# Writing a quick fix

This document describes what a quick fix is and outlines the basic steps for
writing a quick fix.

## Overview

A quick fix is an automated code edit that's associated with a diagnostic. The
intent is to automate the work required to fix the issue being reported by the
diagnostic.

There are three ways for users to apply quick fixes: single-site, fix-in-file,
and bulk. We'll discuss how to implement the support for each of these usages in
its own section below. The sections build on each other, so we recommend reading
them in order.

Through most of this document we'll use a simple example. We'll assume you wrote
a new lint named `could_be_final` that flags fields that could be marked final
but aren't, and that you're adding a fix for it. The fix will simply add the
keyword `final` to the field declaration.

## Single-site fixes

A single-site fix is one in which a single fix can be applied to a single
location at which the associated diagnostic is reported.

When the client asks for quick fixes, the analysis server computes the relevant
diagnostics based on the cursor location. Then, for each diagnostic, it computes
one or more fixes. It then returns the list of computed fixes to the client. The
client typically allows the user to choose a single fix to be applied.

### Design considerations

Because quick fixes are computed on demand, they need to be computed quickly.
(Some clients request quick fixes every time the user repositions the cursor.)
That places a performance requirement on quick fixes, one that requires that the
code to compute a quick fix can't perform any potentially lengthy computations
such as searching all of the user's code or accessing the network. That, in
turn, generally means that fixes can only support localized changes. They can
add or remove text in the file in which the diagnostic was reported, but
generally can't do more than that. 

### Describing the fix

Each fix has an instance of the class `FixKind` associated with it. The existing
fixes for Dart diagnostics are defined in the class `DartFixKind`, fixes for the
analysis options file are defined in the class `AnalysisOptionsFixKind`, and
fixes for the pubspec file are in the class `PubspecFixKind`. A fix kind has an
identifier, a priority, and a message.

The identifier is used by some LSP-based clients to provide user-defined
shortcuts. It's a hierarchical dot-separated identifier and should follow the
pattern seen in the existing fix kinds.

The priority is used to order the list of fixes when presented to the user. The
larger the value the closer to the top of the list it will appear. If you're
implementing a fix for Dart files, you should use one of the constants defined
in `DartFixKindPriority` (typically `DartFixKindPriority.DEFAULT`), or add a new
constant if there's a need for it.

The message is what will be displayed to the user by the client. This should be
a sentence fragment (no terminating period) that would be appropriate as a label
in a menu or on a button. It should describe the change that will be made to the
user's code.

Create a static field, in the appropriate class, whose value is a `FixKind`
describing the fix you're implementing. For our example you might define a new
constant in `DartFixKind` like this:

```dart
static const ADD_FINAL = FixKind(
  'dart.fix.add.final',
  DartFixKindPriority.DEFAULT,
  "Add 'final' modifier",
);
```

### Implementing the fix, part 1

To implement the fix you'll create a subclass of `CorrectionProducer`. Most of
the existing correction producers are in the directory
`analysis_server/lib/src/services/correction/dart`, so we'll start by creating
a file named `add_final.dart` in that directory that contains the following:

```dart
// Copyright (c) 2021, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'package:analysis_server/src/services/correction/dart/abstract_producer.dart';
import 'package:analysis_server/src/services/correction/fix.dart';
import 'package:analyzer_plugin/utilities/change_builder/change_builder_core.dart';
import 'package:analyzer_plugin/utilities/fixes/fixes.dart';

class AddFinal extends CorrectionProducer {
  @override
  FixKind get fixKind => DartFixKind.ADD_FINAL;

  @override
  Future<void> compute(ChangeBuilder builder) async {
  }

  /// Return an instance of this class. Used as a tear-off in `FixProcessor`.
  static AddFinal newInstance() => AddFinal();
}
```

The `compute` method is where the fix will be built. We'll come back to it in
"Implementing the fix, part 2".

