Calculate the caret position based on the next or previous rune (#7432)

The indexes passed to paragraph.getBoxesForRange are based on UTF-16 code
units.  If the caret is at a character that is encoded as multiple code units,
then the start and end indexes should be adjusted accordingly.

packages/flutter/lib/src/painting/text_painter.dart

@@ -212,8 +212,16 @@ class TextPainter {
     canvas.drawParagraph(_paragraph, offset);
   }
 
+  bool _isUtf16Surrogate(int value) {
+    return value & 0xF800 == 0xD800;
+  }
+
   Offset _getOffsetFromUpstream(int offset, Rect caretPrototype) {
-    List<ui.TextBox> boxes = _paragraph.getBoxesForRange(offset - 1, offset);
+    int prevCodeUnit = _text.codeUnitAt(offset - 1);
+    if (prevCodeUnit == null)
+      return null;
+    int prevRuneOffset = _isUtf16Surrogate(prevCodeUnit) ? offset - 2 : offset - 1;
+    List<ui.TextBox> boxes = _paragraph.getBoxesForRange(prevRuneOffset, offset);
     if (boxes.isEmpty)
       return null;
     ui.TextBox box = boxes[0];
@@ -223,7 +231,11 @@ class TextPainter {
   }
 
   Offset _getOffsetFromDownstream(int offset, Rect caretPrototype) {
-    List<ui.TextBox> boxes = _paragraph.getBoxesForRange(offset, offset + 1);
+    int nextCodeUnit = _text.codeUnitAt(offset + 1);
+    if (nextCodeUnit == null)
+      return null;
+    int nextRuneOffset = _isUtf16Surrogate(nextCodeUnit) ? offset + 2 : offset + 1;
+    List<ui.TextBox> boxes = _paragraph.getBoxesForRange(offset, nextRuneOffset);
     if (boxes.isEmpty)
       return null;
     ui.TextBox box = boxes[0];

packages/flutter/lib/src/painting/text_span.dart

@@ -160,6 +160,24 @@ class TextSpan {
     return buffer.toString();
   }
 
+  /// Returns the UTF-16 code unit at the given index in the flattened string.
+  /// Returns null if the index is out of bounds.
+  int codeUnitAt(int index) {
+    if (index < 0)
+      return null;
+    int offset = 0;
+    int result;
+    visitTextSpan((TextSpan span) {
+      if (index - offset < span.text.length) {
+        result = span.text.codeUnitAt(index - offset);
+        return false;
+      }
+      offset += span.text.length;
+      return true;
+    });
+    return result;
+  }
+
   @override
   String toString([String prefix = '']) {
     StringBuffer buffer = new StringBuffer();

packages/flutter/test/painting/text_painter_test.dart

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:ui' as ui;
+
+import 'package:flutter/painting.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test("TextPainter caret test", () {
+    TextPainter painter = new TextPainter();
+
+    String text = 'A';
+    painter.text = new TextSpan(text: text);
+    painter.layout();
+
+    Offset caretOffset = painter.getOffsetForCaret(new ui.TextPosition(offset: 0), ui.Rect.zero);
+    expect(caretOffset.dx, 0);
+    caretOffset = painter.getOffsetForCaret(new ui.TextPosition(offset: text.length), ui.Rect.zero);
+    expect(caretOffset.dx, painter.width);
+
+    // Check that getOffsetForCaret handles a character that is encoded as a surrogate pair.
+    text = 'A\u{1F600}';
+    painter.text = new TextSpan(text: text);
+    painter.layout();
+    caretOffset = painter.getOffsetForCaret(new ui.TextPosition(offset: text.length), ui.Rect.zero);
+    expect(caretOffset.dx, painter.width);
+  });
+}