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Customized Leaf tags in Vapor 4

On this article I will present you create some helpful customized tags for the Leaf template engine, written in Swift.


Learn how to lengthen Leaf?

With the rebirth of Leaf we will actually lengthen the template engine and customized tags are only a factor of the previous. You already know in earlier variations of Leaf every part was referred to as a tag and there was no differentiation between these little bastards beginning with the # image. Now issues have modified. There are lots of completely different entities in Leaf Tau.

  • Blocks (e.g. #for, #whereas, #if, #elseif, #else)
  • Features (e.g. #Date, #Timestamp, and so on.)
  • Strategies (e.g. .rely(), .isEmpty, and so on.)

This can be a great point and on high of this you may create your very personal capabilities, strategies and even blocks. This brings us to a totally extensible template engine that may render every part in a non-blocking asynchronous method. How cool is that? 😎

Did I point out that Leaf you may lengthen the context with customized LeafDataGenerators? Sure, that is a factor now, up to now you would use the userInfo object to set a “international” accessible variable in Leaf, that was effectively accessible in each single template file.

Now there are some particular variables accessible which you can lengthen:

The present context in fact is what you cross to your template utilizing the render technique written in Swift. It’s price to say that self is simply an alias to the present $context, so it does not issues which one you employ. The $app and $req scopes are empty by design, however you may lengthen them. You possibly can even register your personal scope for instance $api and set every part you want globally underneath that variable. I will present you the way to do that in a while.

As you may see there are many choices accessible to increase Leaf. It’s important to assume twice which path you’re taking, nevertheless it’s nice that we now have this many alternatives. Now we’ll stroll by of every of these items and I will present you write customized extensions for Leaf Tau. 🥳

Learn how to lengthen Leaf contexts?

Some of the straightforward method of extending Leaf is to supply customized context variables. We will simply write an extension for the Utility and the Request object and return LeafDataGenerator values with particular keys and in a while we will register these as further context variables.

import Vapor
import Leaf

extension Utility {
    var customLeafVars: [String: LeafDataGenerator] {
            "isDebug": .lazy(LeafData.bool(!self.environment.isRelease && self.environment != .production))

extension Request {
    var customLeafVars: [String: LeafDataGenerator] {
            "url": .lazy([
                        "isSecure": LeafData.bool(self.url.scheme?.contains("https")),
                        "host": LeafData.string(self.url.host),
                        "port": LeafData.int(self.url.port),
                        "path": LeafData.string(self.url.path),
                        "query": LeafData.string(self.url.query)

A LeafDataGenerator object may be lazy or instant. Quick values shall be saved immediately, alternatively lazy values will produce generator blocks which can be going to be referred to as solely when the renderer wants them. Nothing particular, this works just like the lazy key phrase in Swift.

struct ScopeExtensionMiddleware: Middleware {

    func reply(to req: Request, chainingTo subsequent: Responder) -> EventLoopFuture<Response> {
        do {
            attempt req.leaf.context.register(mills: req.utility.customLeafVars, toScope: "app")
            attempt req.leaf.context.register(mills: req.customLeafVars, toScope: "req")
        catch {
            return req.eventLoop.future(error: error)
        return subsequent.reply(to: req)

We want an extension middleware that registers our generator variables to the given scope.

public func configure(_ app: Utility) throws {



Attempt to print these values in a template file, you may entry child-values utilizing the dot notation.




Now we’re going to create a customized context to get some details about the host machine.

ultimate class ServerLeafContextGenerator: LeafContextPublisher {

    var osName: String {
        #if os(macOS)
        return "macOS"
        #elseif os(Linux)
        return "Linux"
        #elseif os(Home windows)
        return "Home windows"
        return "Unknown"

    lazy var leafVariables: [String: LeafDataGenerator] = [
        "os": .lazy([
            "name": LeafData.string(self.osName),
            "version": LeafData.string(ProcessInfo.processInfo.operatingSystemVersionString),
        "cpu-cores": .instant(ProcessInfo.processInfo.processorCount),
        "reminiscence": .instant(ProcessInfo.processInfo.physicalMemory),

We will merely put this line subsequent to the opposite two within the scope extension middleware.

attempt req.leaf.context.register(mills: ServerLeafContextGenerator().leafVariables, toScope: "server")

This fashion we will get some more information in regards to the server in our Leaf templates by utilizing the $server scope. One other method is to increase a scope domestically with a generator.

app.get("server-info") { req -> EventLoopFuture<View> in
    var context: LeafRenderer.Context = [
        "title": "Server info",
    attempt context.register(object: ServerLeafContextGenerator(), toScope: "server")
    return req.leaf.render(template: "server-info", context: context)

The distinction is that within the second case the server scope is just accessible for a single endpoint, but when we register it by the middleware then it may be reached globally in each single Leaf file.

