Use a simpler approach to add contract support

lib/hanami/action.rb

@@ -138,6 +138,11 @@ def self.params(_klass = nil)
             "To use `params`, please add 'hanami/validations' gem to your Gemfile"
     end
 
+    def self.contract
+      raise NoMethodError,
+            "To use `contract`, please add 'hanami/validations' gem to your Gemfile"
+    end
+
     # @overload self.append_before(*callbacks, &block)
     #   Define a callback for an Action.
     #   The callback will be executed **before** the action is called, in the

lib/hanami/action/validatable.rb

@@ -105,6 +105,16 @@ def params(klass = nil, &blk)
 
           @params_class = klass
         end
+
+        def contract(&block)
+          klass = const_set(PARAMS_CLASS_NAME, Class.new(Params))
+
+          klass.class_eval do
+            @_validator = Class.new(Dry::Validation::Contract, &block).new
+          end
+
+          @params_class = klass
+        end
       end
     end
   end

spec/support/fixtures.rb

@@ -1902,3 +1902,49 @@ def call(env)
     end
   end
 end
+
+class ContractAction < Hanami::Action
+  contract do
+    params do
+      required(:birth_date).filled(:date)
+      required(:book).schema do
+        required(:title).filled(:str?)
+      end
+    end
+
+    rule(:birth_date) do
+      key.failure("you must be 18 years or older") if value < Date.today << (12 * 18)
+    end
+  end
+
+  def handle(request, response)
+    if request.params.valid?
+      response.status = 201
+      response.body = JSON.generate(
+        new_name: request.params[:book][:title].upcase
+      )
+    else
+      response.body = {errors: request.params.errors.to_h}
+      response.status = 302
+    end
+  end
+  end
+
+class WhitelistedUploadDslContractAction < Hanami::Action
+  contract do
+    params do
+      required(:id).maybe(:integer)
+      required(:upload).filled
+    end
+  end
+
+  def handle(req, res)
+    res.body = req.params.to_h.inspect
+  end
+end
+
+class RawContractAction < Hanami::Action
+  def handle(req, res)
+    res.body = req.params.to_h.inspect
+  end
+end

spec/unit/hanami/action/contract_spec.rb

@@ -0,0 +1,65 @@
+# frozen_string_literal: true
+
+require "rack"
+
+RSpec.describe "Contracts" do
+  describe "when defined as block in action" do
+    let(:action) { ContractAction.new }
+
+    context "when it has errors" do
+      it "returns them" do
+        response = action.call("birth_date" => "2000-01-01")
+
+        expect(response.status).to eq 302
+        expect(response.body).to eq ["{:errors=>{:book=>[\"is missing\"], :birth_date=>[\"you must be 18 years or older\"]}}"]
+      end
+    end
+
+    context "when it is valid" do
+      it "works" do
+        response = action.call("birth_date" => Date.today - (365 * 15), "book" => {"title" => "Hanami"})
+
+        expect(response.status).to eq 201
+        expect(response.body).to eq ["{\"new_name\":\"HANAMI\"}"]
+      end
+    end
+  end
+
+  describe "#raw" do
+    context "when this feature isn't enabled" do
+      let(:action) { RawContractAction.new }
+
+      it "raw gets all params" do
+        File.open("spec/support/fixtures/multipart-upload.png", "rb") do |upload|
+          response = action.call("id" => "1", "unknown" => "2", "upload" => upload)
+
+          expect(response[:params][:id]).to eq("1")
+          expect(response[:params][:unknown]).to eq("2")
+          expect(FileUtils.cmp(response[:params][:upload], upload)).to be(true)
+
+          expect(response[:params].raw.fetch("id")).to eq("1")
+          expect(response[:params].raw.fetch("unknown")).to eq("2")
+          expect(response[:params].raw.fetch("upload")).to eq(upload)
+        end
+      end
+    end
+
+    context "when this feature is enabled" do
+      let(:action) { WhitelistedUploadDslContractAction.new }
+
+      it "raw gets all params" do
+        Tempfile.create("multipart-upload") do |upload|
+          response = action.call("id" => "1", "unknown" => "2", "upload" => upload, "_csrf_token" => "3")
+
+          expect(response[:params][:id]).to          eq(1)
+          expect(response[:params][:unknown]).to     be(nil)
+          expect(response[:params][:upload]).to      eq(upload)
+
+          expect(response[:params].raw.fetch("id")).to          eq("1")
+          expect(response[:params].raw.fetch("unknown")).to     eq("2")
+          expect(response[:params].raw.fetch("upload")).to      eq(upload)
+        end
+      end
+    end
+  end
+end