Extract DrawCommands tracking in StateBuilder into a helper class.

Summary: @public This is a pure refactoring diff makes `StateBuilder` code a little bit easier to read. This gets increasingly important as new features with similar logic are added to `StateBuilder`.

Reviewed By: sriramramani

Differential Revision: D2564342

ReactAndroid/src/main/java/com/facebook/react/flat/ElementsList.java

@@ -0,0 +1,175 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+package com.facebook.react.flat;
+
+import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.lang.reflect.Array;
+
+/**
+ * Helper class that supports 3 main operations: start(), add() an element and finish().
+ *
+ * When started, it takes a baseline array to compare to. When adding a new element, it checks
+ * whether a corresponding element in baseline array is the same. On finish(), it will return null
+ * if baseline array contains exactly the same elements that were added with a sequence of add()
+ * calls, or a new array the recorded elements:
+ *
+ * Example 1:
+ * -----
+ * start([A])
+ * add(A)
+ * finish() -> null (because [A] == [A])
+ *
+ * Example 2:
+ * ----
+ * start([A])
+ * add(B)
+ * finish() -> [B] (because [A] != [B])
+ *
+ * It is important that start/finish can be nested:
+ * ----
+ * start([A])
+ * add(A)
+ *   start([B])
+ *   add(B)
+ *   finish() -> null
+ * add(C)
+ * finish() -> [A, C]
+ *
+ * StateBuilder is using this class to check if e.g. a DrawCommand list for a given View needs to be
+ * updated.
+ */
+/* package */ final class ElementsList<E> {
+
+  private static final class Scope {
+    Object[] elements;
+    int index;
+    int size;
+  }
+
+  private final ArrayList<Scope> mScopesStack = new ArrayList<>();
+  private final ArrayDeque<E> mElements = new ArrayDeque<>();
+  private final E[] mEmptyArray;
+  private Scope mCurrentScope = null;
+  private int mScopeIndex = 0;
+
+  public ElementsList(E[] emptyArray) {
+    mEmptyArray = emptyArray;
+    mScopesStack.add(mCurrentScope);
+  }
+
+  /**
+   * Starts a new scope.
+   */
+  public void start(Object[] elements) {
+    pushScope();
+
+    Scope scope = getCurrentScope();
+    scope.elements = elements;
+    scope.index = 0;
+    scope.size = mElements.size();
+  }
+
+  /**
+   * Finished current scope, and returns null if there were no changes recorded, or a new array
+   * containing all the recorded elements otherwise.
+   */
+  public E[] finish() {
+    Scope scope = getCurrentScope();
+    popScope();
+
+    E[] result = null;
+    int size = mElements.size() - scope.size;
+    if (scope.index != scope.elements.length) {
+      result = extractElements(size);
+    } else {
+      // downsize
+      for (int i = 0; i < size; ++i) {
+        mElements.pollLast();
+      }
+    }
+
+    // to prevent leaks
+    scope.elements = null;
+
+    return result;
+  }
+
+  /**
+   * Adds a new element to the list. This method can be optimized to avoid inserts on same elements.
+   */
+  public void add(E element) {
+    Scope scope = getCurrentScope();
+
+    if (scope.index < scope.elements.length &&
+        scope.elements[scope.index] == element) {
+      ++scope.index;
+    } else {
+      scope.index = Integer.MAX_VALUE;
+    }
+
+    mElements.add(element);
+  }
+
+  /**
+   * Resets all references to the elements to null to avoid memory leaks.
+   */
+  public void clear() {
+    if (getCurrentScope() != null) {
+      throw new RuntimeException("Must call finish() for every start() call being made.");
+    }
+    mElements.clear();
+  }
+
+  /**
+   * Extracts last size elements into an array.
+   */
+  private E[] extractElements(int size) {
+    if (size == 0) {
+      // avoid allocating empty array
+      return mEmptyArray;
+    }
+
+    E[] elements = (E[]) Array.newInstance(mEmptyArray.getClass().getComponentType(), size);
+    for (int i = size - 1; i >= 0; --i) {
+      elements[i] = mElements.pollLast();
+    }
+
+    return elements;
+  }
+
+  /**
+   * Saves current scope in a stack.
+   */
+  private void pushScope() {
+    ++mScopeIndex;
+    if (mScopeIndex == mScopesStack.size()) {
+      mCurrentScope = new Scope();
+      mScopesStack.add(mCurrentScope);
+    } else {
+      mCurrentScope = mScopesStack.get(mScopeIndex);
+    }
+  }
+
+  /**
+   * Restores last save current scope.
+   */
+  private void popScope() {
+    --mScopeIndex;
+    mCurrentScope = mScopesStack.get(mScopeIndex);
+  }
+
+  /**
+   * Returns current scope.
+   */
+  private Scope getCurrentScope() {
+    return mCurrentScope;
+  }
+}

