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Announcing Dart ii.15

Fast concurrency, constructor tear-offs, improved enums, and more

Today we're releasing version 2.xv of the Sprint SDK, featuring fast concurrency with worker isolates, a new constructor tear-off language feature, improved enum support in the sprint:core library, new features for bundle publishers, and more.

Fast concurrency with worker isolates

Merely about all mod devices accept CPUs with multiple cores, capable of running multiple tasks in parallel. For most Sprint programs, how these cores are used is transparent to you as a programmer: the Dart runtime system past default runs all your Dart code on a unmarried cadre, just then uses additional cores for executing systems-level tasks such as async input/output, similar writing a file or making a network call.

Just your Dart lawmaking itself may demand to run meantime. For instance, you may have a continuous animation and a long-running task such as parsing a large JSON file. If the additional task takes too long, that might cause stutter or lag in the UI. By moving those additional tasks to a separate core, the blitheness can continue to run on the primary thread of execution, uninterrupted.

Dart'southward model for concurrency is based on isolates — independent units of execution that are isolated from i some other — to prevent a big form of concurrency programming bugs related to shared memory, such every bit race conditions like information races. Dart prevents these bugs past not allowing any mutable objects to be shared between isolates, and instead uses a model where isolates exchange land using bulletin passing. In Dart 2.15 nosotros've made a number of substantial enhancements to isolates.

We started by redesigning and reimplementing how isolates work, introducing a new concept: isolate groups. Isolates in an isolate group share various internal information structures representing the running plan. This makes individual isolates in the group much cheaper. It is now more than 100 times faster to beginning an additional isolate in an existing isolate group as we don't need to initialize the plan structures, and those spawned isolates consume between ten–100 times less memory.

While isolate groups still prevent shared access to mutable objects between isolates, the group is implemented with a shared heap, which unlocks further capabilities. We tin can laissez passer objects from one isolate to another, which can be used for worker isolates that perform a task that returns a large slice of memory. An example is a worker isolate that makes a network call to get data, parses that information into a large JSON object graph, and and then returns that JSON graph to the main isolate. Before Dart two.15, that result needed to exist deep-copied, which could itself cause UI jank if the copy took longer than the frame budget.

In 2.15 the worker isolate can phone call Isolate.exit(), passing its consequence as an argument. The Dart runtime then passes the memory containing the result from the worker isolate to the primary isolate without copying, and the primary isolate can receive the upshot in constant time. We've updated the compute() utility office in Flutter ii.viii to take reward of Isolate.exit(). If you're already using compute(), then you'll get these functioning gains automatically afterward upgrading to Flutter ii.8.

Finally, we've reworked how the isolate bulletin passing mechanism is implemented, making passing of pocket-size-to-medium sized messages approximately 8 times faster. Sending is significantly faster, and receiving messages is nearly ever done in abiding time. We've as well expanded the kinds of objects that isolates tin can ship to each other, calculation back up for function types, closures, and stacktrace objects. For details, encounter the API docs for SendPort.send().

To acquire more about how to use isolates, come across the new Concurrency in Dart documentation nosotros added for 2.15. We also accept a number of code samples you lot can check out.

New linguistic communication feature: Constructor tear-offs

In Dart you tin can create a function object, which points to a function on another object, by using the function's name. In the post-obit example, the second line of the primary() method illustrates this syntax when it sets `g` to `yard.greet`:

          grade Greeter {
            concluding Cord proper name;
            Greeter(this.name);                void greet(String who) {
              print('$proper name says: Hello $who!');
              }
}
                    void primary() {
            final g = Greeter('Michael');
            terminal g = g.greet; // g holds a function pointer to yard.greet.
            one thousand('Leaf'); // Invokes and prints "Michael says: Hello Leafage!"
}

Such function pointers — also referred to equally function tear-offs — appear frequently when using the Dart core libraries. Here'southward an case of calling foreach() on an iterable by passing it a office pointer:

          final one thousand = Greeter('Michael');          ['Lasse', 'Bob', 'Erik'].forEach(chiliad.greet);          // Prints "Michael says: Hello Lasse!", "Michael says: Hello Bob!",
// "Michael says: Hi Erik!"

Historically nosotros haven't supported creating tear-offs from a constructor (language issue #216). That's abrasive because in many cases — for example, when edifice Flutter UIs — a constructor tear-off is what you need. As of Dart 2.15, this syntax is now supported. Hither'due south an case of edifice a Column widget containing three Text widgets, by calling .map() and passing it a tear-off to the constructor of Text.

          class FruitWidget extends StatelessWidget {
            @override
            Widget build(BuildContext context) {
            render Column(
            children: ['Apple tree', 'Orange'].map(Text.new).toList());
            }
}

Text.new refers to the default constructor of the Text course. You tin too refer to a named constructor — for example, .map(Text.rich).

