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🦉 Templates 🦉

Content

Overview

Owl templates are describe using the QWeb specification. It is based on the XML format, and used mostly to generate HTML. In OWL, QWeb templates are compiled into functions that generate a virtual dom representation of the HTML. Also, since Owl is a live component system, there are additional directives specific to Owl (such as t-on).

<div>
    <span t-if="somecondition">Some string</span>
    <ul t-else="">
        <li t-foreach="messages" t-as="message">
            <t t-esc="message"/>
        </li>
    </ul>
</div>

Template directives are specified as XML attributes prefixed with t-, for instance t-if for conditionals, with elements and other attributes being rendered directly.

To avoid element rendering, a placeholder element <t> is also available, which executes its directive but doesn’t generate any output in and of itself.

We present in this section the templating language, including its Owl specific extensions.

Directives

For reference, here is a list of all standard QWeb directives:

Name Description
t-esc Outputting safely a value
t-out Outputting value, possibly without escaping
t-set, t-value Setting variables
t-if, t-elif, t-else, conditionally rendering
t-foreach, t-as Loops
t-att, t-attf-*, t-att-* Dynamic attributes
t-call Rendering sub templates
t-debug, t-log Debugging
t-translation Disabling the translation of a node

The component system in Owl requires additional directives, to express various needs. Here is a list of all Owl specific directives:

Name Description
t-component, t-props Defining a sub component
t-ref Setting a reference to a dom node or a sub component
t-key Defining a key (to help virtual dom reconciliation)
t-on-* Event handling
t-portal Portal
t-slot, t-set-slot, t-slot-scope Rendering a slot
t-model Form input bindings
t-tag Rendering nodes with dynamic tag name

QWeb Template Reference

White Spaces

White spaces in a template are handled in a special way:

  • consecutive whitespaces are always condensed to a single whitespace
  • if a whitespace-only text node contains a linebreak, it is ignored
  • the previous rules do not apply if we are in a <pre> tag

Expression Evaluation

QWeb expressions are strings that will be processed at compile time. Each variable in the javascript expression will be replaced with a lookup in the context (so, the component). For example, a + b.c(d) will be converted into:

context["a"] + context["b"].c(context["d"]);

It is useful to explain the various rules that apply on these expressions:

  1. it should be a simple expression which returns a value. It cannot be a statement.

    <div><p t-if="1 + 2 === 3">ok</p></div>

    is valid, but the following is not valid:

    <div><p t-if="console.log(1)">NOT valid</p></div>

  2. it can use anything in the rendering context (typically, the component):

    <p t-if="user.birthday === today()">Happy bithday!</p>

    is valid, and will read the user object from the context, and call the today function.

  3. it can use a few special operators to avoid using symbols such as <, >, & or |. This is useful to make sure that we still write valid XML.

    Word replaced with
    and &&
    or ||
    gt >
    gte >=
    lt <
    lte <=

    So, one can write this:

    <div><p t-if="10 + 2 gt 5">ok</p></div>

Static Html Nodes

Normal, regular html nodes are rendered into themselves:

  <div>hello</div> <!–– rendered as itself ––>

Outputting Data

The t-esc directive is necessary whenever you want to add a dynamic text expression in a template. The text is escaped to avoid security issues.

<p><t t-esc="value"/></p>

rendered with the value value set to 42 in the rendering context yields:

<p>42</p>

The t-out directive is almost the same as t-esc, but possibly without the escaping. The difference is that the value received by the t-out directive will only be not-escaped if it has been marked as such, using the markup utility function:

For example, in the following component:

const { markup, Component, xml } = owl;

class SomeComponent extends Component {
  static template = xml`
    <t t-out="value1"/>
    <t t-out="value2"/>`;

  value1 = "<div>some text 1</div>";
  value2 = markup("<div>some text 2</div>");
}

The first t-out will act as a t-esc directive, which means that the content of value1 will be escaped. However, since value2 has been tagged as a markup, this will be injected as html.

Setting Variables

QWeb allows creating variables from within the template, to memoize a computation (to use it multiple times), give a piece of data a clearer name, ...

