Added a system for running tasks once per frame until the task indicates

that it no longer wants to be run.  This comes up a lot and I was tired
of writing whole services just to get a once-per-frame update.

Author: pspeed42

release-notes.md

@@ -11,6 +11,7 @@ Version 1.8.0 (unreleased)
     of an existing/custom GameSystemManager.
 * Added GameSystemsState.getGameSystemManager() and 
     GameSystemsState.get(class, boolean) methods.
+* Added RecurringTaskSystem for executing general tasks once per frame.    
 
 
 Version 1.7.0 (latest)

src/main/java/com/simsilica/sim/RecurringTaskSystem.java

@@ -0,0 +1,116 @@
+/*
+ * $Id$
+ * 
+ * Copyright (c) 2022, Simsilica, LLC
+ * All rights reserved.
+ * 
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions 
+ * are met:
+ * 
+ * 1. Redistributions of source code must retain the above copyright 
+ *    notice, this list of conditions and the following disclaimer.
+ * 
+ * 2. Redistributions in binary form must reproduce the above copyright 
+ *    notice, this list of conditions and the following disclaimer in 
+ *    the documentation and/or other materials provided with the 
+ *    distribution.
+ * 
+ * 3. Neither the name of the copyright holder nor the names of its 
+ *    contributors may be used to endorse or promote products derived 
+ *    from this software without specific prior written permission.
+ * 
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS 
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT 
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS 
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE 
+ * COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, 
+ * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES 
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR 
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, 
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED 
+ * OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package com.simsilica.sim;
+
+import java.util.*;
+import java.util.function.Function;
+
+import org.slf4j.*;
+
+/**
+ *  A GameSystem that supports recurring tasks.
+ *
+ *  @author    Paul Speed
+ */
+public class RecurringTaskSystem extends AbstractGameSystem {
+ 
+    static Logger log = LoggerFactory.getLogger(RecurringTaskSystem.class);
+ 
+    private List<Function<SimTime, Boolean>> tasks = new ArrayList<>();
+    private volatile Function[] taskArray;
+    private static Function[] EMPTY_ARRAY = new Function[0]; 
+    
+    public RecurringTaskSystem() {
+    }
+ 
+    /**
+     *  Adds a recurring task that will continue to run once a frame
+     *  as long as it returns Boolean.TRUE.
+     */   
+    public void addRecurringTask( Function<SimTime, Boolean> task ) {
+        synchronized(tasks) {
+            tasks.add(task);
+            taskArray = null;
+        }
+    }
+    
+    public void removeRecurringTask( Function<SimTime, Boolean> task ) {
+        synchronized(tasks) {
+            tasks.remove(task);
+            taskArray = null;
+        }
+    }
+
+    protected Function[] getArray() {
+        // Grab the array, unprotected, so it doesn't move underneath us
+        Function[] result = taskArray;
+        if( result != null ) {
+            return result;
+        }
+        
+        // Else a little double-checked locking
+        synchronized(tasks) {
+            // See if one was created in the mean time
+            if( taskArray != null ) {
+                return taskArray;
+            }
+            taskArray = tasks.toArray(EMPTY_ARRAY);
+            return taskArray;
+        }        
+    }
+
+    @Override
+    @SuppressWarnings("unchecked")
+    public void update( SimTime time ) {        
+        for( Function f : getArray() ) {
+            Object result = f.apply(time);
+            if( !Objects.equals(result, Boolean.TRUE) ) {
+                removeRecurringTask((Function<SimTime, Boolean>)f);
+            }
+        }
+    }
+
+    @Override
+    protected void initialize() {
+    }
+
+    @Override
+    protected void terminate() {
+    }
+}
+
+