Yew-Template

A crate for using separate HTML files as Yew objects, with support for seamless localization.

Getting Started

Hello World

<div>
    <p>Hello {{name}}!</p>
</div>

let html = template_html!("templates/hello.html", name="World");

The code above will actually compile to the following code:

let html = yew::html! {
    <div>
        <p>{"Hello World!"}</p>
    </div>
};

Usage

• Variables
• Attributes
• Struct fields
• Expressions
• Example: Yew callbacks
• Components
• Optional variables
• Optional elements
• Iterators
• Minimizing bloat
• Virtual elements
• Localization
• Config
• Features
• Security Notes

Variables

let name = "World";
let html = template_html!("templates/hello.html", name);

Would compile to:

let name = "World";
let html = yew::html! {
    <div>
        <p>{"Hello "}{name}{"!"}</p>
    </div>
};

When the name of your variable isn't the same as the name in the template, you can use the following syntax:

let other_name = "Yew";
let html = template_html!("templates/hello.html", name=other_name);

Attributes

<div style={{style}}>
   <p>Hello {{name}}!</p>
</div>

let html = template_html!(
    "templates/hello.html",
    name="Yew",
    style="color: red;"
);

Yew-template supports a format!-like syntax in attributes, allowing you to do the following:

<div style="background-color: {{bg_color}}; color: {{text_color}};">
   Yew is cool
</div>

Struct fields

Sometimes you want to pass many struct fields as variables to your template, but destructuring the struct would be too verbose. As when using the actual yew macro, you can just pass the struct and access its fields from the template:

<div>
   <p>Hello {{person.first_name}} {{person.last_name}}!</p>
</div>

struct Person {
    first_name: String,
    last_name: String,
}

let person = Person {
    first_name: "Edouard".to_string(),
    last_name: "Foobar".to_string()
};
let html = template_html!("templates/fields.html", person);

Expressions

let name_reversed = String::from("dlroW");
let html = template_html!(
    "templates/hello.html",
    name = {
        let mut name = name_reversed.into_bytes();
        name.reverse();
        let name = String::from_utf8(name).unwrap();
        name
    }
);

Which will also display Hello World! as the Yew-code output is as follows:

let name_reversed = String::from("dlroW");
let html = yew::html! {
    <div>
        <p>
            {"Hello "}{{
            let mut name = name_reversed.into_bytes();
            name.reverse();
            let name = String::from_utf8(name).unwrap();
            name
            }}{"!"}
        </p>
    </div>
};

Note that the curly brackets around expressions are required for expressions.

Example with Yew callbacks

<div onclick={{onclick}}>
   <p>Hello {{name}}!</p>
</div>

let link = ctx.link();
let html = template_html!(
    "templates/hello.html",
    name="World",
    onclick={link.callback(|_| Msg::AddOne)}
);

Components

While yew-template can be used only with raw HTML, it is also possible to use Yew components in your templates. These do not follow the same syntax as in Yew's html macro, and need to be explicitly marked as components using the comp or component tag name.

<comp name="SearchBar"/>
<!-- Which is equivalent to -->
<component name="SearchBar"/>
<!-- Or even -->
<Component name="SearchBar"/>

As you can see, the rust identifier for the component is passed as an attribute.

Other attributes and even children can be passed the regular way provided that your component supports them.

<comp name="SearchBar" placeholder="Search..." onclick={{onclick}}>
    <span>child 1</span>
    <span>child 2</span>
</comp>

Optional variables

Optional variables are marked with an opt_ prefix or an _opt suffix, at your option. Their value is expected to be an Option<T>.

Optional variables work with optional html elements. Mark an element with the opt attribute to make it optional. An optional element will only be rendered if ALL the optional variables it contains are Some. Note that variables contained by smaller optional elements are excluded from this requirement.

<div>
    <p>Hello {{name}}!</p>
    <div opt>
        <h2>Age</h2>
        <p>You are {{opt_age}} years old!</p>
    </div>
</div>

In the example above, the div block will not be shown if opt_age is None.

Let's see how optional elements can be nested.

<div>
    <p>Hello {{name}}!</p>
    <div opt>
        <h2>Age</h2>
        <p>You are {{opt_age}} years old!</p>
        <p opt>And you are born in {{opt_birth_city}}.</p>
    </div>
</div>

Here, both opt_age and opt_birth_city are optional. opt_age would be displayed even if opt_birth_city is None. However, if opt_age is None, opt_birth_city will not be displayed regardless of its value.

From the Rust side, there is no usage difference. Note that curly brackets are required (for now).

let opt_age: Option<u8> = Some(20);
let opt_birth_city: Option<String> = None;
let html = template_html!(
    "templates/opt.html",
    name="John",
    opt_age,
    opt_birth_city
);

In the generated Yew code, if let expressions are used. As a result, optional variables based on expressions behave differently as they are only evaluated once for each optional element using them.

Optional elements

Sometimes optional variables are not suitable for making an element optional. You might need a logic that is more complex than just checking if a variable is Some or None. In this case, you can use optional elements.

Elements can be given a present-if attribute. The value will be evaluated at runtime as a boolean expression. If the expression is true, the element will be rendered. Otherwise, it will be skipped.

