Add possibility to define reusable params.

CHANGELOG.md

@@ -14,6 +14,7 @@ Next Release
 * [#540](https://github.com/intridea/grape/pull/540): Ruby 2.1.0 is now supported - [@salimane](https://github.com/salimane).
 * [#544](https://github.com/intridea/grape/pull/544): The `rescue_from` keyword now handles subclasses of exceptions by default - [@xevix](https://github.com/xevix).
 * [#545](https://github.com/intridea/grape/pull/545): Added `type` (Array or Hash) support to `requires`, `optional` and `group` - [@bwalex](https://github.com/bwalex).
+* [#550](https://github.com/intridea/grape/pull/550): Added possibility to define reusable params - [@dm1try](https://github.com/dm1try).
 * Your contribution here.
 
 #### Fixes

README.md

@@ -576,6 +576,58 @@ class API < Grape::API
 end
 ```
 
+You can define reusable `params` using `helpers`.
+
+```ruby
+class API < Grape::API
+  helpers do
+    params :pagination do
+      optional :page, type: Integer
+      optional :per_page, type: Integer
+    end
+  end
+
+  desc "Get collection"
+  params do
+    use :pagination # aliases: includes, use_scope
+  end
+  get do
+    Collection.page(params[:page]).per(params[:per_page])
+  end
+end
+```
+
+You can also define reusable `params` using shared helpers.
+
+```ruby
+module SharedParams
+  extend Grape::API::Helpers
+
+  params :period do
+    optional :start_date
+    optional :end_date
+  end
+
+  params :pagination do
+    optional :page, type: Integer
+    optional :per_page, type: Integer
+  end
+end
+
+class API < Grape::API
+  helpers SharedParams
+
+  desc "Get collection"
+  params do
+    use :period, :pagination
+  end
+  get do
+    Collection.from(params[:start_date]).to(params[:end_date])
+              .page(params[:page]).per(params[:per_page])
+  end
+end
+```
+
 ## Cookies
 
 You can set, get and delete your cookies very simply using `cookies` method.

lib/grape/api.rb

@@ -268,11 +268,16 @@ def helpers(new_mod = nil, &block)
         if block_given? || new_mod
           mod = settings.peek[:helpers] || Module.new
           if new_mod
+            inject_api_helpers_to_mod(new_mod) if new_mod.is_a?(Helpers)
             mod.class_eval do
               include new_mod
             end
           end
-          mod.class_eval(&block) if block_given?
+          if block_given?
+            inject_api_helpers_to_mod(mod) do
+              mod.class_eval(&block)
+            end
+          end
           set(:helpers, mod)
         else
           mod = Module.new
@@ -509,6 +514,12 @@ def inherit_settings(other_stack)
           e.options[:app].inherit_settings(other_stack) if e.options[:app].respond_to?(:inherit_settings, true)
         end
       end
+
+      def inject_api_helpers_to_mod(mod, &block)
+        mod.extend(Helpers)
+        yield if block_given?
+        mod.api_changed(self)
+      end
     end
 
     def initialize
@@ -581,5 +592,28 @@ def add_head_not_allowed_methods_and_options_methods
         end
       end
     end
+
+    # This module extends user defined helpers
+    # to provide some API-specific functionality
+    module Helpers
+      attr_accessor :api
+      def params(name, &block)
+        @named_params ||= {}
+        @named_params.merge! name => block
+      end
+
+      def api_changed(new_api)
+        @api = new_api
+        process_named_params
+      end
+
+      protected
+
+      def process_named_params
+        if @named_params && @named_params.any?
+          api.imbue(:named_params, @named_params)
+        end
+      end
+    end
   end
 end

lib/grape/validations.rb

@@ -147,6 +147,18 @@ def params(params)
         params
       end
 
+      def use(*names)
+        named_params = @api.settings[:named_params] || {}
+        names.each do |name|
+          params_block = named_params.fetch(name) do
+            raise "Params :#{name} not found!"
+          end
+          instance_eval(&params_block)
+        end
+      end
+      alias_method :use_scope, :use
+      alias_method :includes, :use
+
       def full_name(name)
         return "#{@parent.full_name(@element)}[#{name}]" if @parent
         name.to_s

spec/grape/validations_spec.rb

@@ -698,5 +698,60 @@ def validate_param!(attr_name, params)
         end
       end
     end # end custom validation
+
+    context 'named' do
+      context 'can be defined' do
+        it 'in helpers' do
+          subject.helpers do
+            params :pagination do
+            end
+          end
+        end
+
+        it 'in helper module which kind of Grape::API::Helpers' do
+          module SharedParams
+            extend Grape::API::Helpers
+            params :pagination do
+            end
+          end
+          subject.helpers SharedParams
+        end
+      end
+
+      context 'can be included in usual params' do
+        before do
+          module SharedParams
+            extend Grape::API::Helpers
+            params :period do
+              optional :start_date
+              optional :end_date
+            end
+          end
+          subject.helpers SharedParams
+
+          subject.helpers do
+            params :pagination do
+              optional :page, type: Integer
+              optional :per_page, type: Integer
+            end
+          end
+        end
+
+        it 'by #use' do
+          subject.params do
+            use :pagination
+          end
+          subject.settings[:declared_params].should eq [:page, :per_page]
+        end
+
+        it 'by #use with multiple params' do
+          subject.params do
+            use :pagination, :period
+          end
+          subject.settings[:declared_params].should eq [:page, :per_page, :start_date, :end_date]
+        end
+
+      end
+    end
   end
 end