Flutter: How to Get the Number of Text Lines

Question

How to Get the Number of Text Lines In our Flutter project, we need to determine if the number of lines of text exceeds 3, then we will display a prompt message. How can we use the code to implement it?

Answer 1

If you simply want to check for how many newlines the text contains, you can do a simple

final numLines = '\n'.allMatches(yourText).length + 1;

However, I guess you're more interested in the number of lines that are actually being displayed visually. Here, things get a little more complicated, because you need to know the available space (the BoxConstraints) in order to calculate how many lines the text needs. To do that, you can use a LayoutBuilder to delay the widget building and a TextPainter to do the actual calculation:

return LayoutBuilder(builder: (context, size) {
  final span = TextSpan(text: yourText, style: yourStyle);
  final tp = TextPainter(text: span, maxLines: 3);
  tp.layout(maxWidth: size.maxWidth);

  if (tp.didExceedMaxLines) {
    // The text has more than three lines.
    // TODO: display the prompt message
    return Container(color: Colors.red);
  } else {
    return Text(yourText, style: yourStyle);
  }
});

I extracted some of the code from the auto_size_text pub package, which might also be interesting to you: It sizes its text so it fits in the given space.

Anyhow, be careful when displaying the prompt: Your widget's build method may be called several times per second, resulting in multiple prompts being displayed simultaneously.

Answer 2

Update: this answer is here for historical purposes. See my updated answer here for a much easier solution.

---

If you have a TextPainter object and you have already called layout, then you can get the number of lines by selecting everything and calling getBoxesForSelection. Each box is the size of the line.

TextSelection selection = TextSelection(baseOffset: 0, extentOffset: text.length);
List<TextBox> boxes = textPainter.getBoxesForSelection(selection);
int numberOfLines = boxes.length;

Here is an example:

final text = 'My text line.\nThis line wraps to the next.\nAnother line.';

print(numberOfLines); // 4

Unfortunately, this fails if there is a mix of bidirectional text:

final text = 'My text line.\nThis كلمة makes more boxes.\nAnother line.';

print(numberOfLines); // 6

This could be overcome by only counting the boxes that along one edge. If you notice the data from above, there are only four boxes that have an edge at 0.0.

flutter: TextBox.fromLTRBD(0.0, 0.2, 171.8, 36.0, TextDirection.ltr)
flutter: TextBox.fromLTRBD(0.0, 36.5, 68.4, 72.3, TextDirection.ltr)
flutter: TextBox.fromLTRBD(68.4, 38.2, 111.5, 75.0, TextDirection.rtl)
flutter: TextBox.fromLTRBD(111.5, 36.5, 299.9, 72.3, TextDirection.ltr)
flutter: TextBox.fromLTRBD(0.0, 77.2, 92.2, 113.0, TextDirection.ltr)
flutter: TextBox.fromLTRBD(0.0, 113.2, 179.7, 149.0, TextDirection.ltr)

I wish there were a TextPainter.numberOfLines method, but I haven't found it.