Captain Protons MVC Advanced Example

**Dr Livingston** · Apr 15, 2005, 05:27

The model I have at the moment is still fragile and prone to change until I can get something concrete together to use the model and see what breaks, and where, under stress.

Unit Testing has helped in some way, but I'm not entirely happy as yet. So I've not thought about Javascript. My stance is that you put in the document HEAD the Javascript file, and use the DOM to manipulate the document based on IDs, from the client but...

This is another topic I think, as we now have rich clients using AJAX, JpSpan et al How are these effected by a recursive structure? I don't know myself at the moment.

If parent asked its children for <head> info, the child wouldn't even need to know about its parent, in which case the parent my as well be root.

I am finding it difficult to have one composite controller know about another composite controller as I said earlier in this thread, so this is basically what your asking as well. Doesn't really matter if it's the composite with the HEAD template fragment or not, as in a (composite) recursion all composites need to be treated the same.

**dylanegan** · Apr 15, 2005, 06:36

Originally Posted by Dr Livingston

For the recursion to work properly (on the fly) each composite has to have it's own ID, and it's own parents ID. I leave these as attributes in the XML file, on the otherhand WACT takes another approach for example.

This is what I want to set up. I would probably take a page out of your book relating to your XML layout. Using this I would define a page with certain actions, not the views, because an action could return a different view for success and one for error. I will just put something basic together below

PHP Code:


..... Read the XML file into an array of Actions to be called and with id + parent_id .....

foreach ($actions as $action) {

  $a = new $action['actionname'];

  $a->act();

  $views[$action['actionname']]['view'] = $a->getView();

  $views[$action['actionname']]['id'] = $action['id'];

  $views[$action['actionname']]['parent_id'] = $action['parent_id'];

}



$cv = new CompositeView;

$cv->addViews($views);

$cv->run();



echo $cv->display();

Now as you can see it's pretty basic and yeah basic. I hope it makes a little sense, but it's what I have in mind.

I think the CV should create a big XML file by reading the ids and parent_ids and then the XSL file will be created by using <xsl:include/> and creating one XSL file including all the XSL files from each view. Then just one transform and viola nice XHTML page.

**Ren** · Apr 15, 2005, 07:55

Originally Posted by dylanegan

I think the CV should create a big XML file by reading the ids and parent_ids and then the XSL file will be created by using <xsl:include/> and creating one XSL file including all the XSL files from each view. Then just one transform and viola nice XHTML page.

This is what I toying with atm, havent got very far yet tho.

Each view returns a xsl:template (considering more than one atm, using modes for things like javascript), that is placed in a XSLT.

Implemented so far... given a HTML template

Code:

<html xmlns:tp="urn:templating.experiments">
	<head>
		<title><tp:title /></title>
	</head>
	<body>
		<div id="navigation">
			<tp:navigation param="1" />
		</div>
		<div id="searchbox">
			<tp:searchbox />
		</div>
	</body>
</html>

that gets converted to stylesheet/template

Code:

<?xml version="1.0"?> <xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:php="http://php.net/xsl" xmlns:tp="urn:templating.experiments" version="1.0"><xsl:output method="xml" media-type="application/xhtml+xml" encoding="UTF-8" doctype-public="-//W3C//DTD XHTML 1.0 Strict//EN" doctype-system="http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd" omit-xml-declaration="yes" cdata-section-elements="script style"/><xsl:template match="/"><html> <head> <title><xsl:call-template name="title"/></title> </head> <body> <div id="navigation"> <xsl:call-template name="navigation"><xsl:with-param name="param" select="1"/></xsl:call-template> </div> <div id="searchbox"> <xsl:call-template name="searchbox"/> </div> </body> </html></xsl:template></xsl:stylesheet>

with some fairly trivial DOM manipulation.. the idea being that the navigation & searchbox subviews will eventually also add their own templates.

Also need to investigate the how capable calling PHP from an XSLT transform is, so can attempt something like ruby components run_component

so could have

<tp:runComponent controller="gallery" action="latestImage" /> converted to <xsl:call-template name="runComponent"><xsl:with-param name="controller" name="gallery" /><xsl:with-param name="action" value="latestImage" /></xsl:call-template> with that doing

<xsl:template name="runComponent">
<xsl:value-of select="php:function(some-magic-here)" />
</xsl:template>

which'll also get xsl:include/xsl:import'd into at XSLT building stage.

