controller: add TriggeredBy custom watches to engine

Signed-off-by: Dr. Stefan Schimanski <stefan.schimanski@upbound.io>

pkg/controller/engine.go

@@ -149,9 +149,10 @@ func (e *Engine) done(name string, err error) {
 
 // Watch an object.
 type Watch struct {
-	kind       client.Object
-	handler    handler.EventHandler
-	predicates []predicate.Predicate
+	kind        client.Object
+	handler     handler.EventHandler
+	predicates  []predicate.Predicate
+	customCache cache.Cache
 }
 
 // For returns a Watch for the supplied kind of object. Events will be handled
@@ -160,6 +161,13 @@ func For(kind client.Object, h handler.EventHandler, p ...predicate.Predicate) W
 	return Watch{kind: kind, handler: h, predicates: p}
 }
 
+// TriggeredBy returns a custom watch for secondary resources triggering the
+// controller. Events will be handled by the supplied EventHandler, and may be
+// filtered by the supplied predicates.
+func TriggeredBy(cache cache.Cache, kind client.Object, h handler.EventHandler, p ...predicate.Predicate) Watch {
+	return Watch{kind: kind, handler: h, predicates: p, customCache: cache}
+}
+
 // Start the named controller. Each controller is started with its own cache
 // whose lifecycle is coupled to the controller. The controller is started with
 // the supplied options, and configured with the supplied watches. Start does
@@ -191,6 +199,10 @@ func (e *Engine) Start(name string, o controller.Options, w ...Watch) error {
 	}
 
 	for _, wt := range w {
+		ca := ca
+		if wt.customCache != nil {
+			ca = wt.customCache
+		}
 		if err := ctrl.Watch(source.Kind(ca, wt.kind), wt.handler, wt.predicates...); err != nil {
 			return errors.Wrap(err, errWatch)
 		}