Add callback for UIImplementation layout updates

Differential Revision: D6063839

fbshipit-source-id: ae08fd2fc6c6365813b8077efe14990a0355a5c7

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

@@ -54,6 +54,14 @@ public class UIImplementation {
   private final int[] mMeasureBuffer = new int[4];
 
   private long mLastCalculateLayoutTime = 0;
+  protected @Nullable LayoutUpdateListener mLayoutUpdateListener;
+
+  /** Interface definition for a callback to be invoked when the layout has been updated */
+  public interface LayoutUpdateListener {
+
+    /** Called when the layout has been updated */
+    void onLayoutUpdated(ReactShadowNode root);
+  }
 
   public UIImplementation(
       ReactApplicationContext reactContext,
@@ -693,6 +701,10 @@ public class UIImplementation {
           } finally {
             Systrace.endSection(Systrace.TRACE_TAG_REACT_JAVA_BRIDGE);
           }
+
+          if (mLayoutUpdateListener != null) {
+            mLayoutUpdateListener.onLayoutUpdated(cssRoot);
+          }
         }
       }
     } finally {
@@ -1009,4 +1021,12 @@ public class UIImplementation {
   public void enableLayoutCalculationForRootNode(int rootViewTag) {
     this.mMeasuredRootNodes.add(rootViewTag);
   }
+
+  public void setLayoutUpdateListener(LayoutUpdateListener listener) {
+    mLayoutUpdateListener = listener;
+  }
+
+  public void removeLayoutUpdateListener() {
+    mLayoutUpdateListener = null;
+  }
 }