1 ==========================
2 HTML Templating Mechanisms
3 ==========================
5 :Version: $Revision: 1.7 $
7 Current Situation and Issues
8 ============================
10 Syntax
11 ------
13 Roundup currently uses an element-based HTML-tag-alike templating syntax::
15 <display call="checklist('status')">
17 The templates were initially parsed using recursive regular expression
18 parsing, and since no template tag could be encapsulate itself, the parser
19 worked just fine. Then we got the ``<require>`` tag, which could have other
20 ``<require>`` tags inside. This forced us to move towards a more complete
21 parser, using the standard python sgmllib/htmllib parser. The downside of this
22 switch is that constructs of the form::
24 <tr class="row-<display call="plain('status')">">
26 don't parse as we'd hope. It would be almost impossible to modify the sgmllib
27 parser to parse the above "correctly", so a wholly new parser would be
28 required. That is a large undertaking, and doesn't address another couple of
29 issues that have arisen:
31 1. the template syntax is not well-formed, and therefore is a pain to parse
32 and doesn't play well with other tools, and
33 2. user requirements generally have to be anticipated and accounted for in
34 templating functions (like ``plain()`` and ``checklist()`` above), and
35 we are therefore artificially restrictive.
37 Arguments for switching templating systems:
39 *Pros*
41 - more flexibility in templating control and content
42 - we can be well-formed
44 *Cons*
46 - installed user base (though they'd have to edit their templates with the
47 next release anyway)
48 - current templating system is pretty trivial, and a more flexible system
49 is likely to be more complex
52 Templates
53 ---------
55 We should also take this opportunity to open up the flexibility of the
56 templates through:
58 1. allowing the instance to define a "page" template, which holds the overall
59 page structure, including header and footer
63 Possible approaches
64 ===================
66 Zope's PageTemplates
67 --------------------
69 Using Zope's PageTemplates seems to be the best approach of the lot.
70 In my opinion, it's the peak of HTML templating technology at present. With
71 appropriate infrastructure, the above two examples would read:
73 <span tal:replace="item/status/checklist">status checklist</span>
75 <tr tal:attributes="class string:row-${item/status/name}">
77 ... which doesn't look that much more complicated... honest...
79 Other fun can be had when you start playing with stuff like:
81 <table>
82 <tr tal:repeat="message item/msg/list">
83 <td tal:define="from message/from">
84 <a href="" tal:attributes="href string:mailto:${from/address}"
85 tal:content="from/name">mailto link</a>
86 </td>
87 <td tal:content="message/title">subject</td>
88 <td tal:content="message/created">received date</td>
89 </tr>
90 </table>
92 PageTemplates in a Nutshell
93 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
95 PageTemplates consist of three technologies:
97 TAL - Template Attribute Language
98 This is the syntax which is woven into the HTML using the ``tal:`` tag
99 attributes. A TAL parser pulls out the TAL commands from the attributes
100 runs them using some expression engine.
102 TALES - TAL Expression Language
103 The expression engine used in this case is TALES, which runs the expressions
104 that form the tag attribute values. TALES expressions come in three
105 flavours:
107 Path Expressions - eg. ``foo/bar/frozz``
108 These are object attribute / item accesses. Roughly speaking, the path
109 ``foo/bar/frozz`` is broken into parts ``foo``, ``bar`` and ``frozz``. The
110 ``foo`` part is the root of the expression. We then look for a ``bar``
111 attribute on foo, or failing that, a bar item (as in foo['bar']). If that
112 fails, the path expression fails. When we get to the end, the object we're
113 left with is evaluated to get a string - methods are called, objects are
114 stringified. Path expressions may have an optional ``path:`` prefix, though
115 they are the default expression type, so it's not necessary.
117 String Expressions - eg. ``string:hello ${user/name}``
118 These expressions are simple string interpolations (though they can be just
119 plain strings with no interpolation if you want. The expression in the
120 ``${ ... }`` is just a path expression as above.
122 Python Expressions - eg. ``python: 1+1``
123 These expressions give the full power of Python. All the "root level"
124 variables are available, so ``python:foo.bar.frozz()`` might be equivalent
125 to ``foo/bar/frozz``, assuming that ``frozz`` is a method.