The `fixKind` getter is how you associate the fix kind we created earlier with
the fix produced by the `compute` method.

The static `newInstance` method will be used later in "Registering the fix".

There's another getter you might need to override. The message associated with
the fix kind is actually a template that can be filled in at runtime. The
placeholders in the message are denoted by integers inside curly braces (such as
`{0}`). The integers are indexes into a list of replacement values, and the
getter `fixArguments` returns the list of replacement values. The message we
used above doesn't have any placeholders, but if we'd written the message as
`"Add '{0}' modifier"`, then we could return the replacement values by
implementing:

```dart
@override
List<Object> get fixArguments => ['final'];
```

If you don't implement this getter, then the inherited getter will return an
empty list. The number of elements in the list must match the largest index used
in the message.

### Testing the fix

Before we look at implementing the `compute` method, we should probably write
some tests. Even if you don't normally use a test-driven approach to coding, we
recommend it in this case because writing the tests can help you think of corner
cases that the implementation will need to handle. The corresponding tests are
in the directory `analysis_server/test/src/services/correction/fix`, so we'll
create a file named `add_final_test.dart` that contains the following:

```dart
// Copyright (c) 2021, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'package:analysis_server/src/services/correction/fix.dart';
import 'package:analysis_server/src/services/linter/lint_names.dart';
import 'package:analyzer_plugin/utilities/fixes/fixes.dart';
import 'package:test_reflective_loader/test_reflective_loader.dart';

import 'fix_processor.dart';

void main() {
  defineReflectiveSuite(() {
    defineReflectiveTests(AddFinalTest);
  });
}

@reflectiveTest
class AddFinalTest extends FixProcessorLintTest {
  @override
  FixKind get kind => DartFixKind.ADD_FINAL;

  @override
  String get lintCode => LintNames.could_be_final;
}
```

These two getters tell the test framework to enable the lint being fixed and to
expect a fix of the expected kind.

The test can then be written in a method that looks something like this:

```dart
Future<void> test_withType() async {
  await resolveTestCode('''
class C {
  String name;

  C(this.name);
}
''');
  await assertHasFix('''
class C {
  final String name;

  C(this.name);
}
''');
}
```

The test framework will create a file containing the first piece of code, run
the lint over the code, use our correction producer to build a fix, apply the
fix to the file, and textually compare the results with the second piece of
code.

### Registering the fix

Before we can run the test, we need to register the correction producer so that
it can be run.

The list of fixes is computed by a `FixProcessor`. For each diagnostic passed
to it, it will look up the diagnostic in a table to get a list of the correction
producers it can use to produce fixes. There are three tables:

- The `lintProducerMap` is used for fixes related to lint rules. The table is
  keyed by the name of the lint to which the fix applies.

- The `nonLintProducerMap` is used for all other fixes. The table is keyed by
  the `ErrorCode` associated with the diagnostic.

- The `nonLintMultiProducerMap` is used for multi-producers, which are
  described below in "Multi-fix producers".

Actually, the tables contain lists of functions used to create the producers. We
do that so that producers can't accidentally carry state over from one use to
the next. These functions are usually a tear-off of the static method you
defined in "Implementing the fix, part 1".

The last step is to add your correction producer to the appropriate map. If
you're adding a fix for a lint, then you'd add an entry like

```dart
LintNames.could_be_final: [
  AddFinal.newInstance,
],
```

At this point you should be able to run the test and see it failing.

### Implementing the fix, part 2

We're now at a point where we can finish the implementation of the fix by
implementing the `compute` method.

The correction producer has access to most of the information you should need in
order to write the fix. The change builder passed to `compute` is how you
construct the fix that will be sent back to the client.

The first step in the implementation of any fix is to find the location in the
AST where the diagnostic was reported and verify that all the conditions on
which the fix is predicated are valid. Assuming that the lint has been written
and tested correctly, there's no need to test that the conditions that caused
the lint rule to generate a diagnostic are still true. However, sometimes there
are additional constraints that need to be satisfied before the fix can safely
be applied. For example, for our fix, where we're only going to add a keyword,
we'll need to ensure that there's only one field that would be impacted by the
change, otherwise we might introduce more problems than there were before the
fix.

