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| <h1 class="content-header header-section1" id="dgui_datamodel_types" itemprop="headline">The types</h1> |
| </div></div><div class="page-menu"> |
| <div class="page-menu-title">Page Contents</div> |
| <ul><li><a class="page-menu-link" href="#dgui_datamodel_scalar" data-menu-target="dgui_datamodel_scalar">Scalars</a></li><li><a class="page-menu-link" href="#dgui_datamodel_container" data-menu-target="dgui_datamodel_container">Containers</a></li><li><a class="page-menu-link" href="#autoid_15" data-menu-target="autoid_15">Subroutines</a><ul><li><a class="page-menu-link" href="#dgui_datamodel_method" data-menu-target="dgui_datamodel_method">Methods and functions</a></li><li><a class="page-menu-link" href="#dgui_datamodel_userdefdir" data-menu-target="dgui_datamodel_userdefdir">User-defined directives</a></li><li><a class="page-menu-link" href="#autoid_16" data-menu-target="autoid_16">Function/method versus user-defined directive</a></li></ul></li><li><a class="page-menu-link" href="#autoid_17" data-menu-target="autoid_17">Miscellaneous</a><ul><li><a class="page-menu-link" href="#dgui_datamodel_node" data-menu-target="dgui_datamodel_node">Nodes</a></li></ul></li></ul> </div><p>The suppored types are:</p><ul> |
| <li> |
| <a href="#dgui_datamodel_scalar">Scalars:</a> |
| |
| <ul> |
| <li> |
| String |
| </li> |
| |
| <li> |
| Number |
| </li> |
| |
| <li> |
| Boolean |
| </li> |
| |
| <li> |
| Date |
| </li> |
| </ul> |
| </li> |
| |
| <li> |
| <a href="#dgui_datamodel_container">Containers:</a> |
| |
| <ul> |
| <li> |
| Hash |
| </li> |
| |
| <li> |
| Sequence |
| </li> |
| |
| <li> |
| Collection |
| </li> |
| </ul> |
| </li> |
| |
| <li> |
| Subroutines: |
| |
| <ul> |
| <li> |
| <a href="#dgui_datamodel_method">Methods and |
| functions</a> |
| </li> |
| |
| <li> |
| <a href="#dgui_datamodel_userdefdir">User-defined |
| directives</a> |
| </li> |
| </ul> |
| </li> |
| |
| <li> |
| Miscellaneous/seldom used: |
| |
| <ul> |
| <li> |
| <a href="#dgui_datamodel_node">Node</a> |
| </li> |
| </ul> |
| </li> |
| </ul> |
| |
| |
| |
| |
| <h2 class="content-header header-section2" id="dgui_datamodel_scalar">Scalars</h2> |
| |
| |
| <a name="topic.designer.scalarVariable"></a> |
| |
| <p>These are the basic, simple kind of values. They can |
| be:</p> |
| |
| <ul> |
| <li> |
| <p>String: It is simple text, e.g., the name of a |
| product.</p> |
| |
| <p>If you want to give a string value directly in the |
| template, rather than use a variable that comes from the data |
| model, you write the text between quotation marks, e.g., |
| <code class="inline-code">"green mouse"</code> or <code class="inline-code">'green |
| mouse'</code>. (More details regarding the syntax can be |
| found <a href="dgui_template_exp.html#dgui_template_exp_direct_string">later</a>.)</p> |
| </li> |
| |
| <li> |
| <p>Number: For example the price of a product. |
| <span class="marked-for-programmers">Whole numbers and non-whole |
| numbers are not distinguished; there is only a single number |
| type. So for example 3/2 will be always 1.5, and never 1. Just |
| like if you are using a calculator.</span></p> |
| |
| <p>If you want to give a numerical value directly in the |
| template, then you write for example: <code class="inline-code">150</code> or |
| <code class="inline-code">-90.05</code> or <code class="inline-code">0.001</code>. (More |
| details regarding the syntax can be found <a href="dgui_template_exp.html#dgui_template_exp_direct_number">later</a>.)</p> |
| </li> |
| |
| <li> |
| <p>Boolean: A boolean value represents a logical true |
| or false (yes or no). For example, if a the visitor has been |
| logged in or not. Typically you use booleans as the condition of |
| the <code class="inline-code">if</code> directive, like <code class="inline-code"><#if |
| loggedIn |
| ><em class="code-color">...</em></#if></code> or |
