-skin", the default skin, or "MySkin", that provides one additional CSS. Everything else is taken from "portal-skin".
From my POV this solution is very clear and comprehensible. The aspects "portal functionality" and "skinning" are separated and the used implementations can simply be replaced by other implementations (shown by using the "mySkin" block).
- o -
What does your solution that ueses multiple inheritance look like? (If this is a bad example to show the advantages of MI feel free to enhance it!)
First I will not use the term MI as it doesn't describe what I want to achieve that well and as it also seem to stir all kinds of bad reactions that distracts us from the task at hand.
So I agree with most of what you show in your example, it looks neat. What I lack from it is how to reuse the sitemap in the Portal block.
I would have a sitemap similar to the one in the demo portal in the Portal block. But e.g. the profiles part in the portal-handler configuration would rather be:
Portal Sitemap --------------
...
<profiles>
<copletbasedata-load uri="blocks:/load-global-profile?profile=copletbasedata"/>
<copletdata-global-load uri="blocks:/load-global-profile?profile=copletdata"/>
<copletdata-role-load uri="blocks:/load-role-profile?profile=copletdata"/>
<copletdata-user-load uri="blocks:/load-user-profile?profile=copletdata"/>
<copletinstancedata-global-load uri="blocks:/load-global-profile?profile=copletinstancedata"/>
<copletinstancedata-role-load uri="blocks:/load-role-profile?profile=copletinstancedata"/>
<copletinstancedata-user-load uri="blocks:/load-user-profile?profile=copletinstancedata"/>
<copletinstancedata-user-save uri="blocks:/save-user-profile?profile=copletinstancedata"/>
<layout-global-load uri="blocks:/load-global-profile?profile=layout"/>
<layout-role-load uri="blocks:/load-role-profile?profile=layout"/>
<layout-user-load uri="blocks:/load-user-profile?profile=layout"/>
<layout-user-save uri="blocks:/save-user-profile?profile=layout"/>
</profiles>
...
Meaning that the different configuration pipelines are found through the blocks manager that would ask the extending block (recursively) for the configuration pipelines first, and if they not are found there, the own pipeline would be used.
Then MyPortal could redefine some of the configuration pipelines and reuse the rest from Portal:
MyPortal Sitemap ----------------
... <pipeline> <match pattern="load-user-profile"> ... </match>
<mount uri-prefix="" src="blocks://portal"/> </pipline> ...
Now this mechanism is more limited than real inheritance. map:mount become a two way contract where the mounting sitemap can be asked about services through the block manager, but it doesn't export the interface of the "extended" block. If we have something like the we probably should have some way to differ between mounts that allow the mounted block to ask and those who don't.
(I think) I understand what you want. You called it an "application block" which is the base for all your applications. This application block (e.g. a company portal) provides services that can be used and customized by other blocks.
--- o0o ---
Concerning the skin I find it somewhat burocratic to need to define a new block for beeing able to extend it but I'm ok with it for the time beeing, we will see when we start to use the things. What I would prefer would be to do something like:
MyPortal Sitemap ----------------
... <pipeline> <match pattern="load-user-profile"> ... </match>
<match pattern="skin/one-special.css"> <read src="styles/css/one-special.css"/> </match>
<mount uri-prefix="skin" src="blocks://skin"/>
<mount uri-prefix="" src="blocks://portal"/> </pipline> ...
--- o0o ---
So what do you think about this?
I think I got the idea. Personally, I would solve this by composition; the profiling is just another reference of the block. Of course, as Stefano said, blocks have to be designed for this. If the application block hasn't factored out the profile aspect, then you can't replace implementation A by implementation B:
--------------------------------------------------------------------------- "Application block" (to be reused in many projects) --------------------------------------------------------------------------- <block xmlns="http://apache.org/cocoon/blocks/cob/1.0"; id="http://mycompany.com/blocks/application-block/1.0.0";> <name>Application block</name> <requirements> <requires interface="http://mycompany.com/interfaces/skin/1.0"; name="skin" default="http://mycompany.com/blocks/myskin/1.0.0"/> <requires interface="http://cocoon.apache.org/interfaces/profile/1.0"; name="profile" default="http://cocoon.apache.org/blocks/portal/1.0.0"/> <requires interface="http://cocoon.apache.org/interfaces/portal/1.0"; name="portal" default="http://cocoon.apache.org/blocks/portal/1.0.0"/> </requirements> </block>
What I do like is <mount uri-prefix="" src="blocks://portal"/> which makes it explicit what a block exports. I'm not sure about why this has to be a "two way contract".
If you write <copletdata-role-load uri="blocks:/load-role-profile?profile=copletdata"/> then it means that you make it explicit what can be overriden and what not.
I would use <copletdata-role-load uri="blocks:/profile/load-role-profile?profile=copletdata"/> which requires another reference. Using the proposed <mount>-mechanism, you can reuse the "portal"-pipeline.
- o -
I think for now we shoudn't support these two-way contracts but favour composition by references (incl. <map:mount uri="[another-block]"/>). If this gets too bureaucratic, we can still think about alternatives.
WDYT?
--
Reinhard P�tz Independent Consultant, Trainer & (IT)-Coach
{Software Engineering, Open Source, Web Applications, Apache Cocoon}web(log): http://www.poetz.cc --------------------------------------------------------------------