**DougBTX** · Apr 15, 2005, 09:41

Originally Posted by Dr Livingston

So I've not thought about Javascript.

This is probably because you are over-complicating the problem so much that you forget about solving the problem.

Take it all back down to the fundamentals. What you are writing is a script to take one string, and from that, output a second string. PHP does a good job at distancing you from that initial input string, but for any script you have to remember that that is where it starts, and that is where it ends.

Douglas

**Dr Livingston** · Apr 15, 2005, 10:01

Umm...

I didn't put much thought into Javascript as I'm a serverside developer, and I tend to stay away from Javascript nowadays. That's my excuse anyways

On the point of breaking it all down to the barebones, this is what I'm doing at the moment (see 'refactoring question' thread) but I'm not following your

script to take one string, and from that, output a second string.

Part, can you explain more? Thanks.

**Ren** · Apr 15, 2005, 10:17

Well, I think the problem isn't just entirely javascript. Just used it as an example.
CSS would be the another, sub-component-x requires some css rules to be available in the output.

**Dr Livingston** · Apr 15, 2005, 10:40

I agree with you on this point, and have not found a solution either...

If, during the recursion cycle you have some kind of 'map' to map the CSS to a given composite node, ie An array for example, you could inject this array index(es) into the composite it's self, would this help?

See... http://www.dofactory.com/Patterns/Patterns.aspx

Looking at the Visitor for a moment, the Visitor could visit the composite, passing the array index along with it? This is just a thought though, I have been wrong in the past.

Hope it helps anyways.

**Dr Livingston** · Apr 15, 2005, 12:25

Ren,

I have altered in a quick manner my original classes to accept a visitor as an example, so I hope you can follow this script.

For each composite there is, you basically want the class 'Visitor' to do your thing

Okay, here goes...

PHP Code:


class XmlWalker {

// see my composite post in my sig, as

// this class has no changes

}



// utils

class ApplicationFaultException extends Exception {

        public function __construct( $message ) {

            parent::__construct( $message );

        }

    }

    

    class IllegalParameterException extends Exception {

        public function __construct( $message ) {

            parent::__construct( $message );

        }

    }



interface IParser {

        public function getRoot();

        public function push( $fragment ); // DomElement

    }

    

    class XmlAdaptableCompositeParser implements IParser {

        private $decoratable;

        

        public function __construct( $decoratable ) {

            $this -> decoratable = $decoratable;

        }

        

        public function getRoot() {

            return $this -> decoratable -> getRoot();

        }

        

        public function push( $fragment ) {

            if( $fragment -> nodeName == 'fragment' ) {

                return $this -> decoratable -> push( $fragment );

            }

        }

    }

    

    class XmlCompositeParser implements IParser {

        private $root;

        private $visitor;

        

        public function __construct($fragment ) {

            if( !$fragment instanceof DomElement ) {

                throw new IllegalParameterException( 'thrown exception on, expected parameter(s) not found.' );

                break;

            }

            $this -> root = new Composite( $fragment );

        }

        

        public function getRoot() {

            return $this -> root;

        }

        

        public function push( $fragment ) {

            $composite = new Composite( $fragment );

            if( $tmp = $this -> traverse( $this -> getRoot(), $fragment ) ) {

                

                // added this part to original class

                $this -> visitor -> visit( $composite );

                

                $tmp -> attach( $composite );

            }

        }

        

        // added this part to original class

        public function accept( $visitor ) {

            $this -> visitor = $visitor;

        }

        

        private function traverse( $composite, $fragment ) {

            $parent = $fragment -> getElementsByTagName( 'parent' );

            $parent = $parent -> item( 0 ) -> nodeValue;

            if( $composite -> getId() == $parent ) {

                return $composite;

            } else {

                if( $composite -> hasChildren() ) {

                    $children = $composite -> getChildren();

                    foreach( $children as $child ) {

                        $c = new Composite( $fragment );

                        if( $tmp = $this -> traverse( $child, $fragment ) ) {

                            

                            // added this part to original class

                            $this -> visitor -> visit( $c );

                            

                            $tmp -> attach( $c );

                        }

                    }

                }

            }

        }

    }

See my composite post in my sig for the Component and it's Composite class, as they've not been altered either. Here is how it works,

PHP Code:


...

class Visitor {

        public function __construct() {

        }

        

        public function visit( $composite ) {

            echo( $composite -> getId().'<br />' );

            // Ren, i've only outputing a composites ID but

            // you need to do what you want at this point

        }

    }

    

    $dom = new DomDocument;

    $dom -> load( 'test.xml' );

    $fragment = $dom -> documentElement -> childNodes;

    

    $walker = new XmlWalker( $fragment -> item( 1 ) -> childNodes );

    $parser = new XmlCompositeParser( $fragment -> item( 1 ) );

    // adding a visitor alters interface :eek2:

    $parser -> accept( new Visitor() );

    // require a decorator to filter tags by the name of 'fragment'

    $parser = new XmlAdaptableCompositeParser( $parser );

    $walker -> walk( $parser );

...

I've only just knocked this up a few minutes ago, and it works but tread carefully all the same

**DougBTX** · Apr 15, 2005, 13:24

Originally Posted by Dr Livingston

but I'm not following your...part, can you explain more? Thanks.

Just a statment about what web development is

You get one string in, looks something like this:

Code:

GET /search?q=php HTTP/1.1
Host: www.google.com
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-GB; rv:1.7.6) Gecko/20050226 Firefox/1.0.1
Accept: text/xml,application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5
Accept-Language: en-gb,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 300
Proxy-Connection: keep-alive

And then you return a string, which looks something like this:

Code:

HTTP/1.x 200 OK
Cache-Control: private
Content-Type: text/html
Content-Encoding: gzip
Date: Fri, 15 Apr 2005 20:11:38 GMT

<html>
...
</html>

Fundamentally, that is all you are doing. Different parts of the string are handled by different programs (Apache handles the "GET /search?q=php HTTP/1.1" part for example) but utlimatley that is what you are doing. PHP parses some of the input string into arrays to make life easier for you, but you are still playing the "build a string" game.

I have to admit I didn't fully grap this until I started learning to use Ruby without Rails. Ruby is much more general than PHP, it isn't focused on web development. PHP is becomming more useful for non-web things, just as Ruby and Python are becomming more useful for web things. (With Ruby, this is mostly due to Rails.)

Beyond that, the challange is to build that output string as "fast" as possible. (I've overloaded "fast" a little there. I mean fast rendering of the HTML, fast development time, fast maintenance, everything.)

WRT Javascript and CSS, I treat them much like images. I just put them in a public directory on the server, and link to them from my HTML. I try to avoid building CSS per request, the same as I try to avoid building images per request. Hell, I try and build as little HTML per request as possible too; it all fits into the "render as fast as possible with as little effort on the part of the developer as possible" idea

WRT server-side development... I don't consider myself a server-side developer, more someone who builds web interfaces. In my case, that usually involves some web design and user interface design too, though nothing I would consider "heavy" server side development, like writing database servers or web servers or anything like that.

Douglas

**DougBTX** · Apr 15, 2005, 13:33

Originally Posted by Dr Livingston

On the point of breaking it all down to the barebones, this is what I'm doing at the moment

Looks more like you are just adding layers of complication, though I may have missed what it is you are tring to do. You might want to look at how Propel handles XML. It uses XML config files, but caches them as PHP files for performance reasons.

Personally, I just use PHP files for config anyway

Douglas

**DougBTX** · Apr 15, 2005, 13:35

Originally Posted by Dr Livingston

PHP Code:


// utils

class ApplicationFaultException extends Exception {

        public function __construct( $message ) {

            parent::__construct( $message );

        }

    }

    

    class IllegalParameterException extends Exception {

        public function __construct( $message ) {

            parent::__construct( $message );

        }

    }

I'm wondering if you get a kick out of having more LOC!

Just write the above like this:

PHP Code:


class ApplicationFaultException extends Exception {}

class IllegalParameterException extends Exception {}

No need for 6 lines of code when you can do it with 2

PHP Code:

And that's a bad code smell too. I think you could probably live without that whole class.

PHP Code:


        public function __construct($fragment ) {

            if( !$fragment instanceof DomElement ) {

                throw new IllegalParameterException( 'thrown exception on, expected parameter(s) not found.' );

                break;

            }

            $this -> root = new Composite( $fragment );

        }

Really?

