Basic filter implementation, similar to Sinatra.

spec/middleware/filters_spec.cr

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+require "../spec_helper"
+
+describe "Kemal::Middleware::Filters" do
+  it "executes code before home request" do
+    test_filter = FilterTest.new
+    test_filter.modified = "false"
+
+    filter = Kemal::Middleware::Filter.new
+    filter.add :before, "/greetings", {} of Symbol => String { test_filter.modified = "true" }
+
+    kemal = Kemal::RouteHandler.new
+    kemal.add_route "GET", "/greetings" { test_filter.modified }
+
+    test_filter.modified.should eq("false")
+    request = HTTP::Request.new("GET", "/greetings")
+    create_request_and_return_io(filter, request)
+    io_with_context = create_request_and_return_io(kemal, request)
+    client_response = HTTP::Client::Response.from_io(io_with_context, decompress: false)
+    client_response.body.should eq("true")
+  end
+end
+
+class FilterTest
+  property modified
+end

src/kemal/middleware/filters.cr

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+module Kemal::Middleware
+  # Kemal::Filter handle all code that should be evaluated before and after
+  # every request
+  class Filter < HTTP::Handler
+    def initialize
+      @tree = Radix::Tree.new
+    end
+
+    def add(type, path, options, &block : HTTP::Server::Context -> _)
+      node = radix_path type, path
+      @tree.add node, Block.new &block
+    end
+
+    def call(context)
+      process_filter(context, :before)
+      call_next(context)
+      process_filter(context, :after)
+    end
+
+    private def process_filter(context, type)
+      lookup = @tree.find radix_path(type, context.request.path)
+      if lookup.found? && lookup.payload.is_a? Block
+        block = lookup.payload as Block
+        block.block.call(context)
+      end
+    end
+
+    private def radix_path(type : Symbol, path)
+      "/#{type}#{path}"
+    end
+  end
+
+  class Block
+    property block
+    def initialize(&@block : HTTP::Server::Context -> _)
+    end
+  end
+end
+
+def add_filters
+  Kemal.config.add_handler Kemal::Filters::Filter.new
+end
+
+def before(path = "*", options = {} of Symbol => String, &block : HTTP::Server::Context -> _)
+  filter = Kemal.config.handlers.first as Kemal::Filters::Filter
+  filter.add :before, path, options, &block
+end
+
+def after(path = "*", options = {} of Symbol => String, &block : HTTP::Server::Context -> _)
+  filter = Kemal.config.handlers.first as Kemal::Filters::Filter
+  filter.add :after, path, options, &block
+end