Recycle CSSNodes

Summary: Adds a pool to recycle CSSNodes within UIManager. A follow-up diff will hook this up to a memory pressure listener to drop the pool on memory pressure.

Reviewed By: emilsjolander

Differential Revision: D4189532

fbshipit-source-id: 46583546f71a8c59853e1dd124de31657b3c617b

ReactAndroid/src/main/java/com/facebook/react/common/ClearableSynchronizedPool.java

@@ -0,0 +1,48 @@
+// Copyright 2004-present Facebook. All Rights Reserved.
+
+package com.facebook.react.common;
+
+import android.support.v4.util.Pools;
+
+/**
+ * Like {@link android.support.v4.util.Pools.SynchronizedPool} with the option to clear the pool
+ * (e.g. on memory pressure).
+ */
+public class ClearableSynchronizedPool<T> implements Pools.Pool<T> {
+
+  private final Object[] mPool;
+  private int mSize = 0;
+
+  public ClearableSynchronizedPool(int maxSize) {
+    mPool = new Object[maxSize];
+  }
+
+  @Override
+  public synchronized T acquire() {
+    if (mSize == 0) {
+      return null;
+    }
+    mSize--;
+    final int lastIndex = mSize;
+    T toReturn = (T) mPool[lastIndex];
+    mPool[lastIndex] = null;
+    return toReturn;
+  }
+
+  @Override
+  public synchronized boolean release(T obj) {
+    if (mSize == mPool.length) {
+      return false;
+    }
+    mPool[mSize] = obj;
+    mSize++;
+    return true;
+  }
+
+  public synchronized void clear() {
+    for (int i = 0; i < mSize; i++) {
+      mPool[i] = null;
+    }
+    mSize = 0;
+  }
+}