[proposal][desktop] Tab character (\t) should be displayed as indentation in TextField

[aldrinmathew]

Tab Character \t should be displayed as indentation in EditableText, TextField and all implementations of Text widgets in Desktop

This is a common behaviour in many desktop applications

Flutter Channel: master
Platform: Desktop, Linux
Version: 2.1.0-11.0.pre.138

When Tab key is pressed while the TextField is focused, in a Flutter Desktop Application, by default, the focused TextField is unfocused and the focus shifts to another widget. That is a problem in my case as I want the user to be able to enter tab characters. But I partially solved it by using a RawKeyboardListener to detect when the user presses Tab key, and then inserted a \t to the TextField at the current cursor position through its controller.
The problem is that, the \t character is displayed as a small space, almost half as wide as whitespace, which is counterproductive.
There should be an option to configure how \t is displayed, as it can be of multiple number of spaces' width, and it should be customisable.

I believe this is not exactly a bug. For mobile devices, the Tab key is irrelevant. For web, pressing Tab key is helpful in shifting focus between UI elements.
But for desktop, it makes sense to have an option to customise how a Tab character is displayed in a TextField.

Suggestion

Add an option to disable focus shifting on Tab press on Desktop and add Tab character to the TextField. Provide a way to customise Tab character display, by equating it to a specific number of whitespaces that can be changed if required.

[pedromassangocode]

Yes this is definitely a proposal. Just tested on Slack MacOS app and Tab button move focus to another component.

[TimWhiting]

Found a way to accomplish this:

But I agree that there should be two properties on EditableText and TextField called something like:
bool acceptTabs
int tabIndentation
And TextField or Editable text should handle wrapping in the Action/ Shortcuts widget.

code sample

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData.dark(),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class InsertTabIntent extends Intent {
  const InsertTabIntent(this.numSpaces, this.textController);
  final int numSpaces;
  final TextEditingController textController;
}

class InsertTabAction extends Action {
  @override
  Object invoke(covariant Intent intent) {
    if (intent is InsertTabIntent) {
      final oldValue = intent.textController.value;
      final newComposing = TextRange.collapsed(oldValue.composing.start);
      final newSelection = TextSelection.collapsed(
          offset: oldValue.selection.start + intent.numSpaces);

      final newText = StringBuffer(oldValue.selection.isValid
          ? oldValue.selection.textBefore(oldValue.text)
          : oldValue.text);
      for (var i = 0; i < intent.numSpaces; i++) {
        newText.write(' ');
      }
      newText.write(oldValue.selection.isValid
          ? oldValue.selection.textAfter(oldValue.text)
          : '');
      intent.textController.value = intent.textController.value.copyWith(
        composing: newComposing,
        text: newText.toString(),
        selection: newSelection,
      );
    }
    return '';
  }
}

class _MyHomePageState extends State<MyHomePage> {
  TextEditingController textController;
  @override
  void initState() {
    super.initState();
    textController = TextEditingController();
  }

  @override
  void dispose() {
    textController.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text(widget.title)),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Actions(
              actions: {InsertTabIntent: InsertTabAction()},
              child: Shortcuts(
                shortcuts: {
                  LogicalKeySet(LogicalKeyboardKey.tab):
                      InsertTabIntent(2, textController)
                },
                child: TextField(
                  controller: textController,
                  textInputAction: TextInputAction.newline,
                  maxLines: 30,
                  keyboardType: TextInputType.multiline,
                ),
              ),
            ),
          ],
        ),
      ),
    );
  }
}

[aldrinmathew]

@TimWhiting @pedromassangocode

I tested the implementation by Tim Whiting and it works well. 👍

However, I reviewed the code and found that it is adding the appropriate number of spaces whenever the user presses Tab key. I have already implemented this in the application that I am developing. Is there anyway that we can add the '\t' character in the text and make it look like multiple spaces? I mean, without replacing the tab with spaces?
In theory, we should be able to configure and customise the display of certain characters in the text field. Especially when we consider the possibility that desktop applications developed by flutter might not always be mere "applications", they might be complex enough to be called "software".

Use case

I want the user to be able to add the raw tab character (\t) to the text field. And I want to customise the display of the tab character to resemble the equivalent number of spaces chosen by the user.
I am developing a text/code editor. I think you get where I am going with this. If a user/programmer presses tab to indent their content/code, and if I write code to replace the indentation with actual whitespace, someone's definitely getting angry. I can easily provide them with an option to replace '\t' with spaces, but that shouldn't be the default behaviour in this case.

[aldrinmathew]

@TimWhiting

This is the modified code. This won't run as there is no property called indentSpaceCount for TextField.

code sample

import 'dart:io';

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData.dark(),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class InsertTabIntent extends Intent {
  const InsertTabIntent(this.textController);
  final TextEditingController textController;
}

class InsertTabAction extends Action {
  @override
  Object invoke(covariant Intent intent) {
    if (intent is InsertTabIntent) {
      final oldValue = intent.textController.value;
      final newComposing = TextRange.collapsed(oldValue.composing.start);
      final newSelection = TextSelection.collapsed(
          offset: oldValue.selection.start + 1);

      final newText = StringBuffer(oldValue.selection.isValid
          ? oldValue.selection.textBefore(oldValue.text)
          : oldValue.text);
      newText.write('\t');
      newText.write(oldValue.selection.isValid
          ? oldValue.selection.textAfter(oldValue.text)
          : '');
      intent.textController.value = intent.textController.value.copyWith(
        composing: newComposing,
        text: newText.toString(),
        selection: newSelection,
      );
    }
    return '';
  }
}

class _MyHomePageState extends State<MyHomePage> {
  TextEditingController textController;
  @override
  void initState() {
    super.initState();
    textController = TextEditingController();
  }

  @override
  void dispose() {
    textController.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text(widget.title)),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Actions(
              actions: {InsertTabIntent: InsertTabAction()},
              child: Shortcuts(
                shortcuts: {
                  LogicalKeySet(LogicalKeyboardKey.tab):
                      InsertTabIntent(textController)
                },
                child: TextField(
                  controller: textController,
                  textInputAction: TextInputAction.newline,
                  maxLines: 30,
                  keyboardType: TextInputType.multiline,
                  indentSpaceCount: 4,
                ),
              ),
            ),
          ],
        ),
      ),
    );
  }
}

[TimWhiting]

Yeah, I agree, the properties should be more like:

• bool acceptTabs (with this property set to false which should be the default pressing tab should change focus)
• int indentSpaceCount (this property controls the visual spacing of a literal \t, default should probably be 4)
• bool replaceTabsWithSpaces (this property controls whether to replace a literal \t with indentSpaceCount of spaces, default should be false) or there should be an InputFormatter provided by Flutter to do the same

As a design note, for accessibility there needs to be a way to unfocus the field that accepts tabs to a larger focus scope to allow moving to a different field. I believe this is typically done by the escape key, but I could be wrong.