Bug 1887800 part 2: Support the UIA LabeledBy property. r=morgan

Differential Revision: https://phabricator.services.mozilla.com/D208852

accessible/tests/browser/windows/uia/browser_relationProps.js

@@ -91,3 +91,53 @@ addUiaTask(
     await testUiaRelationArray("none", "FlowsTo", []);
   }
 );
+
+/**
+ * Test the LabeledBy property.
+ */
+addUiaTask(
+  `
+<label id="label">label</label>
+<input id="input" aria-labelledby="label">
+<label id="wrappingLabel">
+  <input id="wrappedInput" value="wrappedInput">
+  <p id="wrappingLabelP">wrappingLabel</p>
+</label>
+<button id="button" aria-labelledby="label">content</button>
+<button id="noLabel">noLabel</button>
+  `,
+  async function testLabeledBy() {
+    await definePyVar("doc", `getDocUia()`);
+    // input's LabeledBy should be label's text leaf.
+    let result = await runPython(`
+      input = findUiaByDomId(doc, "input")
+      label = findUiaByDomId(doc, "label")
+      labelLeaf = uiaClient.RawViewWalker.GetFirstChildElement(label)
+      return uiaClient.CompareElements(input.CurrentLabeledBy, labelLeaf)
+    `);
+    ok(result, "input has correct LabeledBy");
+    // wrappedInput's LabeledBy should be wrappingLabelP's text leaf.
+    result = await runPython(`
+      wrappedInput = findUiaByDomId(doc, "wrappedInput")
+      wrappingLabelP = findUiaByDomId(doc, "wrappingLabelP")
+      wrappingLabelLeaf = uiaClient.RawViewWalker.GetFirstChildElement(wrappingLabelP)
+      return uiaClient.CompareElements(wrappedInput.CurrentLabeledBy, wrappingLabelLeaf)
+    `);
+    ok(result, "wrappedInput has correct LabeledBy");
+    // button has aria-labelledby, but UIA prohibits LabeledBy on buttons.
+    ok(
+      !(await runPython(
+        `bool(findUiaByDomId(doc, "button").CurrentLabeledBy)`
+      )),
+      "button has no LabeledBy"
+    );
+    ok(
+      !(await runPython(
+        `bool(findUiaByDomId(doc, "noLabel").CurrentLabeledBy)`
+      )),
+      "noLabel has no LabeledBy"
+    );
+  },
+  // The IA2 -> UIA proxy doesn't expose LabeledBy properly.
+  { uiaEnabled: true, uiaDisabled: false }
+);

accessible/windows/uia/uiaRawElmProvider.cpp

@@ -22,7 +22,9 @@
 #include "MsaaRootAccessible.h"
 #include "nsAccessibilityService.h"
 #include "nsAccUtils.h"
+#include "nsIAccessiblePivot.h"
 #include "nsTextEquivUtils.h"
+#include "Pivot.h"
 #include "Relation.h"
 #include "RootAccessible.h"
 
@@ -60,6 +62,29 @@ static bool IsRadio(Accessible* aAcc) {
   return r == roles::RADIOBUTTON || r == roles::RADIO_MENU_ITEM;
 }
 
