a11y: expose max character count for text fields (flutter#12269)

lib/ui/semantics.dart

@@ -618,9 +618,16 @@ class SemanticsUpdateBuilder extends NativeFieldWrapperClass2 {
   /// string describes what result an action performed on this node has. The
   /// reading direction of all these strings is given by `textDirection`.
   ///
-  /// The fields 'textSelectionBase' and 'textSelectionExtent' describe the
+  /// The fields `textSelectionBase` and `textSelectionExtent` describe the
   /// currently selected text within `value`.
   ///
+  /// The field `maxValueLength` is used to indicate that an editable text field
+  /// has a limit on the number of characters entered. If it is -1 there is
+  /// no limit on the number of characters entered. The field
+  /// `currentValueLength` indicates how much of that limit has already been
+  /// used up. When `maxValueLength` is set, `currentValueLength` must also be
+  /// set.
+  ///
   /// The field `platformViewId` references the platform view, whose semantics
   /// nodes will be added as children to this node. If a platform view is
   /// specified, `childrenInHitTestOrder` and `childrenInTraversalOrder` must be
@@ -652,6 +659,8 @@ class SemanticsUpdateBuilder extends NativeFieldWrapperClass2 {
     int id,
     int flags,
     int actions,
+    int maxValueLength,
+    int currentValueLength,
     int textSelectionBase,
     int textSelectionExtent,
     int platformViewId,
@@ -683,6 +692,8 @@ class SemanticsUpdateBuilder extends NativeFieldWrapperClass2 {
       id,
       flags,
       actions,
+      maxValueLength,
+      currentValueLength,
       textSelectionBase,
       textSelectionExtent,
       platformViewId,
@@ -713,6 +724,8 @@ class SemanticsUpdateBuilder extends NativeFieldWrapperClass2 {
     int id,
     int flags,
     int actions,
+    int maxValueLength,
+    int currentValueLength,
     int textSelectionBase,
     int textSelectionExtent,
     int platformViewId,

lib/ui/semantics/semantics_node.h

@@ -93,6 +93,8 @@ struct SemanticsNode {
   int32_t id = 0;
   int32_t flags = 0;
   int32_t actions = 0;
+  int32_t maxValueLength = -1;
+  int32_t currentValueLength = -1;
   int32_t textSelectionBase = -1;
   int32_t textSelectionExtent = -1;
   int32_t platformViewId = -1;

lib/ui/semantics/semantics_update_builder.cc

@@ -40,6 +40,8 @@ void SemanticsUpdateBuilder::updateNode(
     int id,
     int flags,
     int actions,
+    int maxValueLength,
+    int currentValueLength,
     int textSelectionBase,
     int textSelectionExtent,
     int platformViewId,
@@ -74,6 +76,8 @@ void SemanticsUpdateBuilder::updateNode(
   node.id = id;
   node.flags = flags;
   node.actions = actions;
+  node.maxValueLength = maxValueLength;
+  node.currentValueLength = currentValueLength;
   node.textSelectionBase = textSelectionBase;
   node.textSelectionExtent = textSelectionExtent;
   node.platformViewId = platformViewId;

lib/ui/semantics/semantics_update_builder.h

@@ -26,6 +26,8 @@ class SemanticsUpdateBuilder
   void updateNode(int id,
                   int flags,
                   int actions,
+                  int maxValueLength,
+                  int currentValueLength,
                   int textSelectionBase,
                   int textSelectionExtent,
                   int platformViewId,

lib/web_ui/lib/src/engine/semantics/semantics.dart

@@ -35,6 +35,8 @@ class SemanticsNodeUpdate {
     this.id,
     this.flags,
     this.actions,
+    this.maxValueLength,
+    this.currentValueLength,
     this.textSelectionBase,
     this.textSelectionExtent,
     this.platformViewId,
@@ -67,6 +69,12 @@ class SemanticsNodeUpdate {
   /// See [ui.SemanticsUpdateBuilder.updateNode].
   final int actions;
 