| <code class="inline-code"><#if price == |
| 0><em class="code-color">...</em></#if></code>; in |
| the last case the result of the <code class="inline-code">price == 0</code> |
| part is a boolean value.</p> |
| |
| <p>In the templates you can directly specify a boolean with |
| the reserved words <code class="inline-code">true</code> and |
| <code class="inline-code">false</code>.</p> |
| </li> |
| |
| <li> |
| <p>Date: A date variable stores date/time related |
| data. It has three variations:</p> |
| |
| <ul> |
| <li> |
| <p>A date with day precision (often referred simply as |
| "date") as April 4, 2003</p> |
| </li> |
| |
| <li> |
| <p>Time of day (without the date part), as 10:19:18 PM. |
| Time is stored with millisecond precision.</p> |
| </li> |
| |
| <li> |
| <p>Date-time (sometimes called "time stamp") as April 4, |
| 2003 10:19:18 PM. The time part is stored with millisecond |
| precision.</p> |
| </li> |
| </ul> |
| |
| <p>Unfortunately, because of the limitations of the Java |
| platform, FreeMarker sometimes can't decide which parts of the |
| date are in use (i.e., if it is date-time, or a time of day, |
| etc.). The solution for this problem is an advanced topic that |
| will be discussed <a href="ref_builtins_date.html#ref_builtin_date_datetype">later</a>.</p> |
| |
| <p>It is possible to define date values directly in |
| templates, but this is an advanced topic that will be explained |
| <a href="ref_builtins_string.html#ref_builtin_string_date">later</a>.</p> |
| </li> |
| </ul> |
| |
| <p>Bear in mind that FreeMarker distinguishes strings from |
| numbers and booleans, so the string <code class="inline-code">"150"</code> and the |
| number <code class="inline-code">150</code> are totally different. A number holds |
| a numerical value. A boolean holds a logical true or false. A string |
| holds an arbitrary sequence of characters.</p> |
| |
| |
| |
| |
| |
| <h2 class="content-header header-section2" id="dgui_datamodel_container">Containers</h2> |
| |
| |
| |
| |
| <p>These are the values whose purpose is to contain other |
| variables; they are just containers. The contained variables are |
| often referred as <em>subvariables</em>. The container |
| types are:</p> |
| |
| <ul> |
| <li> |
| <p>Hash: Associates a unique lookup name with each of |
| its subvariables. The name is an unrestricted string. A hash |
| <em>doesn't define an ordering</em> for the |
| subvariables in it. That is, there is no such thing as the first |
| subvariable, and the second subvariable, etc.; the variables are |
| just accessed by name.</p> |
| </li> |
| |
| <li> |
| <p>Sequence: Associates an integer number with each |
| of its subvariables. The first subvariable is associated with 0, |
| the second with 1, the third to 2, and so on; the subvariables |
| are ordered. These numbers are often called the |
| <em>indexes</em> of the subvariables. Sequences are |
| usually dense, i.e., all indexes up to the index of the last |
| subvariable have an associated subvariable, but it's not |
| strictly necessary. The type of the subvariable values need not |
| be the same.</p> |
| </li> |
| |
| <li> |
| <p>Collection: A collection, from the viewpoint of |
| the template author, is a restricted sequence. You cannot access |
| its size or retrieve its subvariables by index, but they can be |
| still listed with the <a href="ref_directive_list.html#ref.directive.list"><code>list</code> |
| directive</a>.</p> |
| </li> |
| </ul> |
| |
| <p>Note that since <a href="dgui_datamodel_basics.html#topic.multitype">a value can |
| have multiple types</a>, it is possible for a value to be both a |
| hash and a sequence, in which case it would support index-based |
| access as well as access by lookup name. However, typically a |
| container will be either a hash or a sequence, not both.</p> |
| |
| <p>As the value of the variables stored in hashes and sequences |