I believe scopes are very helpful, particularly Request associated ones. Prior to now we needed to create a customized Leaf tag to get the trail, however now we will use a scope extension and this data shall be accessible in every single place. With the lazy load we additionally get some free efficiency enhancements.

Customized Leaf capabilities and strategies

You possibly can create customized capabilities and strategies for Leaf, I would say that this new API is the replacemenet of the previous tag system. There are some variations and at first sight you may assume that it is tougher to create a perform with the brand new instruments, however in time you may get used to it.

public struct Hi there: LeafFunction, StringReturn, Invariant {
    public static var callSignature: [LeafCallParameter] { [.string] }

    public func consider(_ params: LeafCallValues) -> LeafData {
        guard let identify = params[0].string else {
            return .error("`Hi there` have to be referred to as with a string parameter.")
        return .string("Hi there (identify)!")

This can be a very fundamental perform. Each single perform has a name signature, which is only a record of type-safe arguments. Features can have return varieties, happily there are pre-made protocols for these, so you do not have to implement the required stuff, however you may say that this capabilities is e.g. a StringReturn perform. Invariant signifies that the perform will all the time return the identical output for a similar enter. That is what you need more often than not, it additionally lets you keep away from side-effects.

Within the consider perform you will get entry to all of the enter parameters and it’s a must to return with a LeafData sort. If a parameter is lacking or it may’t be casted to the correct sort you may all the time return with an error. Consider is wish to the previous render technique, nevertheless it’s far more superior.

LeafConfiguration.entities.use(Hi there(), asFunction: "Hi there")

You additionally need to register this newly created perform underneath a give identify.

#Hi there("Leaf Tau")

Oh by the way in which strategies are simply particular capabilities so you may construct them the identical method and register them by way of the asMethod: property. If you wish to see extra examples, you need to check out my different submit about what’s new in Leaf Tau or scroll right down to the final part of this text.

Learn how to construct customized Leaf blocks?

This can be a very attention-grabbing and complicated subject. Blocks are particular sort of LeafFunctions, similar to strategies, however issues are just a bit bit extra sophisticated on this case. Instance time:

import Vapor
import Leaf

struct MaybeBlock: LeafBlock, VoidReturn, Invariant {
    static var parseSignatures: ParseSignatures? = nil
    static var evaluable: Bool = false
    var scopeVariables: [String]? = nil

    static var callSignature: [LeafCallParameter] { [.double(labeled: "chance")] }
    static func instantiate(_ signature: String?, _ params: [String]) throws -> MaybeBlock { .init() }

    mutating func evaluateScope(_ params: LeafCallValues, _ variables: inout [String: LeafData]) -> EvalCount {
        params[0].double! > Double.random(in: 0..<1) ? .as soon as : .discard
    mutating func reEvaluateScope(_ variables: inout [String : LeafData]) -> EvalCount {
        fatalError("Error: `Perhaps` blocks cannot be re-evaluated.")

This block has a name signature with a labeled argument referred to as likelihood. It has an instantiate technique which is utilized by the Leaf engine to create this block. It will not have any parseSignatures or scope variables, we’ll depart that for the for block (go and test the supply in LeafKit if you’re curious & courageous sufficient). We set evaluable to false since we do not wish to make it callable by way of the #consider perform. Now let’s discuss scope analysis actual fast.

The evaluateScope technique shall be referred to as first when the block inside your template will get evaluated. It’s important to return an EvalCount on this technique, which is able to resolve what number of instances ought to we print out the contents in between your block (#[name]:THIS PART#finish[name]).

Principally when a LeafBlock is evaluated the primary time, it is by way of evaluateScope. If that returns a consequence fairly than nil, any additional calls will use reEvaluateScope as a substitute. – tdotclare

If EvalCount is about to discard then the contents shall be discarded, in any other case it will be evaluated as many instances as you come. If the rely is .as soon as which means the tip of the story, but when it get’s evaluated a number of instances and you do not want further params for additional analysis, then the reEvaluateScope shall be referred to as for all the opposite cycles.

LeafConfiguration.entities.use(MaybeBlock.self, asBlock: "perhaps")

Remember that we now have to register this block with a given identify earlier than we may use it.

#perhaps(likelihood: 0.5):
    <p>Is that this going to occur? 50-50.</p>

That is it, we have simply prolonged Leaf with a fundamental block, you may attempt to construct your personal A/B testing Chained block if you wish to dig deeper, however that is fairly a sophisticated subject and there aren’t any docs accessible simply but so you’ve got to check out the LeafKit supply recordsdata in many of the circumstances.

Helpful Leaf extensions.

I’ve made a bunch of helpful Leaf extensions accessible underneath the LeafFoundation repository. It is a work-in-progress challenge, however hopefully it’ll include lot extra attention-grabbing extensions by the point Leaf 4 shall be formally launched. PR’s are welcomed. 😬



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