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Leaf Overview

Leaf is a powerful templating language with Swift-inspired syntax. You can use it to generate dynamic HTML pages for a front-end website or generate rich emails to send from an API.

This guide will give you an overview of Leaf's syntax and the available tags.

Template syntax

Here is an example of a basic Leaf tag usage.

There are #count(users) users.

Leaf tags are made up of four elements:

  • Token #: This signals the leaf parser to begin looking for a tag.
  • Name count: that identifies the tag.
  • Parameter List (users): May accept zero or more arguments.
  • Body: An optional body can be supplied to some tags using a semicolon and a closing tag

There can be many different usages of these four elements depending on the tag's implementation. Let's look at a few examples of how Leaf's built-in tags might be used:

#(variable)
#extend("template"): I'm added to a base template! #endextend
#export("title"): Welcome to Vapor #endexport
#import("body")
#count(friends)
#for(friend in friends): <li>#(friend.name)</li> #endfor

Leaf also supports many expressions you are familiar with in Swift.

  • +
  • >
  • ==
  • ||
  • etc.
#if(1 + 1 == 2):
    Hello!
#endif

Context

In the example from Getting Started, we used a [String: String] dictionary to pass data to Leaf. However, you can pass anything that conforms to Encodable. It's actually preferred to use Encodable structs since [String: Any] is not supported. This means you can not pass in an array, and should instead wrap it in a struct:

struct WelcomeContext: Encodable {
    var title: String
    var numbers: [Int]
}
return req.view.render("home", WelcomeContext(title: "Hello!", numbers: [42, 9001]))

That will expose title and numbers to our Leaf template, which can then be used inside tags. For example:

<h1>#(title)</h1>
#for(number in numbers):
    <p>#(number)</p>
#endfor

Usage

Here are some common Leaf usage examples.

Conditions

Leaf is able to evaluate a range of conditions using its #if tag. For example, if you provide a variable it will check that variable exists in its context:

#if(title):
    The title is #(title)
#else:
    No title was provided.
#endif

You can also write comparisons, for example:

#if(title == "Welcome"):
    This is a friendly web page.
#else:
    No strangers allowed!
#endif

If you want to use another tag as part of your condition, you should omit the # for the inner tag. For example:

#if(count(users) > 0):
    You have users!
#else:
    There are no users yet :(
#endif

You can also use #elseif statement.s

#if(title == "Welcome"):
    Hello new user!
#elseif(title == "Welcome back!"):
    Hello old user
#else
    Unexpected page!
#endif

Loops

If you provide an array of items, Leaf can loop over them and let you manipulate each item individually using its #for tag.

For example, we could update our Swift code to provide a list of planets:

struct SolarSystem: Codable {
    let planets = ["Venus", "Earth", "Mars"]
}

return req.view.render("solarSystem", SolarSystem())

We could then loop over them in Leaf like this:

Planets:
<ul>
#for(planet in planets):
    <li>#(planet)</li>
#endfor
</ul>

This would render a view that looks like:

Planets:
- Venus
- Earth
- Mars

Extending templates

Leaf’s #extend tag allows you to copy the contents of one template into another. When using this, you should always omit the template file's .leaf extension.

Extending is useful for copying in a standard piece of content, for example a page footer, advert code or table that's shared across multiple pages:

#extend("footer")

This tag is also useful for building one template on top of another. For example, you might have a layout.leaf file that includes all the code required to lay out your website – HTML structure, CSS and JavaScript – with some gaps in place that represent where page content varies.

Using this approach, you would construct a child template that fills in its unique content, then extends the parent template that places the content appropriately. To do this, you can use the #export and #import tags to store and later retrieve content from the context.

For example, you might create a child.leaf template like this:

#extend("master"):
    #export("body"):
        <p>Welcome to Vapor!</p>
    #endexport
#endextend

We call #export to store some HTML and make it available to the template we're currently extending. We then render master.leaf and use the exported data when required along with any other context variables passed in from Swift. For example, master.leaf might look like this:

<html>
    <head>
        <title>#(title)</title>
    </head>
    <body>#import(body)</body>
</html>

Here we are using #import to fetch the content passed to the #extend tag. When passed ["title": "Hi there!"] from Swift, child.leaf will render as follows:

<html>
    <head>
        <title>Hi there!</title>
    </head>
    <body><p>Welcome to Vapor!</p></body>
</html>

Other tags

#count

The #count tag returns the number of items in an array. For example:

Your search matched #count(matches) pages.

#lowercased

The #lowercased tag lowercases all letters in a string.

#lowercased(name)

#uppercased

The #uppercased tag uppercases all letters in a string.

#uppercased(name)

#capitalized

The #capitalized tag uppercases the first letter in each word of a string and lowercases the others. See String.capitalized for more information.

#capitalized(name)

#contains

The #contains tag accepts an array and a value as its two parameters, and returns true if the array in parameter one contains the value in parameter two.

#if(contains(planets, "Earth")):
    Earth is here!
#elseif
    Earth is not in this array.
#endif

#date

The #date tag formats dates into a readable string. By default it uses ISO8601 formatting.

render(..., ["now": Date()])
The time is #date(now)

You can pass a custom date formatter string as the second argument. See Swift's DateFormatter for more information.

The date is #date(now, "yyyy-MM-dd")