Implement scrollWidth/scrollHeight (#39328)

Summary:
Pull Request resolved: #39328

This adds a new method in Fabric to get the scroll size for an element, and uses it to implement `scrollWidth` and `scrollHeight` as defined in react-native-community/discussions-and-proposals#607

Scroll size determine how much of the content of a node would move if the node was scrollable. If the content does not overflow the padding box of the node, then this is the same as the `client{Width,Height}` (the size of the node without its borders). If the content would overflow the node, then it would be the size of the content that would be scrollable (in other words, what would "move" when you scrolled).

If the element isn't displayed or it has display: inline, it return 0 in both cases.

These APIs provide rounded integers.

NOTE: The current implementation of `ScrollView` has several known bugs and inconsistencies across platforms (Android vs. iOS) and architectures (Paper vs. Fabric) (e.g.: content showing on top of the border on Android, `overflow: visible` only working on Android but not on iOS, etc.). The data that this API reports is the one that aligns with the Web (with a few limitations), and we'll need to fix the implementation to align with this.

NOTE: transforms are not considered correctly for the sake of this API, but also not applied correctly in any of the native platforms. On Web, the scrollable area is the overflow of all the children **with transforms applied** which isn't honored in RN. We''ll fix the data reported by this API when we also fix the user perceived behavior.

Changelog: [internal]

Reviewed By: sammy-SC

Differential Revision: D49058368

fbshipit-source-id: 39a10bf7bddec9afc54f46cc02284d601b6962f3

packages/react-native/Libraries/DOM/Nodes/ReadOnlyElement.js

@@ -135,7 +135,16 @@ export default class ReadOnlyElement extends ReadOnlyNode {
   }
 
   get scrollHeight(): number {
-    throw new Error('Unimplemented');
+    const node = getShadowNode(this);
+
+    if (node != null) {
+      const scrollSize = nullthrows(getFabricUIManager()).getScrollSize(node);
+      if (scrollSize != null) {
+        return scrollSize[1];
+      }
+    }
+
+    return 0;
   }
 
   get scrollLeft(): number {
@@ -169,7 +178,16 @@ export default class ReadOnlyElement extends ReadOnlyNode {
   }
 
   get scrollWidth(): number {
-    throw new Error('Unimplemented');
+    const node = getShadowNode(this);
+
+    if (node != null) {
+      const scrollSize = nullthrows(getFabricUIManager()).getScrollSize(node);
+      if (scrollSize != null) {
+        return scrollSize[0];
+      }
+    }
+
+    return 0;
   }
 
   get tagName(): string {

packages/react-native/Libraries/ReactNative/FabricUIManager.js

@@ -91,6 +91,9 @@ export interface Spec {
   +getScrollPosition: (
     node: Node,
   ) => ?[/* scrollLeft: */ number, /* scrollTop: */ number];
+  +getScrollSize: (
+    node: Node,
+  ) => ?[/* scrollWidth: */ number, /* scrollHeight: */ number];
   +getInnerSize: (node: Node) => ?[/* width: */ number, /* height: */ number];
   +getBorderSize: (
     node: Node,
@@ -141,6 +144,7 @@ const CACHED_PROPERTIES = [
   'getBoundingClientRect',
   'getOffset',
   'getScrollPosition',
+  'getScrollSize',
   'getInnerSize',
   'getBorderSize',
   'getTagName',

packages/react-native/Libraries/ReactNative/__mocks__/FabricUIManager.js

@@ -524,6 +524,34 @@ const FabricUIManagerMock: IFabricUIManagerMock = {
     },
   ),
 
+  getScrollSize: jest.fn(
+    (node: Node): ?[/* scrollLeft: */ number, /* scrollTop: */ number] => {
+      ensureHostNode(node);
+
+      const nodeInCurrentTree = getNodeInCurrentTree(node);
+      const currentProps =
+        nodeInCurrentTree != null ? fromNode(nodeInCurrentTree).props : null;
+      if (currentProps == null) {
+        return null;
+      }
+
+      const scrollForTests: ?{
+        scrollWidth: number,
+        scrollHeight: number,
+        ...
+      } =
+        // $FlowExpectedError[prop-missing]
+        currentProps.__scrollForTests;
+
+      if (scrollForTests == null) {
+        return null;
+      }
+
+      const {scrollWidth, scrollHeight} = scrollForTests;
+      return [scrollWidth, scrollHeight];
+    },
+  ),
+
   getInnerSize: jest.fn(
     (node: Node): ?[/* width: */ number, /* height: */ number] => {
       ensureHostNode(node);

