feat: introduce UnimplementedHook to avoid authors having to define empty functions (#55)

* feat: introduce UnimplementedHook to avoid authors having to define empty functions

Signed-off-by: Skye Gill <[email protected]>

* implemented TestBeforeHookNilContext

Signed-off-by: Skye Gill <[email protected]>

Signed-off-by: Skye Gill <[email protected]>

Author: skyerus

README.md

@@ -33,6 +33,26 @@ func main() {
 }
 ```
 
+### Hooks
+
+Implement your own hook by conforming to the [Hook interface](./pkg/openfeature/hooks.go).
+
+To satisfy the interface all methods (`Before`/`After`/`Finally`/`Error`) need to be defined. To avoid defining empty functions
+make use of the `UnimplementedHook` struct (which already implements all the empty functions).
+
+```go
+type MyHook struct {
+  openfeature.UnimplementedHook
+}
+
+// overrides UnimplementedHook's Error function
+func (h MyHook) Error(hookContext openfeature.HookContext, err error, hookHints openfeature.HookHints) {
+	log.Println(err)
+}
+```
+
+Register the hook at global, client or invocation level.
+
 ## Configuration
 
 ### Logging

pkg/openfeature/client_test.go

@@ -557,3 +557,29 @@ func TestFlattenContext(t *testing.T) {
 		})
 	}
 }
+
+// TestBeforeHookNilContext asserts that when a Before hook returns a nil EvaluationContext it doesn't overwrite the
+// existing EvaluationContext
+func TestBeforeHookNilContext(t *testing.T) {
+	defer t.Cleanup(initSingleton)
+	ctrl := gomock.NewController(t)
+
+	mockProvider := NewMockFeatureProvider(ctrl)
+	mockProvider.EXPECT().Metadata().AnyTimes()
+	mockProvider.EXPECT().Hooks().AnyTimes()
+	SetProvider(mockProvider)
+
+	hookNilContext := UnimplementedHook{}
+
+	client := NewClient("test")
+	attributes := map[string]interface{}{"should": "persist"}
+	evalCtx := EvaluationContext{Attributes: attributes}
+	mockProvider.EXPECT().BooleanEvaluation(gomock.Any(), gomock.Any(), attributes)
+
+	_, err := client.BooleanValue(
+		"foo", false, evalCtx, NewEvaluationOptions([]Hook{hookNilContext}, HookHints{}),
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+}

pkg/openfeature/hooks.go

@@ -65,3 +65,26 @@ func (h HookContext) ProviderMetadata() Metadata {
 func (h HookContext) EvaluationContext() EvaluationContext {
 	return h.evaluationContext
 }
+
+// check at compile time that UnimplementedHook implements the Hook interface
+var _ Hook = UnimplementedHook{}
+
+// UnimplementedHook implements all hook methods with empty functions
+// Include UnimplementedHook in your hook struct to avoid defining empty functions
+// e.g.
+// type MyHook struct {
+//   UnimplementedHook
+// }
+type UnimplementedHook struct{}
+
+func (u UnimplementedHook) Before(hookContext HookContext, hookHints HookHints) (*EvaluationContext, error) {
+	return nil, nil
+}
+
+func (u UnimplementedHook) After(hookContext HookContext, flagEvaluationDetails EvaluationDetails, hookHints HookHints) error {
+	return nil
+}
+
+func (u UnimplementedHook) Error(hookContext HookContext, err error, hookHints HookHints) {}
+
+func (u UnimplementedHook) Finally(hookContext HookContext, hookHints HookHints) {}