Why do this yourself when PHP does it for you:

PHP Code:


function __construct(DomElement $fragment) {

    $this->root = new Composite($fragment);

}

Even if you do need to use exceptions, you never need to break after a throw. A throw is like an uber powerful return.

Douglas

**arborint** · Apr 15, 2005, 13:52

This is an interesting discussion that leads me to think that each component (I called them a "subMVC' earlier) in the heirarchy needs to not just a "view" in the block sense, but a group of related chunks that the parent can apply to build the final output. As Douglas says, the goal is just to output a string that is HTML. Following this conversation a compenent needs to send back at least two strings: one to go in the '<head>' chunk (CSS and Javascript) and one for the '<body>' chunk (HTML). So rather than just a render() function, perhaps at a minimum our View should look like:

PHP Code:


class HTMLView {

function addHead() {}

function addBody() {}

function renderHead() {}

function renderBody() {}

}

Regarding config information, I think you should be able to use either and as long as the object that provides config info has the same interface is shouldn't matter.

**Dr Livingston** · Apr 15, 2005, 14:11

Doug (can I call you Doug?),

I didn't know that extended Exceptions could be curtailed like that... Need to check that now

True, there may be no need for the Composite class, since most of it's functionality resides in the Component class, but looking at the pattern, a Composite is subclassed so I followed with it.

I also like the use of instanceof to catch illegal parameters, rather than leave it for type checking to catch them. Makes me feel safer, that's all. Hope this helps mate.

**Selkirk** · Apr 15, 2005, 16:29

I've been working on this composite view problem for the WACT controllers. I think I might have a reasonable solution soon. My current line of attack uses a ViewComposer object that is passed down the chain of responsibility represented by the controller tree. Its the not exactly the visitor pattern, but rather more like a collecting parameter pattern.

In this architecture, views house the ability to specify if they can "accept" a parent, or "accept" a child.

Movement along the chain of responsibility continues as long as the view composer 'isAccepting'. The ViewComposer continues accepting view building blocks as long as it has a place to put them (either as a parent or a child). Once the View runs out of places to accept new components into, then further processing is short circuited and the view is rendered.

There are roughly four kinds of View objects:

View - Stand alone
MasterView - accepts children
PartialView - accepts a parent
PartialMasterView - accepts a parent or a child.

A fifth kind of view, a DecoratingView, breaks some of the rules. The decorating view can accept a MasterView or a PartialView as a child and takes on some of the characteristics of the child in regards to the algorithm. These are used for meta-things like output caching, model building, and HTTP headers. Some of the things one might use intercepting filters for.

There are still some round peg, square hole type conflict resolution issues to work out. Mostly, I am leaning toward throwing out the old structure and starting a new one when a conflict occurs.

The algorithm I am focusing on is somewhat counter intuitive in that it builds the composite view from the leaves first and root last. It turns out there are some performance advantages to this and perhaps some usability advantages for the framework users. Its early to say.

There are drawbacks. For one, forwards are becoming extremely difficult, if not impossible to implement. In other threads I've postulated that forwards are how nonhierarchical frameworks clumsily simulate hierarchy. Wouldn't it be ironic if turned out that hierarchical frameworks ended up having to clumsily simulate forwards? Anyway, I am hoping that forwards don't prove necessary.

I think the key to pulling this off in the long run is relative addressing. Each component (M, V, or C) must address its neighbors in a relative way or you can never use it out of context and so there would be little gain in busting your application up into tiny little pieces.

There are alot of implementation details to work out, but so far, I think the concept is sound.

**dylanegan** · Apr 15, 2005, 16:34

Heya Ren,

The last bits you mention about calling a particular action or such. I am planning on using PHPTAL for this as it will all be done in the model area so the view only recieves XML data.

An example would be form management (I started a thread a while back) which would use something like <ns:form name="name"/> this would convert the form into the XML that is defined and if there are any current values, validation errors and so on it would grab them and parse them into the form and then once this result is passed to the view the view would have appropriate XSL to transform that into XForms, Web Forms 1/2 or what ever I would like.

**arborint** · Apr 15, 2005, 17:48

I've been working on this composite view problem for the WACT controllers.

It is interesting to hear how you are trying to solve this problem. One of my concerns with frameworks like WACT and Mojavi is they they end up solving a problem so specifically that it becomes a framework only for people who do it that way. They end up with hooks for implementation rather than hooks for concepts. I would more interested if something like WACT implemented a base controller architecture in which you could implement WACT, Mojavi, or other specific controller architectures. Not sure if it's possible, but one of the exercises I have been doing when experimenting with controllers is to see if I can implement the current WACT and Mojavi controllers behavior.