Of course, sometimes you'll end up duplicating some of the work done by the lint
in the course of getting the information you need from the AST. While that's
less than optimal, there isn't any mechanism for passing information from the
diagnostic to the fix, so sometimes it just can't be avoided.

For this example we're going to assume that the lint highlights the name of the
field that could have been final. Finding the AST node is easy because it's done
for you by the fix processor before `compute` is invoked. All you have to do is
use the getter `node` to find the node at the offset of the lint's highlight
region. We're expecting it to be the name of a field, so that's how we'll name
the variable:

```dart
@override
Future<void> compute(ChangeBuilder builder) async {
  var fieldName = node;
}
```

Then we need to verify that this node really is an identifier as expected:

```dart
@override
Future<void> compute(ChangeBuilder builder) async {
  var fieldName = node;
  if (fieldName is! SimpleIdentifier) {
    return;
  }
}
```

If it isn't, then we'll return. Because we haven't used the builder to create a
fix, returning now means that no fix from this producer will be sent to the
client.

Simple identifiers appear in lots of places, so to be extra sure we have what
we're looking for we'll also make sure that
- the identifier is the name being declared by a variable declaration,
- the variable declaration is in a list of variables in a field declaration, and
- there's only one field being declared.

```dart
@override
Future<void> compute(ChangeBuilder builder) async {
  var fieldName = node;
  if (fieldName is! SimpleIdentifier) {
    return;
  }
  var field = fieldName.parent;
  if (field is! VariableDeclaration || field.name != fieldName) {
    return;
  }
  var fieldList = field.parent;
  if (fieldList is! VariableDeclarationList ||
          fieldList.variables.length > 1 ||
          fieldList.parent is! FieldDeclaration) {
    return;
  }
}
```

After all those checks we now know that the `fieldName` really is the name of a
field and that there are no other fields that would be impacted by marking the
list of fields with `final`.

We're now ready to create the actual fix. To do that we're going to use the
`ChangeBuilder` passed to the `compute` method. In the example below we'll
introduce a couple of the methods on `ChangeBuilder`, but for more information
you can read [Creating `SourceChange`s](https://github.com/dart-lang/sdk/blob/main/pkg/analyzer_plugin/doc/tutorial/creating_edits.md).

Fields can be declared with either `final`, `const`, `var`, or a type
annotation, and the change that needs to be made depends on how the field was
declared. To figure that out we'll make use of the fact that the first three
tokens are all accessible from the AST by using the getter `keyword`. If there
is no keyword, then we can safely insert `final `, but if `var` was used then it
has to be replaced. And, of course, if it's already `final` or `const` then we
shouldn't do anything (and presumably the lint rule wouldn't have produced a
lint).

```dart
@override
Future<void> compute(ChangeBuilder builder) async {
  var fieldName = node;
  if (fieldName is! SimpleIdentifier) {
    return;
  }
  var field = fieldName.parent;
  if (field is! VariableDeclaration || field.name != fieldName) {
    return;
  }
  var fieldList = field.parent;
  if (fieldList is! VariableDeclarationList ||
          fieldList.variables.length > 1 ||
          fieldList.parent is! FieldDeclaration) {
    return;
  }
  var keyword = fieldList.keyword;
  if (keyword == null) {
    await builder.addDartFileEdit(file, (builder) {
      builder.addSimpleInsertion(fieldList.offset, 'final ');
    });
  } else if (keyword.type == Keyword.VAR) {
    await builder.addDartFileEdit(file, (builder) {
      builder.addSimpleReplacement(range.token(keyword), 'final');
    });
  }
}
```

In both cases we're using `addDartFileEdit` to create an edit in a `.dart` file.
If we only need to insert the keyword, then we'll use `addSimpleInsertion` to do
the insertion, but if we need to replace the existing keyword, then we'll use
`addSimpleReplacement` to do the replacement.