packages/react-native/ReactCommon/react/renderer/uimanager/UIManagerBinding.cpp

@@ -1209,6 +1209,74 @@ jsi::Value UIManagerBinding::get(
         });
   }
 
+  if (methodName == "getScrollSize") {
+    // This is a method to access the scroll information of a shadow node, to
+    // implement these methods:
+    // * `Element.prototype.scrollWidth`: see
+    // https://developer.mozilla.org/en-US/docs/Web/API/Element/scrollWidth.
+    // * `Element.prototype.scrollHeight`: see
+    // https://developer.mozilla.org/en-US/docs/Web/API/Element/scrollHeight.
+
+    // It uses the version of the shadow node that is present in the current
+    // revision of the shadow tree. If the node is not present or is not
+    // displayed (because any of its ancestors or itself have 'display: none'),
+    // it returns undefined. Otherwise, it returns the scroll size.
+
+    // getScrollSize(shadowNode: ShadowNode):
+    //   ?[
+    //     /* scrollWidth: */ number,
+    //     /* scrollHeight: */ number,
+    //   ]
+    auto paramCount = 1;
+    return jsi::Function::createFromHostFunction(
+        runtime,
+        name,
+        paramCount,
+        [uiManager, methodName, paramCount](
+            jsi::Runtime& runtime,
+            const jsi::Value& /*thisValue*/,
+            const jsi::Value* arguments,
+            size_t count) -> jsi::Value {
+          validateArgumentCount(runtime, methodName, paramCount, count);
+
+          auto shadowNode = shadowNodeFromValue(runtime, arguments[0]);
+
+          auto newestCloneOfShadowNode =
+              uiManager->getNewestCloneOfShadowNode(*shadowNode);
+          // The node is no longer part of an active shadow tree, or it is the
+          // root node
+          if (newestCloneOfShadowNode == nullptr) {
+            return jsi::Value::undefined();
+          }
+
+          // If the node is not displayed (itself or any of its ancestors has
+          // "display: none"), this returns an empty layout metrics object.
+          auto layoutMetrics = uiManager->getRelativeLayoutMetrics(
+              *shadowNode, nullptr, {/* .includeTransform = */ false});
+
+          if (layoutMetrics == EmptyLayoutMetrics ||
+              layoutMetrics.displayType == DisplayType::Inline) {
+            return jsi::Value::undefined();
+          }
+
+          auto layoutableShadowNode =
+              traitCast<YogaLayoutableShadowNode const*>(
+                  newestCloneOfShadowNode.get());
+          // This should never happen
+          if (layoutableShadowNode == nullptr) {
+            return jsi::Value::undefined();
+          }
+
+          Size scrollSize = getScrollSize(
+              layoutMetrics, layoutableShadowNode->getContentBounds());
+
+          return jsi::Array::createWithElements(
+              runtime,
+              jsi::Value{runtime, std::round(scrollSize.width)},
+              jsi::Value{runtime, std::round(scrollSize.height)});
+        });
+  }
+
   if (methodName == "getInnerSize") {
     // This is a method to access the inner size of a shadow node, to implement
     // these methods:

packages/react-native/ReactCommon/react/renderer/uimanager/primitives.h

@@ -221,4 +221,39 @@ inline static void getTextContentInShadowNode(
     getTextContentInShadowNode(*childNode.get(), result);
   }
 }
+
+inline static Rect getScrollableContentBounds(
+    Rect contentBounds,
+    LayoutMetrics layoutMetrics) {
+  auto paddingFrame = layoutMetrics.getPaddingFrame();
+
+  auto paddingBottom =
+      layoutMetrics.contentInsets.bottom - layoutMetrics.borderWidth.bottom;
+  auto paddingLeft =
+      layoutMetrics.contentInsets.left - layoutMetrics.borderWidth.left;
+  auto paddingRight =
+      layoutMetrics.contentInsets.right - layoutMetrics.borderWidth.right;
+
+  auto minY = paddingFrame.getMinY();
+  auto maxY =
+      std::max(paddingFrame.getMaxY(), contentBounds.getMaxY() + paddingBottom);
+
+  auto minX = layoutMetrics.layoutDirection == LayoutDirection::RightToLeft
+      ? std::min(paddingFrame.getMinX(), contentBounds.getMinX() - paddingLeft)
+      : paddingFrame.getMinX();
+  auto maxX = layoutMetrics.layoutDirection == LayoutDirection::RightToLeft
+      ? paddingFrame.getMaxX()
+      : std::max(
+            paddingFrame.getMaxX(), contentBounds.getMaxX() + paddingRight);
+
+  return Rect{Point{minX, minY}, Size{maxX - minX, maxY - minY}};
+}
+
+inline static Size getScrollSize(
+    LayoutMetrics layoutMetrics,
+    Rect contentBounds) {
+  auto scrollableContentBounds =
+      getScrollableContentBounds(contentBounds, layoutMetrics);
+  return scrollableContentBounds.size;
+}
 } // namespace facebook::react