| (and collections) can be anything, it can be a hash or sequence (or |
| collection) as well. This way you can build arbitrarily deep |
| structures.</p> |
| |
| <p>The data-model itself (or better said the root of it) is a |
| hash.</p> |
| |
| |
| |
| |
| |
| <h2 class="content-header header-section2" id="autoid_15">Subroutines</h2> |
| |
| |
| |
| |
| |
| |
| |
| <h3 class="content-header header-section3" id="dgui_datamodel_method">Methods and functions</h3> |
| |
| |
| <a name="topic.designer.methodVariable"></a> |
| |
| |
| |
| <p>A value that is a method or a function is used to calculate |
| another value, influenced by the parameters you give to it.</p> |
| |
| <p><span class="marked-for-programmers">For programmer types: |
| Methods/functions are first-class values, just like in functional |
| programming languages. This means that functions/methods can be |
| the parameters or return values of other functions/methods, you |
| can assign them to variables, and so on.</span></p> |
| |
| <p>Suppose that programmers have put the method variable |
| <code class="inline-code">avg</code> in the data-model that can be used to |
| calculate the average of numbers. If you give the 3 and 5 as |
| parameters when you access <code class="inline-code">avg</code>, then you get |
| the value 4.</p> |
| |
| <p>The usage of methods will be explained <a href="dgui_template_exp.html#dgui_template_exp_methodcall">later</a>, but perhaps |
| this example helps to understand what methods are:</p> |
| |
| |
| |
| <div class="code-wrapper"><pre class="code-block code-template">The average of 3 and 5 is: ${avg(3, 5)} |
| The average of 6 and 10 and 20 is: ${avg(6, 10, 20)} |
| The average of the price of a python and an elephant is: |
| ${avg(animals.python.price, animals.elephant.price)}</pre></div> |
| |
| <p>this will output:</p> |
| |
| |
| |
| <div class="code-wrapper"><pre class="code-block code-output">The average of 3 and 5 is: 4 |
| The average of 6 and 10 and 20 is: 12 |
| The average of the price of a python and an elephant is: |
| 4999.5</pre></div> |
| |
| <p>What is the difference between a method and a function? As |
| far as the template author is concerned, nothing. Well not really |
| nothing, as methods typically come from the data-model (<span class="marked-for-programmers">as they reflect the methods of Java |
| objects</span>), and functions are defined in templates (with |
| the <a href="ref_directive_function.html#ref.directive.function"><code>function</code> |
| directive</a> -- an advanced topic), but both can be used on |
| the same way.</p> |
| |
| |
| |
| |
| |
| |
| |
| <h3 class="content-header header-section3" id="dgui_datamodel_userdefdir">User-defined directives</h3> |
| |
| |
| |
| |
| |
| |
| |
| |
| <p>A value of this type can be used as user-defined directive |
| (with other words, as FreeMarker tag). An user-defined directive |
| is a subroutine, something like a little reusable template |
| fragment. But this is an advanced topic that will be explained |
| <a href="dgui_misc_userdefdir.html">later</a> in its own |
| chapter.</p> |
| |
| <p><span class="marked-for-programmers">For programmer types: |
| user-defined directives (such as macros), are first-class values |
| too, just like functions/methods are.</span></p> |
| |
| <p>Just to get an idea about user-defined directives (so just |
| ignore this if you won't understand), assume we have a variable, |
| <code class="inline-code">box</code>, whose value is a user-defined directive |
| that prints some kind of fancy HTML message box with a title bar |
| and a message in it. The <code class="inline-code">box</code> variable could be |
| used in the template like this (for example):</p> |
| |
| |
| |
| <div class="code-wrapper"><pre class="code-block code-template"><@<strong>box</strong> title="Attention!"> |
| Too much copy-pasting may leads to |
| maintenance headaches. |
