Drop CSSNode pool on low memory when app is backgrounded

Summary:
Depends on D4189532

Drops the CSSNodePool when memory gets low.

Reviewed By: emilsjolander

Differential Revision: D4190264

fbshipit-source-id: 94cd36d877372e0d6ebdd989661af74bde41486d

Author: astreet

ReactAndroid/src/main/java/com/facebook/react/uimanager/UIManagerModule.java

@@ -11,10 +11,13 @@
 
 import javax.annotation.Nullable;
 
-import java.util.List;
 import java.util.HashMap;
+import java.util.List;
 import java.util.Map;
 
+import android.content.ComponentCallbacks2;
+import android.content.res.Configuration;
+
 import com.facebook.common.logging.FLog;
 import com.facebook.react.animation.Animation;
 import com.facebook.react.bridge.Callback;
@@ -78,6 +81,7 @@ public class UIManagerModule extends ReactContextBaseJavaModule implements
   private final EventDispatcher mEventDispatcher;
   private final Map<String, Object> mModuleConstants;
   private final UIImplementation mUIImplementation;
+  private final MemoryTrimCallback mMemoryTrimCallback = new MemoryTrimCallback();
 
   private int mNextRootViewTag = 1;
   private int mBatchId = 0;
@@ -114,6 +118,11 @@ public Map<String, Object> getConstants() {
     return mModuleConstants;
   }
 
+  @Override
+  public void initialize() {
+    getReactApplicationContext().registerComponentCallbacks(mMemoryTrimCallback);
+  }
+
   @Override
   public void onHostResume() {
     mUIImplementation.onHostResume();
@@ -133,6 +142,9 @@ public void onHostDestroy() {
   public void onCatalystInstanceDestroy() {
     super.onCatalystInstanceDestroy();
     mEventDispatcher.onCatalystInstanceDestroyed();
+
+    getReactApplicationContext().unregisterComponentCallbacks(mMemoryTrimCallback);
+    CSSNodePool.get().clear();
   }
 
   private static Map<String, Object> createConstants(List<ViewManager> viewManagerList) {
@@ -549,4 +561,25 @@ public void addUIBlock (UIBlock block) {
   public int resolveRootTagFromReactTag(int reactTag) {
     return mUIImplementation.resolveRootTagFromReactTag(reactTag);
   }
+
+  /**
+   * Listener that drops the CSSNode pool on low memory when the app is backgrounded.
+   */
+  private class MemoryTrimCallback implements ComponentCallbacks2 {
+
+    @Override
+    public void onTrimMemory(int level) {
+      if (level >= TRIM_MEMORY_MODERATE) {
+        CSSNodePool.get().clear();
+      }
+    }
+
+    @Override
+    public void onConfigurationChanged(Configuration newConfig) {
+    }
+
+    @Override
+    public void onLowMemory() {
+    }
+  }
 }