feat(Schema) instrumenters can modify fields

Author: rmosolgo

lib/graphql/schema.rb

@@ -56,6 +56,7 @@ class Schema
       :orphan_types, :resolve_type,
       :object_from_id, :id_from_object,
       directives: ->(schema, directives) { schema.directives = directives.reduce({}) { |m, d| m[d.name] = d; m  }},
+      instrument: -> (schema, type, instrumenter) { schema.instrumenters[type] << instrumenter },
       query_analyzer: ->(schema, analyzer) { schema.query_analyzers << analyzer },
       middleware: ->(schema, middleware) { schema.middleware << middleware },
       rescue_from: ->(schema, err_class, &block) { schema.rescue_from(err_class, &block)}
@@ -65,7 +66,7 @@ class Schema
       :query_execution_strategy, :mutation_execution_strategy, :subscription_execution_strategy,
       :max_depth, :max_complexity,
       :orphan_types, :directives,
-      :query_analyzers, :middleware
+      :query_analyzers, :middleware, :instrumenters
 
     BUILT_IN_TYPES = Hash[[INT_TYPE, STRING_TYPE, FLOAT_TYPE, BOOLEAN_TYPE, ID_TYPE].map{ |type| [type.name, type] }]
     DIRECTIVES = [GraphQL::Directive::IncludeDirective, GraphQL::Directive::SkipDirective, GraphQL::Directive::DeprecatedDirective]
@@ -90,6 +91,7 @@ def initialize
       @resolve_type_proc = nil
       @object_from_id_proc = nil
       @id_from_object_proc = nil
+      @instrumenters = Hash.new { |h, k| h[k] = [] }
       # Default to the built-in execution strategy:
       @query_execution_strategy = GraphQL::Query::SerialExecution
       @mutation_execution_strategy = GraphQL::Query::SerialExecution
@@ -110,10 +112,16 @@ def define(**kwargs, &block)
       super
       types
       @instrumented_field_map = InstrumentedFieldMap.new(self)
+      field_instrumenters = @instrumenters[:field]
       types.each do |type_name, type|
         if type.kind.fields?
           type.all_fields.each do |field_defn|
-            @instrumented_field_map.set(type.name, field_defn.name, field_defn)
+
+            instrumented_field_defn = field_instrumenters.reduce(field_defn) do |defn, inst|
+              inst.instrument(type, defn)
+            end
+
+            @instrumented_field_map.set(type.name, field_defn.name, instrumented_field_defn)
           end
         end
       end

spec/graphql/schema_spec.rb

@@ -182,4 +182,47 @@
       assert_equal GraphQL::Schema::Printer.print_schema(schema), GraphQL::Schema::Printer.print_schema(built_schema)
     end
   end
+
+  describe "#instrument" do
+    class MultiplyInstrumenter
+      def initialize(multiplier)
+        @multiplier = multiplier
+      end
+
+      def instrument(type_defn, field_defn)
+        prev_proc = field_defn.resolve_proc
+        new_resolve_proc = ->(obj, args, ctx) {
+          inner_value = prev_proc.call(obj, args, ctx)
+          inner_value * @multiplier
+        }
+
+        field_defn.redefine do
+          resolve(new_resolve_proc)
+        end
+      end
+    end
+
+    let(:query_type) {
+      GraphQL::ObjectType.define do
+        name "Query"
+        field :int, types.Int do
+          argument :value, types.Int
+          resolve -> (obj, args, ctx) { args[:value] }
+        end
+      end
+    }
+
+    let(:schema) {
+      spec = self
+      GraphQL::Schema.define do
+        query(spec.query_type)
+        instrument(:field, MultiplyInstrumenter.new(3))
+      end
+    }
+
+    it "can modify field definitions" do
+      res = schema.execute(" { int(value: 2) } ")
+      assert_equal 6, res["data"]["int"]
+    end
+  end
 end