This is done via the t-set directive, which takes the name of the variable to create. The value to set can be provided in two ways:

  1. a t-value attribute containing an expression, and the result of its evaluation will be set:

    <t t-set="foo" t-value="2 + 1"/>
<t t-esc="foo"/>

    will print 3. Note that the evaluation is done at rendering time, not at compilte time.

  2. if there is no t-value attribute, the node’s body is saved and its value is set as the variable’s value:

    <t t-set="foo">
    <li>ok</li>
</t>
<t t-esc="foo"/>

    will generate &lt;li&gt;ok&lt;/li&gt; (the content is escaped as we used the t-esc directive)

The t-set directive acts like a regular variable in most programming language. It is lexically scoped (inner nodes are sub scopes), can be shadowed, ...

Conditionals

The t-if directive is useful to conditionally render something. It evaluates the expression given as attribute value, and then acts accordingly.

<div>
    <t t-if="condition">
        <p>ok</p>
    </t>
</div>

The element is rendered if the condition (evaluated with the current rendering context) is true:

<div>
    <p>ok</p>
</div>

but if the condition is false it is removed from the result:

<div>
</div>

The conditional rendering applies to the bearer of the directive, which does not have to be <t>:

<div>
    <p t-if="condition">ok</p>
</div>

will give the same results as the previous example.

Extra conditional branching directives t-elif and t-else are also available:

<div>
    <p t-if="user.birthday == today()">Happy bithday!</p>
    <p t-elif="user.login == 'root'">Welcome master!</p>
    <p t-else="">Welcome!</p>
</div>

Dynamic Attributes

One can use the t-att- directive to add dynamic attributes. Its main use is to evaluate an expression (at rendering time) and bind an attribute to its result:

For example, if we have id set to 32 in the rendering context,

<div t-att-data-action-id="id"/> <!-- result: <div data-action-id="32"></div> -->

If an expression evaluates to a falsy value, it will not be set at all:

<div t-att-foo="false"/>  <!-- result: <div></div> -->

It is sometimes convenient to format an attribute with string interpolation. In that case, the t-attf- directive can be used. It is useful when we need to mix literal and dynamic elements, such as css classes. The dynamic elements can be specified with either {{...}} or #{...}:

<div t-attf-foo="a {{value1}} is #{value2} of {{value3}} ]"/>
<!-- result if values are set to 1,2 and 3: <div foo="a 0 is 1 of 2 ]"></div> -->

If we need completely dynamic attribute names, then there is an additional directive: t-att, which takes either an object (with keys mapping to their values) or a pair [key, value]. For example:

<div t-att="{'a': 1, 'b': 2}"/> <!-- result: <div a="1" b="2"></div> -->

<div t-att="['a', 'b']"/> <!-- <div a="b"></div> -->

Dynamic class attribute

For convenience, Owl supports a special case for the t-att-class case: one can use an object with keys describing the classes, and values boolean value denoting if the class is or is not present:

<div t-att-class="{'a': true, 'b': true}"/> <!-- result: <div class="a b"></div> -->

<div t-att-class="{'a b': true, 'c': true}"/> <!-- result: <div class="a b c"></div> -->

Note that it can be combined with normal class attribute:

<div class="a" t-att-class="{'b': true}"/> <!-- result: <div class="a b"></div> -->

Dynamic tag names

When writing generic components or templates, the specific concrete tag for an HTML element is not known yet. In those situations, the t-tag directive is useful. It simply evaluates dynamically an expression to use as a tag name. The template:

<t t-tag="tag">
    <span>content</span>
</t>

will be rendered as <div><span>content</span></div> if the tag context key is set to div.

Loops

QWeb has an iteration directive t-foreach which take an expression returning the collection to iterate on, and a second parameter t-as providing the name to use for the current item of the iteration:

<t t-foreach="[1, 2, 3]" t-as="i" t-key="i">
    <p><t t-esc="i"/></p>
</t>

will be rendered as:

<p>1</p>
<p>2</p>
<p>3</p>

Like conditions, t-foreach applies to the element bearing the directive’s attribute, and

<p t-foreach="[1, 2, 3]" t-as="i" t-key="i">
    <t t-esc="i"/>
</p>

is equivalent to the previous example.

