Tasks: only execute the event if the plugin is interested in it

t/Event/BuildFinished.t

@@ -54,6 +54,28 @@ my $jobset = createBaseJobset("basic", "basic.nix", $ctx{jobsdir});
 ok(evalSucceeds($jobset),               "Evaluating jobs/basic.nix should exit with return code 0");
 is(nrQueuedBuildsForJobset($jobset), 3, "Evaluating jobs/basic.nix should result in 3 builds");
 
+subtest "interested" => sub {
+    my $event = Hydra::Event::BuildFinished->new(123, []);
+
+    subtest "A plugin which does not implement the API" => sub {
+        my $plugin = {};
+        my $mock = mock_obj $plugin => ();
+
+        is($event->interestedIn($plugin), 0, "The plugin is not interesting.");
+    };
+
+    subtest "A plugin which does implement the API" => sub {
+        my $plugin = {};
+        my $mock = mock_obj $plugin => (
+            add => [
+                "buildFinished" => sub {}
+            ]
+        );
+
+        is($event->interestedIn($plugin), 1, "The plugin is interesting.");
+    };
+};
+
 subtest "load" => sub {
     my ($build, $dependent_a, $dependent_b) = $db->resultset('Builds')->search(
       { },

t/Event/BuildQueued.t

@@ -39,6 +39,28 @@ subtest "Parsing build_queued" => sub {
     );
 };
 
+subtest "interested" => sub {
+    my $event = Hydra::Event::BuildQueued->new(123, []);
+
+    subtest "A plugin which does not implement the API" => sub {
+        my $plugin = {};
+        my $mock = mock_obj $plugin => ();
+
+        is($event->interestedIn($plugin), 0, "The plugin is not interesting.");
+    };
+
+    subtest "A plugin which does implement the API" => sub {
+        my $plugin = {};
+        my $mock = mock_obj $plugin => (
+            add => [
+                "buildQueued" => sub {}
+            ]
+        );
+
+        is($event->interestedIn($plugin), 1, "The plugin is interesting.");
+    };
+};
+
 subtest "load" => sub {
     my $build = $builds->{"empty_dir"};
 

t/Event/BuildStarted.t

@@ -45,6 +45,28 @@ subtest "Parsing build_started" => sub {
     );
 };
 
+subtest "interested" => sub {
+    my $event = Hydra::Event::BuildStarted->new(123, []);
+
+    subtest "A plugin which does not implement the API" => sub {
+        my $plugin = {};
+        my $mock = mock_obj $plugin => ();
+
+        is($event->interestedIn($plugin), 0, "The plugin is not interesting.");
+    };
+
+    subtest "A plugin which does implement the API" => sub {
+        my $plugin = {};
+        my $mock = mock_obj $plugin => (
+            add => [
+                "buildStarted" => sub {}
+            ]
+        );
+
+        is($event->interestedIn($plugin), 1, "The plugin is interesting.");
+    };
+};
+
 subtest "load" => sub {
     my $build = $db->resultset('Builds')->search(
       { },

t/Event/StepFinished.t

@@ -64,6 +64,28 @@ subtest "Parsing step_finished" => sub {
     );
 };
 
+subtest "interested" => sub {
+    my $event = Hydra::Event::StepFinished->new(123, []);
+
+    subtest "A plugin which does not implement the API" => sub {
+        my $plugin = {};
+        my $mock = mock_obj $plugin => ();
+
+        is($event->interestedIn($plugin), 0, "The plugin is not interesting.");
+    };
+
+    subtest "A plugin which does implement the API" => sub {
+        my $plugin = {};
+        my $mock = mock_obj $plugin => (
+            add => [
+                "stepFinished" => sub {}
+            ]
+        );
+
+        is($event->interestedIn($plugin), 1, "The plugin is interesting.");
+    };
+};
+
 subtest "load" => sub {
 
     my $step = $db->resultset('BuildSteps')->search(