The View is very dependent on the template system used and the Model on the pattern/system used in the data layer. But the Controller has more dependencies which makes it interesting.

**Azmo** · Apr 16, 2005, 04:49

Originally Posted by Selkirk

In this architecture, views house the ability to specify if they can "accept" a parent, or "accept" a child.

An alternative might be to specify whether a View can be a parent, or be a child.

Originally Posted by Selkirk

There are roughly four kinds of View objects:

View - Stand alone
MasterView - accepts children
PartialView - accepts a parent
PartialMasterView - accepts a parent or a child.

I use Actions instead of Views. Each Action either can either fetch or process data, and has one or more ways of being used:

Fetch

Data - the Action returns its filtered data.
Block - the data is also parsed into a template, and the output is returned.
Main - data is parsed, a document device is fetched, and the block output is merged into the document template. The document is returned.

Process

Main - see above.
Subprocess - the Action is performed, but nothing is returned.

There's a document device for each type of action: document_fetch, and document_process. Fetch uses the regular website template, process uses a redirection template. The data of each processing Action contains a reference to where the user should be taken next.

Originally Posted by Selkirk

I think the key to pulling this off in the long run is relative addressing. Each component (M, V, or C) must address its neighbors in a relative way or you can never use it out of context and so there would be little gain in busting your application up into tiny little pieces.

I use named addressing to fetch application objects (which I call devices). Each device is identified by its name and module, and is built by passing the configuration file for that device to an object of the type defined in that configuration file. Code:

PHP Code:


class smsDeviceFactory

{

    public static function buildDevice( $deviceData )

    {

        $class_name            = $deviceData['config']['class']['name'];

        $device_name        = $deviceData['config']['device']['name'];

        $device_module        = $deviceData['config']['device']['module'];



        if ( !class_exists( $class_name ) )

        {

            $class_module        = $deviceData['config']['class']['module'];



            $class_path            = 'app/'. $class_module .'/source/';



            smsPath::readClassFile( $class_name, $class_path );

        }



        // Create device

        $device_instance    = new $class_name( $deviceData['config'] );



        return( $device_instance );

    }

}

All my devices extend the smsDevice type, which contains a method getDevice(). All I need to do to give one device access to another, is add a line like:

PHP Code:


$UserStorage = $this->getDevice('storage_user');

The really cool thing is that I can map devices to other devices, so any request for 'storage_user' will actually return the device 'storage' in the 'core' module. This should make it a lot easier to use multiple databases etc within a single application.

This solution is rather complicated, but I no longer have to pass devices from one to the other, because everything can be pulled. It's also possible to use only parts of the application, because each device is completely self-sufficient: it can solve its own dependencies, and gains all the knowledge it requires from its configuration.

**McGruff** · Apr 16, 2005, 09:28

There's something about parent-child designs which makes me uneasy. Would you say that knowledge about the structure of the browser page is leaking out of the presentation layer?

**arborint** · Apr 16, 2005, 09:49

There's something about parent-child designs which makes me uneasy. Would you say that knowledge about the structure of the browser page is leaking out of the presentation layer?

Not sure why it's making you uneasy. I think most presentation output lends itself to a hierarchical organization, both to promote reuse and manage complexity. As long as all the nodes remain in the presentation layer and access the Model layer appropriately is should not matter their number or organization.

I think the comments on "relative addressing" are interesting. I have been using a Service Locator for some of this and found it works well.

**McGruff** · Apr 16, 2005, 11:53

I'll need to waffle a bit before making my point.

In Cartesian dualism, a distinction is made between internal mind and an external reality. In programming, clear Cartesian boundaries between an internal application mind and external inputs/outputs are I think fundamental to good design. The data access layer is one and the view is another. With well-defined boundaries, the application "core" can use a variety of data sources and output in a variety of formats.

Composite views use template fragments rather than a unified, whole page template but all formatting knowledge, I think, belongs on the other side of the view boundary. The application can present a set of data without knowing anything about how that will be used to build a page.