+// Used to search for a text leaf descendant for the LabeledBy property.
+class LabelTextLeafRule : public PivotRule {
+ public:
+  virtual uint16_t Match(Accessible* aAcc) override {
+    if (aAcc->IsTextLeaf()) {
+      nsAutoString name;
+      aAcc->Name(name);
+      if (name.IsEmpty() || name.EqualsLiteral(" ")) {
+        // An empty or white space text leaf isn't useful as a label.
+        return nsIAccessibleTraversalRule::FILTER_IGNORE;
+      }
+      return nsIAccessibleTraversalRule::FILTER_MATCH;
+    }
+    if (!nsTextEquivUtils::HasNameRule(aAcc, eNameFromSubtreeIfReqRule)) {
+      // Don't descend into things that can't be used as label content; e.g.
+      // text boxes.
+      return nsIAccessibleTraversalRule::FILTER_IGNORE |
+             nsIAccessibleTraversalRule::FILTER_IGNORE_SUBTREE;
+    }
+    return nsIAccessibleTraversalRule::FILTER_IGNORE;
+  }
+};
+
 ////////////////////////////////////////////////////////////////////////////////
 // uiaRawElmProvider
 ////////////////////////////////////////////////////////////////////////////////
@@ -602,6 +627,15 @@ uiaRawElmProvider::GetPropertyValue(PROPERTYID aPropertyId,
           (acc->State() & states::FOCUSABLE) ? VARIANT_TRUE : VARIANT_FALSE;
       return S_OK;
 
+    case UIA_LabeledByPropertyId:
+      if (Accessible* target = GetLabeledBy()) {
+        aPropertyValue->vt = VT_UNKNOWN;
+        RefPtr<IRawElementProviderSimple> uia = MsaaAccessible::GetFrom(target);
+        uia.forget(&aPropertyValue->punkVal);
+        return S_OK;
+      }
+      break;
+
     case UIA_LevelPropertyId:
       aPropertyValue->vt = VT_I4;
       aPropertyValue->lVal = acc->GroupPosition().level;
@@ -1262,6 +1296,46 @@ SAFEARRAY* uiaRawElmProvider::AccRelationsToUiaArray(
   return AccessibleArrayToUiaArray(targets);
 }
 
+Accessible* uiaRawElmProvider::GetLabeledBy() const {
+  // Per the UIA documentation, some control types should never get a value for
+  // the LabeledBy property.
+  switch (GetControlType()) {
+    case UIA_ButtonControlTypeId:
+    case UIA_CheckBoxControlTypeId:
+    case UIA_DataItemControlTypeId:
+    case UIA_MenuControlTypeId:
+    case UIA_MenuBarControlTypeId:
+    case UIA_RadioButtonControlTypeId:
+    case UIA_ScrollBarControlTypeId:
+    case UIA_SeparatorControlTypeId:
+    case UIA_StatusBarControlTypeId:
+    case UIA_TabItemControlTypeId:
+    case UIA_TextControlTypeId:
+    case UIA_ToolBarControlTypeId:
+    case UIA_ToolTipControlTypeId:
+    case UIA_TreeItemControlTypeId:
+      return nullptr;
+  }
+
+  Accessible* acc = Acc();
+  MOZ_ASSERT(acc);
+  // Even when LabeledBy is supported, it can only return a single "static text"
+  // element.
+  Relation rel = acc->RelationByType(RelationType::LABELLED_BY);
+  LabelTextLeafRule rule;
+  while (Accessible* target = rel.Next()) {
+    // If target were a text leaf, we should return that, but that shouldn't be
+    // possible because only an element (not a text node) can be the target of a
+    // relation.
+    MOZ_ASSERT(!target->IsTextLeaf());
+    Pivot pivot(target);
+    if (Accessible* leaf = pivot.Next(target, rule)) {
+      return leaf;
+    }
+  }
+  return nullptr;
+}
+
 SAFEARRAY* a11y::AccessibleArrayToUiaArray(const nsTArray<Accessible*>& aAccs) {
   if (aAccs.IsEmpty()) {
     // The UIA documentation is unclear about this, but the UIA client

accessible/windows/uia/uiaRawElmProvider.h

@@ -191,6 +191,7 @@ class uiaRawElmProvider : public IAccessibleEx,
   bool HasSelectionItemPattern();
   SAFEARRAY* AccRelationsToUiaArray(
       std::initializer_list<RelationType> aTypes) const;
+  Accessible* GetLabeledBy() const;
 };
 
 SAFEARRAY* AccessibleArrayToUiaArray(const nsTArray<Accessible*>& aAccs);