We don't have a test case for the branch where the keyword `var` is used. We'll
leave adding such a test as an exercise for the reader.

In this example we're just adding a single keyword, so we're avoiding any need
to worry about formatting. As a general principle we don't attempt to format the
code after it's been modified, but we do make an effort to leave the code in a
reasonably readable state. There's a getter (`eol`) that you can use to get the
end-of-line marker that should be used in the file, and there's another getter
(`utils`) that will return an object with several utility methods that help with
things like getting the right indentation for nested code.

If we were adding a fix for a non-lint diagnostic, then there would be a couple
of minor differences. First, we'd register the correction producer using the
diagnostic's error code. Second, the test class would be a subclass of
`FixProcessorTest` and wouldn't specify the name of the lint.

### Multi-fix producers

We skipped over the map named `nonLintMultiProducerMap` earlier, promising that
we'd return to it later. You'll probably never have a need for it, but in case
you do this section will hopefully tell you what you need to know.

There's a subclass of `CorrectionProducer` named `MultiCorrectionProducer` and
this map is how you register one of them. That class exists for rare cases where
you need to use a single correction producer to produce multiple fixes. This is
generally only needed when you can't know in advance the maximum number of
fixes that might need to be produced. For example, if there is an undefined
identifier, and it might be possible to add an import to fix the problem,
there's no way to know in advance how many libraries might define the name, but
we'd want to propose adding an import for each such library.

If you are able to enumerate the possible fixes ahead of time, then you're
better off to create one subclass of `CorrectionProducer` for each of the fixes.
For example, taking the case of an undefined identifier again, another way to
fix the problem is to add a declaration of the name. There are a finite number
of kinds of declarations a user might want: class, mixin, variable, etc. Even
though some kinds of declarations might not make sense because of how the
identifier is being used, it's better to have a separate correction producer for
each kind of declaration and have each one determine whether to generate a fix.

And, we don't currently have support for associating a `MultiCorrectionProducer`
with a lint, so if you're writing fixes for a lint then this option isn't
available to you.

## Fix-in-file fixes

A fix-in-file fix is one in which a single fix can be applied that will fix all
the locations within a single file at which a single kind of diagnostic has been
reported.

When the client asks for quick fixes, in addition to computing the single-site
fixes, the analysis server looks to see whether any of the diagnostics at the
cursor location are also reported at other places in the same file. If any of
them are, and if the fix supports being used as a fix-in-file fix, then the
analysis server will attempt to apply the fix in each of those locations and to
build another fix that will apply edits to all the locations at which the
diagnostic was reported.

The fix-in-file fix will only be shown when there is more than one diagnostic
for which your fix is able to produce an edit. For example, if there are three
fields that could all be made final, but the fix only produces an edit for one
of those places (maybe because the other two are in lists of fields), then the
fix-in-file fix won't be shown.

If a fix-in-file fix can be created then it will be returned along with any of
the single-site fixes computed at the cursor location.

In other words, most of the support for doing this comes for free; the only
requirement is that you declare that your fix can be used this way.

### Design considerations

So why isn't this the default? Because it isn't always appropriate to support a
fix-in-file fix. We believe that it's better for users to sometimes not have a
fix-in-file fix that would have been appropriate than it is for them to have one
that isn't appropriate. While there isn't a complete list of situations in which
a fix-in-file fix isn't appropriate, let's look at a couple of the cases to
consider.

First, some fixes, by nature, should only be applied after due consideration.
For example, there's a fix that will create a class declaration in response to
seeing a reference to an undefined name. While it's useful to have a way to
declare the name, how the name should be defined depends on the user's intent,
so it probably isn't appropriate to create class declarations for all undefined
identifiers without considering each case.