127 PageTemplates
128 The PageTemplates module glues together TAL and TALES.
131 Implementation
132 ~~~~~~~~~~~~~~
134 I'm envisaging an infrastructure layer where each template has the following
135 "root level" (that is, driectly accessible in the TALES namespace) variables
136 defined:
138 *user*
139 the current user node as an HTMLItem instance
140 *class*
141 the current class of node being displayed as an HTMLClass instance
142 *item*
143 the current node from the database, if we're viewing a specific node, as an
144 HTMLItem instance
145 (*classname*)
146 the current node is also available under its classname, so a *user* node
147 would also be available under the name *user*. This is also an HTMLItem
148 instance.
149 *form*
150 the current CGI form information as a mapping of form argument name to value
151 *instance*
152 the current instance
153 *db*
154 the current open database
155 *config*
156 the current instance config
157 *util*
158 utility methods
159 *modules*
160 Python modules made available (XXX: not sure what's actually in there tho)
162 Accesses through a class (either through *class* or *db.<classname>*):
164 class HTMLClass:
165 def __getattr__(self, attr):
166 ''' return an HTMLItem instance '''
167 def classhelp(self, ...)
168 def list(self, ...)
170 Accesses through an *item*::
172 class HTMLItem:
173 def __getattr__(self, attr):
174 ''' return an HTMLItem instance '''
175 def history(self, ...)
176 def classhelp(self, ...)
177 def remove(self, ...)
179 String, Number, Date, Interval HTMLProperty
180 a wrapper object which may be stringified for the current plain() behaviour
181 and has methods emulating all the current display functions, so
182 ``item/name/plain`` would emulate the current ``call="plain()``". Also,
183 ``python:item.name.plain(name=value)`` would work just fine::
185 class HTMLProperty:
186 def __init__(self, instance, db, ...)
187 def __str__(self):
188 return self.plain()
190 class StringHTMLProperty(HTLProperty):
191 def plain(self, ...)
192 def field(self, ...)
193 def stext(self, ...)
194 def multiline(self, ...)
195 def email(self, ...)
197 class NumberHTMLProperty(HTMLProperty):
198 def plain(self, ...)
199 def field(self, ...)
201 class BooleanHTMLProperty(HTMLProperty):
202 def plain(self, ...)
203 def field(self, ...)
205 class DateHTMLProperty(HTMLProperty):
206 def plain(self, ...)
207 def field(self, ...)
208 def reldate(self, ...)
210 class IntervalHTMLProperty(HTMLProperty):
211 def plain(self, ...)
212 def field(self, ...)
214 Link HTMLProperty
215 the wrapper object would include the above as well as being able to access
216 the class information. Stringifying the object itself would result in the
217 value from the item being displayed. Accessing attributes of this object
218 would result in the appropriate entry from the class being queried for the
219 property accessed (so item/assignedto/name would look up the user entry
220 identified by the assignedto property on item, and then the name property of
221 that user)::
223 class LinkHTMLProperty(HTMLProperty):
224 ''' Be a HTMLItem too '''
225 def __getattr__(self, attr):
226 ''' return a new HTMLProperty '''
227 def download(self, ...)
228 def checklist(self, ...)
230 Multilink HTMLProperty
231 the wrapper would also be iterable, returning a wrapper object like the Link
232 case for each entry in the multilink::
234 class MultilinkHTMLProperty(HTMLProperty):
235 def __len__(self):
236 ''' length of the multilink '''
237 def __getitem(self, num):
238 ''' return a new HTMLItem '''
239 def checklist(self, ...)
240 def list(self, ...)
242 *util*
243 the util object will handle::
245 class Util:
246 def __init__(self, ...)
247 def filterspec(self, ...)
248 def note(self, ...)
249 def submit(self, ...)
251 Action
252 ======
254 1. Investigate how PageTemplates would be integrated into Roundup:
256 - we could go for a fully-divorced-from-Zope approach, which would involve
257 bundling PageTemplates/TAL/ZTUtils in with Roundup, with all the
258 Zope-specific bits removed.
259 - we could try to coexist with a Zope installation, but there the problem
260 would be that Zope includes its own copy of PageTemplates/TAL/ZTUtils and
261 we'd be installing a version in site-packages, which would be bad.
263 The latter may allow nicer integration with Zope itself, giving Zope
264 Roundup users access to acquired information in their templates. We could
265 get around that by modifying the ZRoundup interface to use the "real Zope"
266 ZPT. Maybe.
268 2. Implement the Roundup infrastructure described in the `implementation`_
269 above.