Flutter TextField - how to shrink the font if the text entered overflows

Question

I have a TextField (not a Text) widget that must remain on one line. I want to reduce it's font size if the text entered is too large for the TextField box, ie shrink it if it overflows. How can I do this?

I have written some code like this in a stateful component

if (textLength < 32) { newAutoTextVM.fontSize = 35.0; } else if (textLength < 42) { newAutoTextVM.fontSize = 25.0;

In the view

fontSize: 25.0,

but it isn't very intelligent, it doesn't cope with resizing, also, because the font size isn't monospaced (courier etc), different characters take up different amounts of space.

boformer · Accepted Answer · 2018-07-29 23:51:20Z

Use a TextPainter to calculate the width of your text. Use a GlobalKey to get the size of your widget (A LayoutBuilder might be better to handle screen rotation).

import 'package:flutter/material.dart'; main() => runApp(MaterialApp(home: Home())); class Home extends StatefulWidget { @override _HomeState createState() => _HomeState(); } const textFieldPadding = EdgeInsets.all(8.0); const textFieldTextStyle = TextStyle(fontSize: 30.0); class _HomeState extends State<Home> { final TextEditingController _controller = TextEditingController(); final GlobalKey _textFieldKey = GlobalKey(); double _textWidth = 0.0; double _fontSize = textFieldTextStyle.fontSize; @override void initState() { super.initState(); _controller.addListener(_onTextChanged); } void _onTextChanged() { // substract text field padding to get available space final inputWidth = _textFieldKey.currentContext.size.width - textFieldPadding.horizontal; // calculate width of text using text painter final textPainter = TextPainter( textDirection: TextDirection.ltr, text: TextSpan( text: _controller.text, style: textFieldTextStyle, ), ); textPainter.layout(); var textWidth = textPainter.width; var fontSize = textFieldTextStyle.fontSize; // not really efficient and doesn't find the perfect size, but you got all you need! while (textWidth > inputWidth && fontSize > 1.0) { fontSize -= 0.5; textPainter.text = TextSpan( text: _controller.text, style: textFieldTextStyle.copyWith(fontSize: fontSize), ); textPainter.layout(); textWidth = textPainter.width; } setState(() { _textWidth = textPainter.width; _fontSize = fontSize; }); } @override Widget build(BuildContext context) { return Scaffold( appBar: AppBar( title: Text('Autosize TextField'), ), body: Padding( padding: EdgeInsets.all(16.0), child: Column( crossAxisAlignment: CrossAxisAlignment.stretch, children: <Widget>[ TextField( key: _textFieldKey, controller: _controller, decoration: InputDecoration( border: InputBorder.none, fillColor: Colors.orange, filled: true, contentPadding: textFieldPadding, ), style: textFieldTextStyle.copyWith(fontSize: _fontSize), ), Text('Text width:'), Container( padding: textFieldPadding, color: Colors.orange, child: Row( children: <Widget>[ Container(width: _textWidth, height: 20.0, color: Colors.blue), ], ), ) ], ), ), ); } }

That's great, I was just experimenting with TextPainter but you beat me to it :-)
I'm not sure if my code handles screen rotation or other width changes correctly. You probably want to use a LayoutBuilder to get the width of the TextField

Andrea Grippi · Accepted Answer · 2018-07-29 14:00:14Z

I have searched through the docs and found a couple of solutions that could come at your help:

L̶o̶o̶k̶ ̶a̶t̶ ̶t̶h̶e̶ ̶o̶f̶f̶i̶c̶i̶a̶l̶ ̶d̶o̶c̶s̶[̶1̶]̶,̶ ̶i̶n̶ ̶p̶a̶r̶t̶i̶c̶u̶l̶a̶r̶e̶ ̶a̶t̶ ̶t̶h̶e̶s̶e̶ ̶p̶r̶o̶p̶e̶r̶t̶i̶e̶s̶:̶ ̶ ̶m̶a̶x̶L̶i̶n̶e̶s̶,̶ ̶o̶v̶e̶r̶f̶l̶o̶w̶ ̶a̶n̶d̶ ̶s̶o̶f̶t̶W̶r̶a̶p̶ (These are TextBox properties, not TextFields)
Have a look at this thread where they suggest to wrap the TextBox/TextFeld with a Flexible Widget

Depending on the rest of your code one of these solutions could be better, try tweaking around.
Hope it helps.

thank you but TextField doesn't have those properties (these all apply to Text)
You're right, I misread the type of Text you were referring too. But the Flexible solution should still apply: stackoverflow.com/questions/44579918/…

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