ReactAndroid/src/main/java/com/facebook/react/flat/StateBuilder.java

@@ -9,8 +9,6 @@
 
 package com.facebook.react.flat;
 
-import java.util.ArrayDeque;
-
 /**
  * Shadow node hierarchy by itself cannot display UI, it is only a representation of what UI should
  * be from JavaScript perspective. StateBuilder is a helper class that can walk the shadow node tree
@@ -21,10 +19,8 @@ import java.util.ArrayDeque;
 
   private final FlatUIViewOperationQueue mOperationsQueue;
 
-  // DrawCommands
-  private final ArrayDeque<DrawCommand> mDrawCommands = new ArrayDeque<>();
-  private DrawCommand[] mPreviousDrawCommands = DrawCommand.EMPTY_ARRAY;
-  private int mPreviousDrawCommandsIndex;
+  private final ElementsList<DrawCommand> mDrawCommands =
+      new ElementsList(DrawCommand.EMPTY_ARRAY);
 
   /* package */ StateBuilder(FlatUIViewOperationQueue operationsQueue) {
     mOperationsQueue = operationsQueue;
@@ -42,14 +38,7 @@ import java.util.ArrayDeque;
    * Adds a DrawCommand for current mountable node.
    */
   /* package */ void addDrawCommand(AbstractDrawCommand drawCommand) {
-    if (mPreviousDrawCommandsIndex < mPreviousDrawCommands.length &&
-        mPreviousDrawCommands[mPreviousDrawCommandsIndex] == drawCommand) {
-      ++mPreviousDrawCommandsIndex;
-    } else {
-      mPreviousDrawCommandsIndex = mPreviousDrawCommands.length + 1;
-    }
-
-    mDrawCommands.addLast(drawCommand);
+    mDrawCommands.add(drawCommand);
   }
 
   /**
@@ -86,47 +75,17 @@ import java.util.ArrayDeque;
       int tag,
       float width,
       float height) {
-    // save
-    int d = mDrawCommands.size();
-    DrawCommand[] previousDrawCommands = mPreviousDrawCommands;
-    int previousDrawCommandsIndex = mPreviousDrawCommandsIndex;
-
-    // reset
-    mPreviousDrawCommands = node.getDrawCommands();
-    mPreviousDrawCommandsIndex = 0;
+    mDrawCommands.start(node.getDrawCommands());
 
     collectStateRecursively(node, 0, 0, width, height);
 
-    if (mPreviousDrawCommandsIndex != mPreviousDrawCommands.length) {
-      // DrawCommands changes, need to re-mount them and re-draw the View.
-      DrawCommand[] drawCommands = extractDrawCommands(d);
+    final DrawCommand[] drawCommands = mDrawCommands.finish();
+    if (drawCommands != null) {
+      // DrawCommands changed, need to re-mount them and re-draw the View.
       node.setDrawCommands(drawCommands);
 
       mOperationsQueue.enqueueUpdateMountState(tag, drawCommands);
     }
-
-    // restore
-    mPreviousDrawCommandsIndex = previousDrawCommandsIndex;
-    mPreviousDrawCommands = previousDrawCommands;
-  }
-
-  /**
-   * Returns all DrawCommands collectes so far starting from a given index.
-   */
-  private DrawCommand[] extractDrawCommands(int lowerBound) {
-    int upperBound = mDrawCommands.size();
-    int size = upperBound - lowerBound;
-    if (size == 0) {
-      // avoid allocating empty array
-      return DrawCommand.EMPTY_ARRAY;
-    }
-
-    DrawCommand[] drawCommands = new DrawCommand[size];
-    for (int i = 0; i < size; ++i) {
-      drawCommands[i] = mDrawCommands.pollFirst();
-    }
-
-    return drawCommands;
   }
 
   /**