An important difference should be made with the usual QWeb behaviour: Owl requires the presence of a t-key directive, to be able to properly reconcile renderings.

t-foreach can iterate on an array (the current item will be the current value) or an object (the current item will be the current key).

In addition to the name passed via t-as, t-foreach provides a few other variables for various data points (note: $as will be replaced with the name passed to t-as):

  • $as_value: the current iteration value, identical to $as for lists and integers, but for objects, it provides the value (where $as provides the key)
  • $as_index: the current iteration index (the first item of the iteration has index 0)
  • $as_first: whether the current item is the first of the iteration (equivalent to $as_index == 0)
  • $as_last: whether the current item is the last of the iteration (equivalent to $as_index + 1 == $as_size), requires the iteratee’s size be available

These extra variables provided and all new variables created into the t-foreach are only available in the scope of the t-foreach. If the variable exists outside the context of the t-foreach, the value is copied at the end of the foreach into the global context.

<t t-set="existing_variable" t-value="false"/>
<!-- existing_variable now False -->

<p t-foreach="Array(3)" t-as="i" t-key="i">
    <t t-set="existing_variable" t-value="true"/>
    <t t-set="new_variable" t-value="true"/>
    <!-- existing_variable and new_variable now true -->
</p>

<!-- existing_variable always true -->
<!-- new_variable undefined -->

Even though Owl tries to be as declarative as possible, the DOM does not fully expose its state declaratively in the DOM tree. For example, the scrolling state, the current user selection, the focused element or the state of an input are not set as attribute in the DOM tree. This is why we use a virtual dom algorithm to make sure we keep the actual DOM node instead of replacing it with a new one.

Consider the following situation: we have a list of two items [{text: "a"}, {text: "b"}] and we render them in this template:

<p t-foreach="items" t-as="item" t-key="item_index"><t t-esc="item.text"/></p>

The result will be two <p> tags with text a and b. Now, if we swap them, and rerender the template, Owl needs to know what the intent is:

  • should Owl actually swap the DOM nodes,
  • or should it keep the DOM nodes, but with an updated text content?

This might look trivial, but it actually matters. These two possibilities lead to different results in some cases. For example, if the user selected the text of the first p, swapping them will keep the selection while updating the text content will not.

There are many other cases where this is important: input tags with their value, css classes and animations, scroll position...

So, the t-key directive is used to give an identity to an element. It allows Owl to understand if different elements of a list are actually different or not.

The above example could be modified by adding an ID: [{id: 1, text: "a"}, {id: 2, text: "b"}]. Then, the template could look like this:

<p t-foreach="items" t-as="item" t-key="item.id"><t t-esc="item.text"/></p>

The t-key directive is useful for lists (t-foreach). A key should be a unique number or string (objects will not work: they will be cast to the "[object Object]" string, which is obviously not unique).

Also, the key can be set on a t tag or on its children. The following variations are all equivalent:

<p t-foreach="items" t-as="item" t-key="item.id">
  <t t-esc="item.text"/>
</p>

<t t-foreach="items" t-as="item" t-key="item.id">
  <p t-esc="item.text"/>
</t>

<t t-foreach="items" t-as="item">
  <p t-key="item.id" t-esc="item.text"/>
</t>

If there is no t-key directive, Owl will use the index as a default key.

Note: the t-foreach directive only accepts arrays (lists) or objects. It does not work with other iterables, such as Set. However, it is only a matter of using the ... javascript operator. For example:

<t t-foreach="...items" t-as="item">...</t>

The ... operator will convert the Set (or any other iterables) into a list, which will work with Owl QWeb.

Sub Templates

QWeb templates can be used for top level rendering, but they can also be used from within another template (to avoid duplication or give names to parts of templates), using the t-call directive:

<div t-name="other-template">
    <p><t t-value="var"/></p>
</div>

<div t-name="main-template">
    <t t-set="var" t-value="owl"/>
    <t t-call="other-template"/>
</div>

will be rendered as <div><p>owl</p></div>. This example shows that the sub template is rendered with the execution context of the parent. The sub template is actually inlined in the main template, but in a sub scope: variables defined in the sub template do not escape.

