Use thread pools (#601)

Reuse threads from pool instead of creating new thread for every task

Author: jeffdgr8

library/src/main/java/com/google/maps/android/clustering/algo/PreCachingAlgorithmDecorator.java

@@ -23,6 +23,8 @@
 
 import java.util.Collection;
 import java.util.Set;
+import java.util.concurrent.Executor;
+import java.util.concurrent.Executors;
 import java.util.concurrent.locks.ReadWriteLock;
 import java.util.concurrent.locks.ReentrantReadWriteLock;
 
@@ -35,6 +37,7 @@ public class PreCachingAlgorithmDecorator<T extends ClusterItem> extends Abstrac
     // TODO: evaluate maxSize parameter for LruCache.
     private final LruCache<Integer, Set<? extends Cluster<T>>> mCache = new LruCache<Integer, Set<? extends Cluster<T>>>(5);
     private final ReadWriteLock mCacheLock = new ReentrantReadWriteLock();
+    private final Executor mExecutor = Executors.newCachedThreadPool();
 
     public PreCachingAlgorithmDecorator(Algorithm<T> algorithm) {
         mAlgorithm = algorithm;
@@ -80,10 +83,10 @@ public Set<? extends Cluster<T>> getClusters(float zoom) {
         Set<? extends Cluster<T>> results = getClustersInternal(discreteZoom);
         // TODO: Check if requests are already in-flight.
         if (mCache.get(discreteZoom + 1) == null) {
-            new Thread(new PrecacheRunnable(discreteZoom + 1)).start();
+            mExecutor.execute(new PrecacheRunnable(discreteZoom + 1));
         }
         if (mCache.get(discreteZoom - 1) == null) {
-            new Thread(new PrecacheRunnable(discreteZoom - 1)).start();
+            mExecutor.execute(new PrecacheRunnable(discreteZoom - 1));
         }
         return results;
     }

library/src/main/java/com/google/maps/android/clustering/view/DefaultClusterRenderer.java

@@ -64,6 +64,8 @@
 import java.util.Queue;
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.Executor;
+import java.util.concurrent.Executors;
 import java.util.concurrent.locks.Condition;
 import java.util.concurrent.locks.Lock;
 import java.util.concurrent.locks.ReentrantLock;
@@ -77,6 +79,7 @@ public class DefaultClusterRenderer<T extends ClusterItem> implements ClusterRen
     private final ClusterManager<T> mClusterManager;
     private final float mDensity;
     private boolean mAnimate;
+    private final Executor mExecutor = Executors.newSingleThreadExecutor();
 
     private static final int[] BUCKETS = {10, 20, 50, 100, 200, 500, 1000};
     private ShapeDrawable mColoredCircleBackground;
@@ -290,7 +293,7 @@ public void run() {
             });
             renderTask.setProjection(projection);
             renderTask.setMapZoom(mMap.getCameraPosition().zoom);
-            new Thread(renderTask).start();
+            mExecutor.execute(renderTask);
         }
 
         public void queue(Set<? extends Cluster<T>> clusters) {