When you include a template, you cross the boundary. Now you're in a whole new realm with a different skill set - html design if it's an html page. Html designers have their own tools to manage shared layout elements: the app doesn't need to concern itself with that at all. There's still some more php to come in looping through rows, conditionals, or simple echoes but the template engine is really a whole other app in its own right.

Composite data-gathering code would be OK and that's where I'd stop. All knowledge about shared headers, footers or etc should perhaps exist only on the other side of the boundary. Apart from code design issues, it makes life a whole lot easier for designers who no longer have to work with fragmented pages.

We're talking without reference to any real code of course: I don't know WACT well enough to say if this is a valid concern.

**Dr Livingston** · Apr 16, 2005, 12:45

Composite views use template fragments rather than a unified, whole page template but all formatting knowledge, I think, belongs on the other side of the view boundary.

If by the boundary I assume you mean the View, as apposed to the composite node yes? Well, I suppose I'd have to agree with you on that point, or are you talking about something else?

Please, waffle on...

**Ren** · Apr 16, 2005, 13:20

Originally Posted by McGruff

Composite views use template fragments rather than a unified, whole page template but all formatting knowledge, I think, belongs on the other side of the view boundary. The application can present a set of data without knowing anything about how that will be used to build a page.

Don't think templates necessarily imply fragments..

Im currently toying with having templates (even sub components) being entire/complete/validatable html page. So each component can have its own javascript & css etc, which eventually all gets merged.

So a designer can open any template in some sort of wysiwg tool.

Also opens up the possibility of using iframes for giving previews to composite views.

**arborint** · Apr 16, 2005, 13:29

I'll need to waffle a bit before making my point.

I know you are of the opinion of one template per page as you have stated that several times. I agree in principle, but in practice I find one template per page unworkable from a maintanence point of view. I have two sites for a car manufacturer with two divisions. It has a hundred templates and both sites run off the same codebase. If I used a template per page I would have 200 templates. Instead I have 100 plus a handfull of site specific templates.

Composite views use template fragments rather than a unified, whole page template but all formatting knowledge, I think, belongs on the other side of the view boundary. The application can present a set of data without knowing anything about how that will be used to build a page.

Really not sure where you are going here. What specifically is the "view boundry"? I don't see how single, multiple, or hierarchical views make any difference to model/view separation. What is the difference between a single template made of blocks where the view might have a method to deal with each block, or splitting the blocks into multiple files and having a distinct view object deal with each block. Same code, same separation, different orginazation.

**auricle** · Apr 16, 2005, 14:04

Originally Posted by Selkirk

I've been working on this composite view problem for the WACT controllers.

This was my impetus for copying a lot of WACT's Controller class but not using it as it is. Correct me if I'm wrong, but it seems that dispatchEvents() usually calls dispatchChild() which invokes execution of one child. If there were a mechanism for invoking execution of more than one child and adding their outputs to the response model, one could (would?) have the makings of a composite view.

I think the key to pulling this off in the long run is relative addressing. Each component (M, V, or C) must address its neighbors in a relative way or you can never use it out of context and so there would be little gain in busting your application up into tiny little pieces.

This may be what comes back to bite me (we'll see!).

**McGruff** · Apr 16, 2005, 14:19

Originally Posted by arborint

If I used a template per page I would have 200 templates. Instead I have 100 plus a handfull of site specific templates.

That could be handled by Dreamweaver libraries.

Originally Posted by arborint

What is the difference between a single template made of blocks where the view might have a method to deal with each block, or splitting the blocks into multiple files and having a distinct view object deal with each block.

Suppose for example you wanted to use the same application on the command line. You'd print the data in paragraphs not blocks. The Cartesian boundary cuts across the view between data (with no knowledge of how it will be output) on one side and formatting on the other. Template fragments, I think, are the smell which shows this boundary has been broken: information about page structure has crossed the line. The difference would be that you gather the same overall set of data in one object, then map it to page elements in the template, on the other side of the boundary.

Often you'll find that bits of data cut across page elements: eg a page title might be output in a header and in the < head > html. Gathering data for a page in a structure which mirrors the page layout can make it more awkward to share.

Incidentally you could identify another boundary - an input boundary - in the presentation layer which might provide the flex point for an easy switch between http and CLI. Any place where there is input or output there's a Cartesian boundary.

PS: I started off by wondering if information about page structure was leaking out of the presentation layer. What I meant to say was is it extending too far into the view.

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