add factory.ChainableModifier (#25)

factory/chainable_modifier.go

@@ -0,0 +1,33 @@
+package factory
+
+// ChainableModifier creates a factory that produces an initial value from initFactory, then lets you change modifiers to modify that value.
+// You're then returned the value once all modifiers have ran over it. This can be particularly useful when building mock data in tests.
+//
+//	func example() {
+//		chain := ChainableModifier(func() *pb.MyRequest {
+//			return &pb.MyRequest{Foo: true, Bar: false}
+//		})
+//
+//		firstRequest := chain() // returns &pb.MyRequest{Foo: true, Bar: false}
+//
+//		secondRequestWithBarTrue := chain(func(v *pb.MyRequest) {
+//			v.Bar = true
+//		}) // returns &pb.MyRequest{Foo: true, Bar: true}
+//
+//		thirdRequestWithValuesSwapped := chain(func(v *pb.MyRequest) {
+//			v.Foo = false
+//		}, func(v *pb.MyRequest) {
+//			v.Bar = true
+//		}) // returns &pb.MyRequest{Foo: false, Bar: true}
+//	}
+func ChainableModifier[T any](initFactory func() T) func(modifiers ...func(T)) T {
+	return func(modifiers ...func(T)) T {
+		out := initFactory()
+
+		for _, fn := range modifiers {
+			fn(out)
+		}
+
+		return out
+	}
+}

factory/chainable_modifier_test.go

@@ -0,0 +1,30 @@
+package factory_test
+
+import (
+	"testing"
+
+	"github.com/aidenwallis/go-utils/factory"
+	"github.com/aidenwallis/go-utils/internal/assert"
+)
+
+func TestChainableModifier(t *testing.T) {
+	t.Parallel()
+
+	type testValue struct {
+		Foo bool
+		Bar bool
+	}
+
+	chain := factory.ChainableModifier(func() *testValue {
+		return &testValue{
+			Foo: true,
+			Bar: false,
+		}
+	})
+
+	out := chain(func(v *testValue) {
+		v.Foo = false
+	})
+	assert.False(t, out.Foo)
+	assert.False(t, out.Bar)
+}