Add trigger interface.

diff --git a/src/main/java/net/pterodactylus/reactor/Trigger.java b/src/main/java/net/pterodactylus/reactor/Trigger.java
new file mode 100644 (file)
index 0000000..449b4fe
--- /dev/null
+++ b/src/main/java/net/pterodactylus/reactor/Trigger.java
@@ -0,0 +1,46 @@
+/*
+ * Reactor - Trigger.java - Copyright © 2013 David Roden
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package net.pterodactylus.reactor;
+
+import net.pterodactylus.reactor.states.FileState;
+
+/**
+ * A trigger determines whether two different states actually warrant a change
+ * trigger. For example, two {@link FileState}s might contain different file
+ * sizes but a trigger might only care about whether the file appeared or
+ * disappeared since the last check.
+ *
+ * @param <S>
+ *            The type of the state
+ * @author <a href="mailto:bombe@pterodactylus.net">David ‘Bombe’ Roden</a>
+ */
+public interface Trigger<S extends State> {
+
+       /**
+        * Checks whether the given states warrant a change trigger.
+        *
+        * @param currentState
+        *            The current state of a system
+        * @param previousState
+        *            The previous state of the system
+        * @return {@code true} if the given states warrant a change trigger,
+        *         {@code false} otherwise
+        */
+       boolean triggers(S currentState, S previousState);
+
+}