Sometimes, one might want to pass information to the sub template. In that case, the content of the body of the t-call directive is available as a special magic variable 0:

<t t-name="other-template">
    This template was called with content:
    <t t-raw="0"/>
</t>

<div t-name="main-template">
    <t t-call="other-template">
        <em>content</em>
    </t>
</div>

will result in :

<div>
    This template was called with content:
    <em>content</em>
</div>

This can be used to define variables scoped to a sub template:

<t t-call="other-template">
    <t t-set="var" t-value="1"/>
</t>
<!-- "var" does not exist here -->

Note: by default, the rendering context for a sub template is simply the current rendering context (so, the current component). However, it may be useful to be able to specify a specific object as context. This can be done by using the t-call-context directive:

<t t-call="other-template" t-call-context="obj"/>

Dynamic sub templates

The t-call directive can also be used to dynamically call a sub template, using string interpolation. For example:

<div t-name="main-template">
    <t t-call="{{template}}">
        <em>content</em>
    </t>
</div>

Here, the name of the template is obtained from the template value in the template rendering context.

Debugging

The javascript QWeb implementation provides two useful debugging directives:

t-debug adds a debugger statement during template rendering:

<t t-if="a_test">
    <t t-debug=""/>
</t>

will stop execution if the browser dev tools are open.

t-log takes an expression parameter, evaluates the expression during rendering and logs its result with console.log:

<t t-set="foo" t-value="42"/>
<t t-log="foo"/>

will print 42 to the console.

Fragments

Owl 2 supports templates with an arbitrary number of root elements, or even just a text node. So, the following templates are all valid:

hello owl. This is just a text node!
<div>hello</div>
<div>hello</div>
<div>ola</div>
<div t-if="someCondition"><SomeChildComponent/></div>
<t t-if="someCondition"><SomeChildComponent/></t>

Inline templates

Most real applications will define their templates in a XML file, to benefit from the XML ecosystem, and to do some additional processing, such as translating them. However, in some cases, it is convenient to be able to define a template inline. To do so, one can use the xml helper function:

const { Component, xml } = owl;

class MyComponent extends Component {
  static template = xml`
      <div>
          <span t-if="somecondition">text</span>
          <button t-on-click="someMethod">Click</button>
      </div>
  `;

    ...
}

mount(MyComponent, document.body);

This function simply generates an unique string id, and register the template under that id in the internals of Owl, then return the id.

Rendering svg

Owl components can be used to generate dynamic SVG graphs:

class Node extends Component {
  static template = xml`
        <g>
            <circle t-att-cx="props.x" t-att-cy="props.y" r="4" fill="black"/>
            <text t-att-x="props.x - 5" t-att-y="props.y + 18"><t t-esc="props.node.label"/></text>
            <t t-set="childx" t-value="props.x + 100"/>
            <t t-set="height" t-value="props.height/(props.node.children || []).length"/>
            <t t-foreach="props.node.children || []" t-as="child">
                <t t-set="childy" t-value="props.y + child_index*height"/>
                <line t-att-x1="props.x" t-att-y1="props.y" t-att-x2="childx" t-att-y2="childy" stroke="black" />
                <Node x="childx" y="childy" node="child" height="height"/>
            </t>
        </g>
    `;
  static components = { Node };
}

class RootNode extends Component {
  static template = xml`
        <svg height="180">
            <Node node="graph" x="10" y="20" height="180"/>
        </svg>
    `;
  static components = { Node };
  graph = {
    label: "a",
    children: [
      { label: "b" },
      { label: "c", children: [{ label: "d" }, { label: "e" }] },
      { label: "f", children: [{ label: "g" }] },
    ],
  };
}

This RootNode component will then display a live SVG representation of the graph described by the graph property. Note that there is a recursive structure here: the Node component uses itself as a subcomponent.

Important note: Owl needs to properly set the namespace for each svg elements. Since Owl compile each template separately, it is not able to determine easily if a template is supposed to be included in a svg namespace or not. Therefore, Owl depends on a heuristic: if a tag is either svg, g or path, then it will be considered as svg. In practice, this means that each component or each sub templates (included with t-call) should have one of these tag as root tag.

Restrictions

Note that Owl templates forbid the use of tag and or attributes starting with the block- string. This restriction prevents name collision with the internal code of Owl.

<div><block-1>this will not be accepted by Owl</block-1></div>