7d22d44e04cea614c0fb2bca0925a8c0d04f0bb5 - dart_style.git

commit	7d22d44e04cea614c0fb2bca0925a8c0d04f0bb5	[log] [tgz]
author	Bob Nystrom <rnystrom@google.com>	Fri Apr 26 16:45:38 2024 -0700
committer	GitHub <noreply@github.com>	Fri Apr 26 16:45:38 2024 -0700
tree	78457af1fd2dfa548f6f35a30f51185413d77de2
parent	585fc7798fbe07ab1a4caca1a5ea3570f9d3aef9 [diff]

Don't allow splitting in interpolation expressions. (#1448)

The old formatter doesn't allow splitting inside interpolation because it almost never looks good. It's better to just let the line overflow, especially if it's a multi-line string. If the user wants it to fit, they're better off splitting the string themselves instead of letting the formatter split inside the interpolation.

Also, a single string can have an unbounded number of arbitrarily large interpolation expressions. Since the expressions can't be separately formatted (because they appear in the middle of a line), this leads to combinatorial performance. It's fairly rare, but it's quite bad when it happens. When I format the Flutter repo, one of the slowest files is packages/flutter/lib/src/material/toggle_buttons.dart. It's spending almost all of its time in:

```
  @override
  String toString() {
    return '''
    {
      selected:
        hovered: $hoverColor, otherwise: ${colorScheme?.primary.withOpacity(0.04)},
        focused: $focusColor, otherwise: ${colorScheme?.primary.withOpacity(0.12)},
        pressed: $splashColor, otherwise: ${colorScheme?.primary.withOpacity(0.16)},
      unselected:
        hovered: $hoverColor, otherwise: ${colorScheme?.onSurface.withOpacity(0.04)},
        focused: $focusColor, otherwise: ${colorScheme?.onSurface.withOpacity(0.12)},
        pressed: $splashColor, otherwise: ${colorScheme?.onSurface.withOpacity(0.16)},
      otherwise: null,
    }
    ''';
  }
```

I added a benchmark containing three of the worst examples I found in the Flutter repo. Before this change:

```
Benchmark (tall)                fastest   median  slowest  average  baseline
-----------------------------  --------  -------  -------  -------  --------
interpolation                  1225.785 1241.865 1242.435  1236.695    (none)
```

With this PR, the benchmark is over 10,000x faster:

```
Benchmark (tall)                fastest   median  slowest  average  baseline
-----------------------------  --------  -------  -------  -------  --------
interpolation                     0.115    0.122    0.303    0.132    (none)
```

Since large string interpolations are rare in practice, this doesn't have a huge impact on overall format times of a large corpus. (On my machine, it takes formatting the Flutter repo from 25.43s to 25.01s.) But it does address a significant hot spot and I also think the resulting output looks better.

7 files changed

tree: 78457af1fd2dfa548f6f35a30f51185413d77de2

README.md

The dart_style package defines an automatic, opinionated formatter for Dart code. It replaces the whitespace in your program with what it deems to be the best formatting for it. Resulting code should follow the Dart style guide but, moreso, should look nice to most human readers, most of the time.

The formatter handles indentation, inline whitespace, and (by far the most difficult) intelligent line wrapping. It has no problems with nested collections, function expressions, long argument lists, or otherwise tricky code.

The formatter turns code like this:

// BEFORE formatting
if (tag=='style'||tag=='script'&&(type==null||type == TYPE_JS
      ||type==TYPE_DART)||
  tag=='link'&&(rel=='stylesheet'||rel=='import')) {}

into:

// AFTER formatting
if (tag == 'style' ||
  tag == 'script' &&
      (type == null || type == TYPE_JS || type == TYPE_DART) ||
  tag == 'link' && (rel == 'stylesheet' || rel == 'import')) {}

The formatter will never break your code—you can safely invoke it automatically from build and presubmit scripts.

Style fixes

The formatter can also apply non-whitespace changes to make your code consistently idiomatic. You must opt into these by passing either --fix which applies all style fixes, or any of the --fix--prefixed flags to apply specific fixes.

For example, running with --fix-named-default-separator changes this:

greet(String name, {String title: "Captain"}) {
  print("Greetings, $title $name!");
}

into:

greet(String name, {String title = "Captain"}) {
  print("Greetings, $title $name!");
}

Using the formatter

The formatter is part of the unified dart developer tool included in the Dart SDK, so most users get it directly from there. That has the latest version of the formatter that was available when the SDK was released.

IDEs and editors that support Dart usually provide easy ways to run the formatter. For example, in WebStorm you can right-click a .dart file and then choose Reformat with Dart Style.

Here's a simple example of using the formatter on the command line:

$ dart format test.dart

This command formats the test.dart file and writes the result to the file.

dart format takes a list of paths, which can point to directories or files. If the path is a directory, it processes every .dart file in that directory or any of its subdirectories.

By default, it formats each file and write the formatting changes to the files. If you pass --output show, it prints the formatted code to stdout.

You may pass a -l option to control the width of the page that it wraps lines to fit within, but you're strongly encouraged to keep the default line length of 80 columns.

Validating files

If you want to use the formatter in something like a presubmit script or commit hook, you can pass flags to omit writing formatting changes to disk and to update the exit code to indicate success/failure:

$ dart format --output=none --set-exit-if-changed .

Running other versions of the formatter CLI command

If you need to run a different version of the formatter, you can globally activate the package from the dart_style package on pub.dev:

$ pub global activate dart_style
$ pub global run dart_style:format ...

Using the dart_style API

The package also exposes a single dart_style library containing a programmatic API for formatting code. Simple usage looks like this:

import 'package:dart_style/dart_style.dart';

main() {
  final formatter = DartFormatter();

  try {
    print(formatter.format("""
    library an_entire_compilation_unit;

    class SomeClass {}
    """));

    print(formatter.formatStatement("aSingle(statement);"));
  } on FormatterException catch (ex) {
    print(ex);
  }
}

Other resources

Before sending an email, see if you are asking a frequently asked question.
Before filing a bug, or if you want to understand how work on the formatter is managed, see how we track issues.