Second, some fixes cause changes that impact other diagnostics of the same kind.
In such a case the automated support for fix-in-file fixes might well cause an
invalid edit to be produced, and fixes should never corrupt the user's code. For
example, there's a fix to add `const ` before a constructor invocation. If some
arguments to the invocation are also constructor invocations then the framework
could add an unnecessary keyword, which would be undesirable.

### Enabling the fix

If it _is_ appropriate for your fix to be used as a fix-in-file fix, then there
are two things you'll need to do to enable the fix-in-file support. The first
step is to define another `FixKind` (in the same file in which you defined the
first fix kind) that looks something like the following:

```dart
static const ADD_FINAL_MULTI = FixKind(
  'dart.fix.add.final.multi',
  DartFixKindPriority.IN_FILE,
  "Add 'final' modifier everywhere in file",
);
```

This is different than the original fix kind in the following ways:
- both the name of the field and the identifier have a suffix added,
- the priority is lower than the single location fixes, and
- the message is worded in a way that makes it clear that multiple locations
  will be changed.

The second step is to implement a couple of additional getters in your
`CorrectionProducer`. First, let the framework know that your fix can be used in
this way:

```dart
@override
bool get canBeAppliedToFile => true;
```

Then specify the fix kind associated with the fix-in-file fix:

```dart
@override
FixKind get multiFixKind => DartFixKind.ADD_FINAL_MULTI;
```

And, if the message takes arguments (which the example above doesn't), provide
them using something like the following:

```dart
@override
List<Object> get multiFixArguments => ['first', 'second'];
```

That's it; the fix-in-file fix should now appear.

You should, of course, write tests for the fix. Those tests will be similar in
structure to those written to test the single application use case, but will be
in a subclass of `FixInFileProcessorTest`. They should be added to the test file
you created earlier for the single-site fixes
(`analysis_server/test/src/services/correction/fix/add_final_test.dart`).

The most important distinction between the two is that for the fix-in-file tests
you'll need to ensure that there are at least two diagnostics produced for the
test code. If possible, those two diagnostics ought to be overlapping in terms
of the region of code impacted by the fix. For example, if the fix reverses the
order of two arguments in an argument list (such as by converting `m(a, b)` to
`m(b, a)`), then make one of the arguments be an invocation of `m` whose
arguments would also need to be swapped (such as `m(a, m(b, c)`).

## Bulk fixes

A bulk fix is one in which all of the fixes for all of the diagnostics that have
been reported in one or more files, such as an entire package, are applied in a
single operation. This kind of fix is currently available through the `dart fix`
command-line tool.

A bulk fix is computed in a manner similar to the way a fix-in-file fix is
computed except that all of the diagnostics across all of the specified files
are iterated over. Unlike the fix-in-file fixes, when bulk fixes are requested
only the one bulk fix that was computed is returned.

### Design considerations

The considerations for why you might not want to enable the fix for bulk
application using `dart fix` include all of those given above for fix-in-file
fixes, but there are a couple of other considerations to keep in mind.

First, `dart fix` is a command-line tool, so unlike changes made in an editor
there is no support for rolling back the changes it makes (other than whatever
support the user has from their source control system).

Second, the tool applies fixes to more than just a single diagnostic, so there
are likely to be more edits applied. That means that there's an even higher
probability of having conflicts between edits. The framework handles some kinds
of conflicts automatically, but can't handle everything.

### Enabling the fix

If you decide that you do want your fix to be supported by `dart fix`, the only
change that's required is to implement the following getter:

```dart
@override
bool get canBeAppliedInBulk => true;
```

You should, of course, write tests for bulk application of the fix. Those tests
will be identical in nature to those written to test the fix-in-file use case,
but will be in a subclass of `BulkFixProcessorTest`. They should also be added
to the test file you created earlier for the single-site and fix-in-file fixes
(`analysis_server/test/src/services/correction/fix/add_final_test.dart`).