Add a constructor for GlyphInfo. (#48971)

Needed for flutter/flutter#139717

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: LongCatIsLooong

lib/ui/text.dart

@@ -1201,6 +1201,9 @@ class FontVariation {
 ///  * [Paragraph.getClosestGlyphInfoForOffset], which finds the [GlyphInfo] of
 ///    the glyph(s) onscreen that's closest to the given [Offset].
 final class GlyphInfo {
+  /// Creates a [GlyphInfo] with the specified values.
+  GlyphInfo(this.graphemeClusterLayoutBounds, this.graphemeClusterCodeUnitRange, this.writingDirection);
+
   GlyphInfo._(double left, double top, double right, double bottom, int graphemeStart, int graphemeEnd, bool isLTR)
     : graphemeClusterLayoutBounds = Rect.fromLTRB(left, top, right, bottom),
       graphemeClusterCodeUnitRange = TextRange(start: graphemeStart, end: graphemeEnd),
@@ -1209,7 +1212,7 @@ final class GlyphInfo {
   /// The layout bounding rect of the associated character, in the paragraph's
   /// coordinates.
   ///
-  /// This is **not** the tight bounding box that encloses the character's outline.
+  /// This is **not** a tight bounding box that encloses the character's outline.
   /// The vertical extent reported is derived from the font metrics (instead of
   /// glyph metrics), and the horizontal extent is the horizontal advance of the
   /// character.
@@ -3017,18 +3020,24 @@ abstract class Paragraph {
   List<TextBox> getBoxesForPlaceholders();
 
   /// Returns the text position closest to the given offset.
+  ///
+  /// This method always returns a [TextPosition] for any given [offset], even
+  /// when the [offset] is not close to any text, or when the paragraph is empty.
+  /// This is useful for determining the text to select when the user drags the
+  /// text selection handle.
+  ///
+  /// See also:
+  ///
+  ///  * [getClosestGlyphInfoForOffset], which returns more information about
+  ///    the closest character to an [Offset].
   TextPosition getPositionForOffset(Offset offset);
 
   /// Returns the [GlyphInfo] of the glyph closest to the given `offset` in the
-  /// paragraph coordinate system, or null if the glyph is not in the visible
-  /// range.
+  /// paragraph coordinate system, or null if if the text is empty, or is
+  /// entirely clipped or ellipsized away.
   ///
   /// This method first finds the line closest to `offset.dy`, and then returns
   /// the [GlyphInfo] of the closest glyph(s) within that line.
-  ///
-  /// This method can be used to implement per-glyph hit-testing. The returned
-  /// [GlyphInfo] can help determine whether the given `offset` directly hits a
-  /// glyph in the paragraph.
   GlyphInfo? getClosestGlyphInfoForOffset(Offset offset);
 
   /// Returns the [GlyphInfo] located at the given UTF-16 `codeUnitOffset` in

testing/dart/text_test.dart

@@ -193,6 +193,18 @@ void testTextRange() {
   });
 }
 
+void testGlyphInfo() {
+  test('constructor', () {
+    const Rect testRect = Rect.fromLTWH(1, 2, 3, 4);
+    const TextRange testRange = TextRange(start: 5, end: 6);
+    const TextDirection testDirection = TextDirection.ltr;
+    final GlyphInfo info = GlyphInfo(testRect, testRange, testDirection);
+    expect(info.graphemeClusterLayoutBounds, testRect);
+    expect(info.graphemeClusterCodeUnitRange, testRange);
+    expect(info.writingDirection, testDirection);
+  });
+}
+
 void testLoadFontFromList() {
   test('loadFontFromList will send platform message after font is loaded', () async {
     late String message;
@@ -336,6 +348,7 @@ void main() {
   testTextStyle();
   testTextHeightBehavior();
   testTextRange();
+  testGlyphInfo();
   testLoadFontFromList();
   testFontFeatureClass();
   testFontVariation();