Added callback line of sight methods. Thanks xZise!

By: EvilSeph <evilseph@gmail.com>

paper-api/src/main/java/org/bukkit/util/Callback.java

@@ -0,0 +1,19 @@
+package org.bukkit.util;
+
+/**
+ * Primitive callback class, to allow specific callback handling. For example to determine if a block in sight is invisible.
+ *
+ * @param <Result> This is the type it will return.
+ * @param <Parameter> This is the type it has as parameter.
+ */
+public interface Callback<Result, Parameter> {
+
+    /**
+     * This method will be called on each step.
+     *
+     * @param parameter The parameter of this step.
+     * @return The result of the step.
+     */
+    public Result call(Parameter parameter);
+
+}

paper-api/src/main/java/org/bukkit/util/TransparentCallback.java

@@ -0,0 +1,34 @@
+package org.bukkit.util;
+
+import java.util.Set;
+
+import org.bukkit.block.Block;
+
+/**
+ * Default transparent call back. This class as callback acts like the normal
+ * line of sight methods.
+ *
+ * To implement own handler override {@link TransparentCallback#call(Block)} and
+ * call it via super.
+ */
+public class TransparentCallback implements Callback<Boolean, Block> {
+
+    private final Set<Byte> transparent;
+
+    /**
+     * Creates a new callback class which returns by default for every block in
+     * the transparent list true. Otherwise false. Could be expanded by override
+     * the {@link Callback#call(Block)} method.
+     *
+     * @param transparent
+     *            The list of transparent blocks.
+     */
+    public TransparentCallback(Set<Byte> transparent) {
+        this.transparent = transparent;
+    }
+
+    public Boolean call(Block parameter) {
+        return this.transparent.contains((byte) parameter.getTypeId());
+    }
+
+}