` 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: ```js 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. ```xml``` is valid, but the following is not valid: ```xmlok
``` 2. it can use anything in the rendering context (which typically contains the properties of the component): ```xmlNOT valid
Happy bithday!
``` 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: ```xml``` ### Static Html Nodes Normal, regular html nodes are rendered into themselves: ```xmlok
hello``` ### 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. ```xml``` rendered with the value `value` set to `42` in the rendering context yields: ```html
42
``` 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: ```js const { markup, Component, xml } = owl; class SomeComponent extends Component { static template = xml``; value1 = " some text 1"; value2 = markup("some text 2"); } ``` 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. `markup` can also be used as a tag function, allowing the interpolated values to be safely escaped: ```js const maliciousInput = ""; // <script>alert('💥💥')</script> const value = markup`${maliciousInput}`; ``` ### 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: ```xml``` 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: ```xml ok ``` will generate `<li>ok</li>` (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. ```xml ``` The element is rendered if the condition (evaluated with the current rendering context) is true: ```xmlok
``` but if the condition is false it is removed from the result: ```xmlok
``` The conditional rendering applies to the bearer of the directive, which does not have to be ``: ```xml ``` will give the same results as the previous example. Extra conditional branching directives `t-elif` and `t-else` are also available: ```xmlok
``` ### 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, ```xml ``` If an expression evaluates to a falsy value, it will not be set at all: ```xml ``` 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 `#{...}`: ```xml ``` 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: ```xml ``` ### 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: ```xml ``` Note that it can be combined with normal class attribute: ```xml ``` ### 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: ```xmlHappy bithday!
Welcome master!
Welcome!
content ``` will be rendered as `content` 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: ```xml``` will be rendered as: ```xml
1
2
3
``` Like conditions, `t-foreach` applies to the element bearing the directive’s attribute, and ```xml``` 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 any iterable, and also has special support for objects and maps, it will expose the key of the current iteration as the contents of the `t-as`, and the corresponding value with the same name and the suffix `_value`. In addition to the name passed via t-as, `t-foreach` provides a few other useful variables (note: `$as` will be replaced with the name passed to `t-as`): - `$as_value`: the current iteration value, identical to `$as` for arrays and other iterables, but for objects and maps, 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. ```xml
``` 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: ```xml
``` The result will be two `
` 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: ```xml
``` 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: ```xml
``` 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: ```xml ... ``` 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: ```xml
``` will be rendered as `owl
This template was called with content: content This template was called with content: content``` This can be used to define variables scoped to a sub template: ```xml``` Note: by default, the rendering context for a sub template is simply the current rendering context. 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: ```xml ``` ### Dynamic sub templates The `t-call` directive can also be used to dynamically call a sub template, using string interpolation. For example: ```xml ``` 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: ```xmlcontent ``` 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: ```xml ``` will print 42 to the console. ### Custom Directives Owl 2 supports the declaration of custom directives. To use them, an Object of functions needs to be configured on the owl APP: ```js new App(..., { customDirectives: { test_directive: function (el, value) { el.setAttribute("t-on-click", value); } } }); ``` The functions will be called when a custom directive with the name of the function is found. The original element will be replaced with the one modified by the function. This : ```xml ``` will be replaced by : ```xml ``` ## 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: ```xml hello owl. This is just a text node! ``` ```xml hello``` ```xmlhelloola``` ```xml``` ```xml``` ## 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: ```js const { Component, xml } = owl; class MyComponent extends Component { static template = xml` text`; ... } 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: ```js class Node extends Component { static template = xml``; static components = { Node }; } class RootNode extends Component { static template = xml` `; 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. ```xml this will not be accepted by Owl