Use the engine's TextOverflow API to implement ellipsizing (#6104)

Fixes https://github.com/flutter/flutter/issues/4478

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

@@ -33,8 +33,9 @@ class TextPainter {
   TextPainter({
     TextSpan text,
     TextAlign textAlign,
-    double textScaleFactor: 1.0
-  }) : _text = text, _textAlign = textAlign, _textScaleFactor = textScaleFactor {
+    double textScaleFactor: 1.0,
+    String ellipsis,
+  }) : _text = text, _textAlign = textAlign, _textScaleFactor = textScaleFactor, _ellipsis = ellipsis {
     assert(text == null || text.debugAssertIsValid());
     assert(textScaleFactor != null);
   }
@@ -80,6 +81,21 @@ class TextPainter {
     _needsLayout = true;
   }
 
+  /// The string used to ellipsize overflowing text.  Setting this to a nonempty
+  /// string will cause this string to be substituted for the remaining text
+  /// if the text can not fit within the specificed maximum width.
+  String get ellipsis => _ellipsis;
+  String _ellipsis;
+  set ellipsis(String value) {
+    assert(value == null || value.isNotEmpty);
+    if (_ellipsis == value)
+      return;
+    _ellipsis = value;
+    _paragraph = null;
+    _needsLayout = true;
+  }
+
+
   // Unfortunately, using full precision floating point here causes bad layouts
   // because floating point math isn't associative. If we add and subtract
   // padding, for example, we'll get different values when we estimate sizes and
@@ -162,7 +178,11 @@ class TextPainter {
     if (_paragraph == null) {
       ui.ParagraphBuilder builder = new ui.ParagraphBuilder();
       _text.build(builder, textScaleFactor: textScaleFactor);
-      ui.ParagraphStyle paragraphStyle = _text.style?.getParagraphStyle(textAlign: textAlign, textScaleFactor: textScaleFactor);
+      ui.ParagraphStyle paragraphStyle = _text.style?.getParagraphStyle(
+        textAlign: textAlign,
+        textScaleFactor: textScaleFactor,
+        ellipsis: _ellipsis,
+      );
       paragraphStyle ??= new ui.ParagraphStyle();
       _paragraph = builder.build(paragraphStyle);
     }

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

@@ -229,14 +229,19 @@ class TextStyle {
   }
 
   /// The style information for paragraphs, encoded for use by `dart:ui`.
-  ui.ParagraphStyle getParagraphStyle({ TextAlign textAlign, double textScaleFactor: 1.0 }) {
+  ui.ParagraphStyle getParagraphStyle({
+      TextAlign textAlign,
+      double textScaleFactor: 1.0,
+      String ellipsis,
+   }) {
     return new ui.ParagraphStyle(
       textAlign: textAlign,
       fontWeight: fontWeight,
       fontStyle: fontStyle,
       fontFamily: fontFamily,
       fontSize: fontSize == null ? null : fontSize * textScaleFactor,
-      lineHeight: height
+      lineHeight: height,
+      ellipsis: ellipsis,
     );
   }
 

packages/flutter/lib/src/rendering/paragraph.dart

@@ -22,6 +22,8 @@ enum TextOverflow {
   ellipsis,
 }
 
+const String _kEllipsis = '\u2026';
+
 /// A render object that displays a paragraph of text
 class RenderParagraph extends RenderBox {
   /// Creates a paragraph render object.
@@ -34,7 +36,12 @@ class RenderParagraph extends RenderBox {
     double textScaleFactor: 1.0
   }) : _softWrap = softWrap,
        _overflow = overflow,
-       _textPainter = new TextPainter(text: text, textAlign: textAlign, textScaleFactor: textScaleFactor) {
+       _textPainter = new TextPainter(
+           text: text,
+           textAlign: textAlign,
+           textScaleFactor: textScaleFactor,
+           ellipsis: overflow == TextOverflow.ellipsis ? _kEllipsis : null,
+       ) {
     assert(text != null);
     assert(text.debugAssertIsValid());
     assert(softWrap != null);
@@ -51,7 +58,6 @@ class RenderParagraph extends RenderBox {
     if (_textPainter.text == value)
       return;
     _textPainter.text = value;
-    _overflowPainter = null;
     _overflowShader = null;
     markNeedsLayout();
   }
@@ -86,6 +92,7 @@ class RenderParagraph extends RenderBox {
     if (_overflow == value)
       return;
     _overflow = value;
+    _textPainter.ellipsis = value == TextOverflow.ellipsis ? _kEllipsis : null;
     markNeedsPaint();
   }
 