<div present-if={{condition}}>
    <p>1+1 = 3</p>
</div>
<div present-if=!{{condition}}> <!-- Negation is supported -->
    <p>1+1 != 3</p>
</div>

let html = template_html!("templates/present_if.html", condition={ 1+1==3 });

Iterators

Iterators work similarly to optional variables. The iterator variables are marked with an iter_ prefix or an _iter suffix, at your option. The looping html element is marked with the iter attribute. The element will reproduce until one of the iterators it depends on is empty.

<div>
    <h2>Contributors:</h2>
    <ul>
        <li iter>{{contributors_iter}} ({{commits_iter}} commits)</li>
    </ul>
</div>

let contributors = vec!["John", "Jane", "Jack"]; // Owned values need to be declared as `let` or they would be freed before the template is rendered.
let html = template_html!(
    "templates/iter.html",
    contributors_iter = {contributors.iter()},
    commits_iter = {[42, 21, 7].iter()}
);

The code above will act as the following for Yew:

let contributors = vec!["John", "Jane", "Jack"];
let html = yew::html! {
    <div>
        <h2>{"Contributors:"}</h2>
        <ul>
            {{
                let mut contributors_iter = { contributors.iter() };
                let mut commits_iter = { [42, 21, 7].iter() };
                let mut fragments = Vec::new();
                while let (Some(contributor), Some(commits)) = (contributors_iter.next(), commits_iter.next()) {
                    fragments.push(html! { <li>{contributor}{" ("}{commits}{" commits)"}</li> });
                }
                fragments.into_iter().collect::<Html>()
            }}
        </ul>
   </div>
};

As of now, Yew item references in lists are not supported. This will be inmplemented in the future as the Yew documentation recommends, though the performance impact has been found to be negligible in most cases.

Minimizing bloat

The whole point of using this crate is making your code more readable than when using Yew directly. However, you will still find yourself writing lines of code that do not carry that much meaning. We already saw that variable_ident=variable_ident can be shortened to variable_ident. But it could even be completely omitted! Add ... at the end of your macro call to tell that undefined variables should be retrieved from local variables with the same name. Taking the "Hello world" example:

<div>
    <p>Hello {{name}}!</p>
</div>

let name = "World";
let html = template_html!("templates/hello.html", ...);

This behavior is disabled by default because missing variables are often mistakes. If you want to enable it without have to add ... to every macro call, please set auto_default to true in your config.

Virtual elements

Yew-template often requires you to add attributes on html elements such as iter, opt or present-if. In rare cases, you don't have any suitable element to add these attributes to, and adding a wrapper element would break your CSS. In this case, you can use virtual elements. The virtual elements tag will be removed from the final HTML but it allows you to add special attributes where they are needed.

<virtual opt>
    {{opt_name}}
</virtual>

let opt_name = Some("John".to_string());
let html = template_html!("templates/virtual.html", opt_name);

On Yew side, this will be seen as:

let opt_name = Some("John".to_string());
let html = yew::html! {
   <>
      if let Some(opt_name) = opt_name { {opt_name} }
  </>
};

And Yew will produce the following HTML:

John

Localization

Yew-template supports localization. It is able to extract localization data from .po files and automatically embed them in the generated code. Enabling this feature is as simple as putting .po files in a directory.

The i18n cargo feature needs to be enabled (it is enabled by default).

By default, the locale directory is set to locales. You can change this by setting locale_directory in your config. Yew template will automatically generate an up-to-date .pot file in this directory. Use it in your translation software as a template to generate .po files.

When done translating, put your .po files in the locale directory. Support for the added locales will automatically be enabled.

In order to select the locale to be rendered at runtime, you need to pass a locale variable to template-html macro calls. This variable will be matched against the filenames of the .po files in the locale directory (exluding the .po extension). If no match is found, the string will be left as it appears in your template.

Instead of using a locale variable, you can decide to evaluate any Rust expression. See the locale_code option in the config section.

Yew-template prevents code injection from localized strings. This is done by escaping double quotes and backslashes. It is SAFE to delegate translation to unknown peers. However, these strings can include variable references, which could break compilation if referenced variables are not defined. Yew-template will take care of this issue in the future.

Config

You can specify various settings in a yew-template.toml file at the crate root. This requires the config cargo feature to be enabled (it is enabled by default).

This is the default configuration:

# Whether to attempt to capture local variables instead of aborting when arguments required by the template are missing.
auto_default = false

# Where to look for templates (relative to crate root)
template_directory = './'

# Where to look for locales (relative to crate root)
locale_directory = './locales/'

# Rust code to evaluate as locale. Should evaluate to a &str.
# If will be inserted in generated code like this: `match locale_code {`.
locale_code = 'locale.as_str()'

# Two strings marking the beginning and end of a variable in a template.
variable_separator = ["{{", "}}"]

Features

All features are enabled by default. There currently two features:

• config: Allows you to use yew-template.toml settings
• i18n: Enables support for localization

Security Notes

• It is safe to display all kinds of strings. They will be escaped appropriately, preventing both HTML and Rust injection.
• Localized strings are harmless in the generated code, but they could break compilation.
• Do not use untrusted template files.
• Do not use untrusted yew-template.toml files.

License: MIT