| </@<strong>box</strong>></pre></div> |
| |
| |
| |
| |
| |
| |
| |
| <h3 class="content-header header-section3" id="autoid_16">Function/method versus user-defined directive</h3> |
| |
| |
| <p>This is for advanced users again (so ignore it if you don't |
| understand). It's a frequent dilemma if you should use a |
| function/method or an user-defined directive to implement |
| something. The rule of thumb is: Implement the facility as |
| user-defined directive instead of as function/method if:</p> |
| |
| <ul> |
| <li> |
| <p>... the output (the return value) is markup (HTML, XML, |
| etc.). The main reason is that the result of functions are |
| subject to automatic XML-escaping (due to the nature of |
| <code class="inline-code">${<em class="code-color">...</em>}</code>), while |
| the output of user-defined directives are not (due to the |
| nature of |
| <code class="inline-code"><@<em class="code-color">...</em>></code>; |
| its output is assumed to be markup, and hence already |
| escaped).</p> |
| </li> |
| |
| <li> |
| <p>... it's the side-effect that is important and not the |
| return value. For example, a directive whose purpose is to add |
| an entry to the server log is like that. (In fact you can't |
| have a return value for a user-defined directive, but some |
| kind of feedback is still possible by setting non-local |
| variables.)</p> |
| </li> |
| |
| <li> |
| <p>... it will do flow control (like for example |
| <code class="inline-code">list</code> or <code class="inline-code">if</code> directives |
| do). You just can't do that with a function/method |
| anyway.</p> |
| </li> |
| </ul> |
| |
| <p>The Java methods of FreeMarker-unaware Java objects are |
| normally visible as methods in templates, regardless of the nature |
| of the Java method. That said, you have no choice there.</p> |
| |
| |
| |
| |
| |
| |
| <h2 class="content-header header-section2" id="autoid_17">Miscellaneous</h2> |
| |
| |
| |
| |
| |
| |
| |
| <h3 class="content-header header-section3" id="dgui_datamodel_node">Nodes</h3> |
| |
| |
| |
| |
| <p>Node variables represent a node in a tree structure, and are |
| used mostly with <a href="xgui.html">XML processing</a>, which |
| is an advanced, and specialized topic.</p> |
| |
| <p>Still, a quick overview <em>for advanced |
| users</em>: A node is similar to a sequence that stores |
| other nodes, which are often referred as the children nodes. A |
| node stores a reference to its container node, which is often |
| referred as the parent node. The main point of being a node is the |
| topological information; other data must be stored by utilizing |
| that a value can have multiple types. Like, a value may be both a |
| node and a number, in which case it can store a number as the |
| "pay-load". Apart from the topological information, a node can |
| store some metainformation as well: a node name, a node type |
| (string), and a node namespace (string). For example, if the node |
| symbolizes a <code class="inline-code">h1</code> element in an XHTML document, |
| then its name could be <code class="inline-code">"h1"</code>, it's node type |
| could be <code class="inline-code">"element"</code>, and it's namespace could be |
| <code class="inline-code">"http://www.w3.org/1999/xhtml"</code>. But it's up to |
| the designer of the data-model if what meaning these |
| metainformations have, and if they are used at all. The way of |
| retrieving the topological and metainformations is described <a href="ref_builtins_node.html">in a later chapter</a> (that you |
| don't have to understand at this point).</p> |
| |
| <div class="bottom-pagers-wrapper"><div class="pagers bottom"><a class="paging-arrow previous" href="dgui_datamodel_basics.html"><span>Previous</span></a><a class="paging-arrow next" href="dgui_template.html"><span>Next</span></a></div></div></div></div> </div> |
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