@@ -99,13 +106,13 @@ class RenderParagraph extends RenderBox {
     if (_textPainter.textScaleFactor == value)
       return;
     _textPainter.textScaleFactor = value;
-    _overflowPainter = null;
     _overflowShader = null;
     markNeedsLayout();
   }
 
   void _layoutText({ double minWidth: 0.0, double maxWidth: double.INFINITY }) {
-    _textPainter.layout(minWidth: minWidth, maxWidth: _softWrap ? maxWidth : double.INFINITY);
+    bool wrap = _softWrap || _overflow == TextOverflow.ellipsis;
+    _textPainter.layout(minWidth: minWidth, maxWidth: wrap ? maxWidth : double.INFINITY);
   }
 
   @override
@@ -160,7 +167,6 @@ class RenderParagraph extends RenderBox {
   }
 
   bool _hasVisualOverflow = false;
-  TextPainter _overflowPainter;
   ui.Shader _overflowShader;
 
   @override
@@ -182,20 +188,16 @@ class RenderParagraph extends RenderBox {
     if (didOverflowWidth) {
       switch (_overflow) {
         case TextOverflow.clip:
-          _overflowPainter = null;
+        case TextOverflow.ellipsis:
           _overflowShader = null;
           break;
         case TextOverflow.fade:
-        case TextOverflow.ellipsis:
-          _overflowPainter ??= new TextPainter(
+          TextPainter fadeWidthPainter = new TextPainter(
             text: new TextSpan(style: _textPainter.text.style, text: '\u2026'),
             textScaleFactor: textScaleFactor
           )..layout();
-          final double overflowUnit = _overflowPainter.width;
-          double fadeEnd = size.width;
-          if (_overflow == TextOverflow.ellipsis)
-            fadeEnd -= overflowUnit / 2.0;
-          final double fadeStart = fadeEnd - _overflowPainter.width;
+          final double fadeEnd = size.width;
+          final double fadeStart = fadeEnd - fadeWidthPainter.width;
           // TODO(abarth): This shader has an LTR bias.
           _overflowShader = new ui.Gradient.linear(
             <Point>[new Point(fadeStart, 0.0), new Point(fadeEnd, 0.0)],
@@ -204,7 +206,6 @@ class RenderParagraph extends RenderBox {
           break;
       }
     } else {
-      _overflowPainter = null;
       _overflowShader = null;
     }
   }
@@ -225,7 +226,7 @@ class RenderParagraph extends RenderBox {
     final Canvas canvas = context.canvas;
     if (_hasVisualOverflow) {
       final Rect bounds = offset & size;
-      if (_overflowPainter != null)
+      if (_overflowShader != null)
         canvas.saveLayer(bounds, new Paint());
       else
         canvas.save();
@@ -239,11 +240,6 @@ class RenderParagraph extends RenderBox {
           ..transferMode = TransferMode.modulate
           ..shader = _overflowShader;
         canvas.drawRect(Point.origin & size, paint);
-        if (_overflow == TextOverflow.ellipsis) {
-          // TODO(abarth): This paint offset has an LTR bias.
-          Offset ellipseOffset = new Offset(size.width - _overflowPainter.width, 0.0);
-          _overflowPainter.paint(canvas, ellipseOffset);
-        }
       }
       canvas.restore();
     }