Implement clientTop/clientLeft in ReadOnlyElement (facebook#39308)

Summary:
Pull Request resolved: facebook#39308

This adds a new method in Fabric to get the border size for an element, and uses it to implement the following methods as defined in react-native-community/discussions-and-proposals#607 :
* `clientLeft`: left border width of the element.
* `clientTop`: top border width of the element.
If the element isn't displayed or it has display: inline, it return 0 in both cases.

These APIs provide rounded integers.

Changelog: [internal]

Reviewed By: mdvacca

Differential Revision: D49009140

fbshipit-source-id: aec2d2f907339aa45e28271100c89b954f816cef

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

@@ -46,11 +46,29 @@ export default class ReadOnlyElement extends ReadOnlyNode {
   }
 
   get clientLeft(): number {
-    throw new TypeError('Unimplemented');
+    const node = getShadowNode(this);
+
+    if (node != null) {
+      const borderSize = nullthrows(getFabricUIManager()).getBorderSize(node);
+      if (borderSize != null) {
+        return borderSize[3];
+      }
+    }
+
+    return 0;
   }
 
   get clientTop(): number {
-    throw new TypeError('Unimplemented');
+    const node = getShadowNode(this);
+
+    if (node != null) {
+      const borderSize = nullthrows(getFabricUIManager()).getBorderSize(node);
+      if (borderSize != null) {
+        return borderSize[0];
+      }
+    }
+
+    return 0;
   }
 
   get clientWidth(): number {

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

@@ -92,6 +92,14 @@ export interface Spec {
     node: Node,
   ) => ?[/* scrollLeft: */ number, /* scrollTop: */ number];
   +getInnerSize: (node: Node) => ?[/* width: */ number, /* height: */ number];
+  +getBorderSize: (
+    node: Node,
+  ) => ?[
+    /* topWidth: */ number,
+    /* rightWidth: */ number,
+    /* bottomWidth: */ number,
+    /* leftWidth: */ number,
+  ];
   +getTagName: (node: Node) => string;
 
   /**
@@ -134,6 +142,7 @@ const CACHED_PROPERTIES = [
   'getOffset',
   'getScrollPosition',
   'getInnerSize',
+  'getBorderSize',
   'getTagName',
   'hasPointerCapture',
   'setPointerCapture',

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

@@ -552,6 +552,48 @@ const FabricUIManagerMock: IFabricUIManagerMock = {
     },
   ),
 
+  getBorderSize: jest.fn(
+    (
+      node: Node,
+    ): ?[
+      /* topWidth: */ number,
+      /* rightWidth: */ number,
+      /* bottomWidth: */ number,
+      /* leftWidth: */ number,
+    ] => {
+      ensureHostNode(node);
+
+      const nodeInCurrentTree = getNodeInCurrentTree(node);
+      const currentProps =
+        nodeInCurrentTree != null ? fromNode(nodeInCurrentTree).props : null;
+      if (currentProps == null) {
+        return null;
+      }
+
+      const borderSizeForTests: ?{
+        topWidth?: number,
+        rightWidth?: number,
+        bottomWidth?: number,
+        leftWidth?: number,
+        ...
+      } =
+        // $FlowExpectedError[prop-missing]
+        currentProps.__borderSizeForTests;
+
+      if (borderSizeForTests == null) {
+        return null;
+      }
+
+      const {
+        topWidth = 0,
+        rightWidth = 0,
+        bottomWidth = 0,
+        leftWidth = 0,
+      } = borderSizeForTests;
+      return [topWidth, rightWidth, bottomWidth, leftWidth];
+    },
+  ),
+
   getTagName: jest.fn((node: Node): string => {
     ensureHostNode(node);
     return 'RN:' + fromNode(node).viewName;

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

@@ -1261,6 +1261,60 @@ jsi::Value UIManagerBinding::get(
         });
   }
 
+  if (methodName == "getBorderSize") {
+    // This is a method to access the border size of a shadow node, to implement
+    // these methods:
+    // * `Element.prototype.clientLeft`: see
+    // https://developer.mozilla.org/en-US/docs/Web/API/Element/clientLeft.
+    // * `Element.prototype.clientTop`: see
+    // https://developer.mozilla.org/en-US/docs/Web/API/Element/clientTop.
+
+    // 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, it is not
+    // displayed (because any of its ancestors or itself have 'display: none'),
+    // or it has an inline display, it returns undefined.
+    // Otherwise, it returns its border size.
+
+    // getBorderSize(shadowNode: ShadowNode):
+    //   ?[
+    //     /* topWidth: */ number,
+    //     /* rightWidth: */ number,
+    //     /* bottomWidth: */ number,
+    //     /* leftWidth: */ 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]);
+
+          // 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 = */ true});
+
+          if (layoutMetrics == EmptyLayoutMetrics ||
+              layoutMetrics.displayType == DisplayType::Inline) {
+            return jsi::Value::undefined();
+          }
+
+          return jsi::Array::createWithElements(
+              runtime,
+              jsi::Value{runtime, std::round(layoutMetrics.borderWidth.top)},
+              jsi::Value{runtime, std::round(layoutMetrics.borderWidth.right)},
+              jsi::Value{runtime, std::round(layoutMetrics.borderWidth.bottom)},
+              jsi::Value{runtime, std::round(layoutMetrics.borderWidth.left)});
+        });
+  }
+
   if (methodName == "getTagName") {
     // This is a method to access the normalized tag name of a shadow node, to
     // implement `Element.prototype.tagName` (see