Prototype View recycling for View

Summary:
Prototype of View Recycling for View + generic APIs.

Changelog: [Added][Android] Adding experimental View Recycling for Fabric on Android.

Reviewed By: mdvacca

Differential Revision: D36608419

fbshipit-source-id: c469ce2fe12ef9332d3def591118befc4a619870

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

@@ -16,13 +16,16 @@ import com.facebook.react.bridge.ReactApplicationContext;
 import com.facebook.react.bridge.ReadableArray;
 import com.facebook.react.bridge.ReadableMap;
 import com.facebook.react.common.mapbuffer.MapBuffer;
+import com.facebook.react.config.ReactFeatureFlags;
 import com.facebook.react.touch.JSResponderHandler;
 import com.facebook.react.touch.ReactInterceptingViewGroup;
 import com.facebook.react.uimanager.annotations.ReactProp;
 import com.facebook.react.uimanager.annotations.ReactPropGroup;
 import com.facebook.react.uimanager.annotations.ReactPropertyHolder;
 import com.facebook.yoga.YogaMeasureMode;
+import java.util.HashMap;
 import java.util.Map;
+import java.util.Stack;
 
 /**
  * Class responsible for knowing how to create and update catalyst Views of a given type. It is also
@@ -33,6 +36,30 @@ import java.util.Map;
 public abstract class ViewManager<T extends View, C extends ReactShadowNode>
     extends BaseJavaModule {
 
+  /**
+   * For View recycling: we store a Stack of unused, dead Views. This is null by default, and when
+   * null signals that View Recycling is disabled. `enableViewRecycling` must be explicitly called
+   * in a concrete constructor to enable View Recycling per ViewManager.
+   */
+  private HashMap<Integer, Stack<T>> mRecyclableViews = null;
+
+  /** Call in constructor of concrete ViewManager class to enable. */
+  protected void enableViewRecycling() {
+    if (ReactFeatureFlags.enableViewRecycling) {
+      mRecyclableViews = new HashMap<>();
+    }
+  }
+
+  private @Nullable Stack<T> getRecyclableViewStack(int surfaceId) {
+    if (mRecyclableViews == null) {
+      return null;
+    }
+    if (!mRecyclableViews.containsKey(surfaceId)) {
+      mRecyclableViews.put(surfaceId, new Stack<>());
+    }
+    return mRecyclableViews.get(surfaceId);
+  }
+
   /**
    * For the vast majority of ViewManagers, you will not need to override this. Only override this
    * if you really know what you're doing and have a very unique use-case.
@@ -137,7 +164,13 @@ public abstract class ViewManager<T extends View, C extends ReactShadowNode>
       @NonNull ThemedReactContext reactContext,
       @Nullable ReactStylesDiffMap initialProps,
       @Nullable StateWrapper stateWrapper) {
-    T view = createViewInstance(reactContext);
+    T view = null;
+    @Nullable Stack<T> recyclableViews = getRecyclableViewStack(reactContext.getSurfaceId());
+    if (recyclableViews != null && !recyclableViews.empty()) {
+      view = recycleView(reactContext, recyclableViews.pop());
+    } else {
+      view = createViewInstance(reactContext);
+    }
     view.setId(reactTag);
     addEventEmitters(reactContext, view);
     if (initialProps != null) {
@@ -157,7 +190,27 @@ public abstract class ViewManager<T extends View, C extends ReactShadowNode>
    * Called when view is detached from view hierarchy and allows for some additional cleanup by the
    * {@link ViewManager} subclass.
    */
-  public void onDropViewInstance(@NonNull T view) {}
+  public void onDropViewInstance(@NonNull T view) {
+    @Nullable
+    Stack<T> recyclableViews =
+        getRecyclableViewStack(((ThemedReactContext) view.getContext()).getSurfaceId());
+    // By default we treat views as recyclable
+    if (recyclableViews != null) {
+      recyclableViews.push(prepareToRecycleView((ThemedReactContext) view.getContext(), view));
+    }
+  }
+
+  /**
+   * Called when a View is removed from the hierachy. This should be used to reset any properties.
+   */
+  protected T prepareToRecycleView(@NonNull ThemedReactContext reactContext, @NonNull T view) {
+    return view;
+  }
+
+  /** Called when a View is going to be reused. */
+  protected T recycleView(@NonNull ThemedReactContext reactContext, @NonNull T view) {
+    return view;
+  }
 
   /**
    * Subclasses can override this method to install custom event emitters on the given View. You
@@ -370,4 +423,24 @@ public abstract class ViewManager<T extends View, C extends ReactShadowNode>
    * components support setting padding, the default implementation of this method does nothing.
    */
   public void setPadding(T view, int left, int top, int right, int bottom) {}
+
+  /**
+   * Lifecycle method: called when a surface is stopped. Currently only used for View Recycling
+   * cleanup. There is no corresponding startSurface lifecycle event for ViewManagers because we
+   * currently only need this for recycling cleanup. Only called in Fabric.
+   */
+  public void onSurfaceStopped(int surfaceId) {
+    if (mRecyclableViews != null) {
+      mRecyclableViews.remove(surfaceId);
+    }
+  }
+
+  /** With even slight memory pressure, we immediately evict all recyclable Views. */
+  /* package */ void trimMemory() {
+    // Wipe out all existing recyclable Views, but do not disable View Recycling entirely.
+    // We only take any action if View Recycling is already enabled.
+    if (mRecyclableViews != null) {
+      mRecyclableViews = new HashMap<>();
+    }
+  }
 }