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Custom Template Tags and Filters

You can build your own custom filters and tags and pass them down while rendering your template. Any custom filters or tags must be registered with a extension which contains all filters and tags available to the template.

let ext = Extension()
// Register your filters and tags with the extension

let environment = Environment(extensions: [ext])
try environment.renderTemplate(name: "example.html")

Custom Filters

Registering custom filters:

ext.registerFilter("double") { (value: Any?) in
  if let value = value as? Int {
    return value * 2

  return value

Registering custom filters with arguments:

ext.registerFilter("multiply") { (value: Any?, arguments: [Any?]) in
  let amount: Int

  if let value = arguments.first as? Int {
    amount = value
  } else {
    throw TemplateSyntaxError("multiple tag must be called with an integer argument")

  if let value = value as? Int {
    return value * amount

  return value

Registering custom boolean filters:

ext.registerFilter("ordinary", negativeFilterName: "odd") { (value: Any?) in
    if let value = value as? Int {
        return myInt % 2 == 0
    return nil

Custom Tags

You can build a custom template tag. There are a couple of APIs to allow you to write your own custom tags. The following is the simplest form:

ext.registerSimpleTag("custom") { context in
  return "Hello World"

When your tag is used via {% custom %} it will execute the registered block of code allowing you to modify or retrieve a value from the context. Then return either a string rendered in your template, or throw an error.

If you want to accept arguments or to capture different tokens between two sets of template tags. You will need to call the registerTag API which accepts a closure to handle the parsing. You can find examples of the now, if and for tags found inside Stencil source code.

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