extension: Implement an EventBroker (#315)

Signed-off-by: James Hillyerd <james@hillyerd.com>

Signed-off-by: James Hillyerd <james@hillyerd.com>

pkg/extension/broker.go

@@ -0,0 +1,57 @@
+package extension
+
+import "sync"
+
+// EventBroker maintains a list of listeners interested in a specific type
+// of event.
+type EventBroker[E any, R comparable] struct {
+	sync.RWMutex
+	listenerNames []string     // Ordered listener names.
+	listenerFuncs []func(E) *R // Ordered listener functions.
+}
+
+// Emit sends the provided event to each registered listener in order, until
+// one returns a non-nil result.  That result will be returned to the caller.
+func (eb *EventBroker[E, R]) Emit(event *E) *R {
+	eb.RLock()
+	defer eb.RUnlock()
+
+	for _, l := range eb.listenerFuncs {
+		// Events are copied to minimize the risk of mutation.
+		if result := l(*event); result != nil {
+			return result
+		}
+	}
+
+	return nil
+}
+
+// AddListener registers the named listener, replacing one with a duplicate
+// name if present.  Listeners should be added in order of priority, most
+// significant first.
+func (eb *EventBroker[E, R]) AddListener(name string, listener func(E) *R) {
+	eb.Lock()
+	defer eb.Unlock()
+
+	eb.lockedRemoveListener(name)
+	eb.listenerNames = append(eb.listenerNames, name)
+	eb.listenerFuncs = append(eb.listenerFuncs, listener)
+}
+
+// RemoveListener unregisters the named listener.
+func (eb *EventBroker[E, R]) RemoveListener(name string) {
+	eb.Lock()
+	defer eb.Unlock()
+
+	eb.lockedRemoveListener(name)
+}
+
+func (eb *EventBroker[E, R]) lockedRemoveListener(name string) {
+	for i, entry := range eb.listenerNames {
+		if entry == name {
+			eb.listenerNames = append(eb.listenerNames[:i], eb.listenerNames[i+1:]...)
+			eb.listenerFuncs = append(eb.listenerFuncs[:i], eb.listenerFuncs[i+1:]...)
+			break
+		}
+	}
+}

pkg/extension/broker_test.go

@@ -0,0 +1,134 @@
+package extension_test
+
+import (
+	"testing"
+
+	"github.com/inbucket/inbucket/pkg/extension"
+)
+
+func testBrokerEmitCallsOneListener(t *testing.T) {
+	broker := &extension.EventBroker[string, bool]{}
+
+	// Setup listener.
+	var got string
+	listener := func(s string) *bool {
+		got = s
+		return nil
+	}
+	broker.AddListener("x", listener)
+
+	want := "bacon"
+	broker.Emit(&want)
+	if got != want {
+		t.Errorf("Emit got %q, want %q", got, want)
+	}
+}
+
+func testBrokerEmitCallsMultipleListeners(t *testing.T) {
+	broker := &extension.EventBroker[string, bool]{}
+
+	// Setup listeners.
+	var first_got, second_got string
+	first := func(s string) *bool {
+		first_got = s
+		return nil
+	}
+	second := func(s string) *bool {
+		second_got = s
+		return nil
+	}
+
+	broker.AddListener("1", first)
+	broker.AddListener("2", second)
+
+	want := "hi"
+	broker.Emit(&want)
+	if first_got != want {
+		t.Errorf("first got %q, want %q", first_got, want)
+	}
+	if second_got != want {
+		t.Errorf("second got %q, want %q", second_got, want)
+	}
+}
+
+func testBrokerEmitCapturesFirstResult(t *testing.T) {
+	broker := &extension.EventBroker[struct{}, string]{}
+
+	// Setup listeners.
+	makeListener := func(result *string) func(struct{}) *string {
+		return func(s struct{}) *string { return result }
+	}
+	first := "first"
+	second := "second"
+	broker.AddListener("0", makeListener(nil))
+	broker.AddListener("1", makeListener(&first))
+	broker.AddListener("2", makeListener(&second))
+
+	want := first
+	got := broker.Emit(&struct{}{})
+	if got == nil {
+		t.Errorf("Emit got nil, want %q", want)
+	} else if *got != want {
+		t.Errorf("Emit got %q, want %q", *got, want)
+	}
+}
+
+func testBrokerAddingDuplicateNameReplacesPrevious(t *testing.T) {
+	broker := &extension.EventBroker[string, bool]{}
+
+	// Setup listeners.
+	var first_got, second_got string
+	first := func(s string) *bool {
+		first_got = s
+		return nil
+	}
+	second := func(s string) *bool {
+		second_got = s
+		return nil
+	}
+
+	broker.AddListener("dup", first)
+	broker.AddListener("dup", second)
+
+	want := "hi"
+	broker.Emit(&want)
+	if first_got != "" {
+		t.Errorf("first got %q, want empty string", first_got)
+	}
+	if second_got != want {
+		t.Errorf("second got %q, want %q", second_got, want)
+	}
+}
+
+func testBrokerRemovingListenerSuccessful(t *testing.T) {
+	broker := &extension.EventBroker[string, bool]{}
+
+	// Setup listeners.
+	var first_got, second_got string
+	first := func(s string) *bool {
+		first_got = s
+		return nil
+	}
+	second := func(s string) *bool {
+		second_got = s
+		return nil
+	}
+
+	broker.AddListener("1", first)
+	broker.AddListener("2", second)
+	broker.RemoveListener("1")
+
+	want := "hi"
+	broker.Emit(&want)
+	if first_got != "" {
+		t.Errorf("first got %q, want empty string", first_got)
+	}
+	if second_got != want {
+		t.Errorf("second got %q, want %q", second_got, want)
+	}
+}
+
+func testBrokerRemovingMissingListener(t *testing.T) {
+	broker := &extension.EventBroker[string, bool]{}
+	broker.RemoveListener("doesn't crash")
+}