+  /// See [ui.SemanticsUpdateBuilder.updateNode].
+  final int maxValueLength;
+
+  /// See [ui.SemanticsUpdateBuilder.updateNode].
+  final int currentValueLength;
+
   /// See [ui.SemanticsUpdateBuilder.updateNode].
   final int textSelectionBase;
 

lib/web_ui/lib/src/ui/semantics.dart

@@ -648,6 +648,8 @@ class SemanticsUpdateBuilder {
     int id,
     int flags,
     int actions,
+    int maxValueLength,
+    int currentValueLength,
     int textSelectionBase,
     int textSelectionExtent,
     int platformViewId,
@@ -676,6 +678,8 @@ class SemanticsUpdateBuilder {
       id: id,
       flags: flags,
       actions: actions,
+      maxValueLength: maxValueLength,
+      currentValueLength: currentValueLength,
       textSelectionBase: textSelectionBase,
       textSelectionExtent: textSelectionExtent,
       scrollChildren: scrollChildren,

shell/platform/android/io/flutter/view/AccessibilityBridge.java

@@ -564,6 +564,14 @@ public AccessibilityNodeInfo createAccessibilityNodeInfo(int virtualViewId) {
                 granularities |= AccessibilityNodeInfo.MOVEMENT_GRANULARITY_WORD;
             }
             result.setMovementGranularities(granularities);
+            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP && semanticsNode.maxValueLength >= 0) {
+                // Account for the fact that Flutter is counting Unicode scalar values and Android
+                // is counting UTF16 words.
+                final int length = semanticsNode.value == null ? 0 : semanticsNode.value.length();
+                int a = length - semanticsNode.currentValueLength + semanticsNode.maxValueLength;
+                result.setMaxTextLength(length - semanticsNode.currentValueLength + semanticsNode.maxValueLength);
+            }
+
         }
 
         // These are non-ops on older devices. Attempting to interact with the text will cause Talkback to read the
@@ -1719,6 +1727,8 @@ private static boolean nullableHasAncestor(SemanticsNode target, Predicate<Seman
 
         private int flags;
         private int actions;
+        private int maxValueLength;
+        private int currentValueLength;
         private int textSelectionBase;
         private int textSelectionExtent;
         private int platformViewId;
@@ -1858,6 +1868,8 @@ private void updateWith(@NonNull ByteBuffer buffer, @NonNull String[] strings) {
 
             flags = buffer.getInt();
             actions = buffer.getInt();
+            maxValueLength = buffer.getInt();
+            currentValueLength = buffer.getInt();
             textSelectionBase = buffer.getInt();
             textSelectionExtent = buffer.getInt();
             platformViewId = buffer.getInt();

shell/platform/android/platform_view_android.cc

@@ -227,7 +227,7 @@ void PlatformViewAndroid::DispatchSemanticsAction(JNIEnv* env,
 void PlatformViewAndroid::UpdateSemantics(
     flutter::SemanticsNodeUpdates update,
     flutter::CustomAccessibilityActionUpdates actions) {
-  constexpr size_t kBytesPerNode = 39 * sizeof(int32_t);
+  constexpr size_t kBytesPerNode = 41 * sizeof(int32_t);
   constexpr size_t kBytesPerChild = sizeof(int32_t);
   constexpr size_t kBytesPerAction = 4 * sizeof(int32_t);
 
@@ -261,6 +261,8 @@ void PlatformViewAndroid::UpdateSemantics(
       buffer_int32[position++] = node.id;
       buffer_int32[position++] = node.flags;
       buffer_int32[position++] = node.actions;
+      buffer_int32[position++] = node.maxValueLength;
+      buffer_int32[position++] = node.currentValueLength;
       buffer_int32[position++] = node.textSelectionBase;
       buffer_int32[position++] = node.textSelectionExtent;
       buffer_int32[position++] = node.platformViewId;