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Custom Tags

You can create custom Leaf tags using the TagRenderer protocol.

To demonstrate this, let's take a look at creating a custom tag #now that prints the current timestamp. The tag will also support a single, optional parameter for specifying the date format.

Tag Renderer

First create a class called NowTag and conform it to TagRenderer.

final class NowTag: TagRenderer {
    init() { }

    func render(tag: TagContext) throws -> EventLoopFuture<TemplateData> {

Now let's implement the render(tag:) method. The TagContext context passed to this method has everything we should need.

let formatter = DateFormatter()
switch tag.parameters.count {
case 0: formatter.dateFormat = "yyyy-MM-dd HH:mm:ss"
case 1:
    guard let string = tag.parameters[0].string else {
        throw ...
    formatter.dateFormat = string
    throw ...

let string = formatter.string(from: .init())
return tag.container.future(.string(string))

Configure Tag

Now that we've implemented NowTag, we just need to configure it. You can configure any TagRenderer like this--even if they come from a separate package.

services.register { container -> LeafTagConfig in
    var config = LeafTagConfig.default()
    config.use(NowTag(), as: "now")
    return config

And that's it! We can now use our custom tag in Leaf.

The time is #now()