Related language changes

While we were implementing constructor tear-offs, nosotros took the opportunity to fix some inconsistencies in our existing support for part pointers. You lot can at present specialize a generic method to create a non-generic method:

          T id<T>(T value) => value;
var intId = id<int>; // New in 2.fifteen.
int Part(int) intId = id; // Pre-2.15 workaround.

You can even specialize a generic function object to create a non-generic function object:

          const fo = id; // Tear off `id`, creating a role object.
const c1 = fo<int>; // New in 2.15; fault before.

Lastly, we cleaned upwardly blazon literals involving generics:

          var y = List; // Already supported.
var z = List<int>; // New in 2.15.
var z = typeOf<List<int>>(); // Pre-2.15 workaround.

Improved enums in the dart:core library

Nosotros've made a number of convenience additions to the enum APIs in the dart:core library (language issue #1511). You can now get the String value for each enum value using .proper name:

          enum MyEnum {
            one, two, iii
}          void main() {
            impress(MyEnum.1.proper name);  // Prints "one".
}

You can also look upwardly an enum value by proper name:

          impress(MyEnum.values.byName('two') == MyEnum.2);  // Prints "true".

Finally, y'all can get a map of all proper name-value pairs:

          last map = MyEnum.values.asNameMap();            
impress(map['three'] == MyEnum.iii);  // Prints "true".

For an instance of using these new APIs, run into this Palpitate PR.

Compressed pointers

Dart 2.15 adds back up for compressed pointers, a technique where a 64-bit SDK can utilise a more space-efficient representation of pointers if only a 32-bit accost space needs to be supported (upwards to 4 GB of memory). Compressed pointers result in a significant retentivity reduction; in our internal testing with the GPay app, we saw an approximately ten% reduction of the Dart heap size.

Because compressed pointers imply not being able to address whatsoever available RAM above four GB, the feature is behind a configuration option in the Dart SDK that can only be toggled by embedders of the Dart SDK when the SDK is built. The Flutter SDK version 2.8 has enabled this configuration for Android builds, and the Palpitate team is considering also enabling it for iOS builds in a future release.

Sprint DevTools included in the Dart SDK

The DevTools suite of debugging and functioning tools previously wasn't in the Sprint SDK; you lot had to download it separately. Starting with Sprint 2.15, you now go DevTools when you lot download the Dart SDK, with no farther installation steps required. For more than information on using DevTools with Sprint command-line apps, see the DevTools documentation.

New pub features for package publishers

Dart ii.15 SDK also has two new features in the dart pub developer command and the pub.dev packet repository.

Offset, there's a new security feature for package publishers. The aim is to detect when a publisher accidentally publishes secrets — for example Cloud or CI credentials — inside pub packages. Nosotros were inspired to add this leak detection after learning that inside GitHub repositories, thousands of secrets are leaked every day.

Leak detection runs equally role of the pre-publish validation run in the dart pub publish command. If it detects a potential undercover in the files well-nigh to be published, the publish control exits without publishing, and prints output like this:

          Publishing my_package 1.0.0 to https://pub.dartlang.org:
Package validation plant the following errors:
* line one, column 1 of lib/primal.pem: Potential leak of Private Key detected.
╷
ane │ ┌ - - -Brainstorm PRIVATE Fundamental - - -
ii │ │ H0M6xpM2q+53wmsN/eYLdgtjgBd3DBmHtPilCkiFICXyaA8z9LkJ
3 │ └ - - -END Individual Key - - -
╵
* line 2, column 23 of lib/my_package.dart: Potential leak of Google OAuth Refresh Token detected.
╷
2 │ terminal refreshToken = "ane//042ys8uoFwZrkCgYIARAAGAQSNwF-L9IrXmFYE-sfKefSpoCnyqEcsHX97Y90KY-p8TPYPPnY2IPgRXdy0QeVw7URuF5u9oUeIF0";

On rare occasions this detection might take false positives, flagging potential leaks for content or files that y'all do in fact intend to publish. In those cases, you can add the files to an allowlist.

Second, we've added another characteristic for publishers that supports retracting a package version that's already been published. When a faulty packet version is published, we normally recommend publishing a new version with a minor increment that fixes the unintended result. In rare cases — for example when you don't have such a set yet, or where you accidentally published a new major version but intended to publish a new small-scale version — you can use the new packet retraction characteristic as a final resort. This functionality is available in the Admin UI on pub.dev:

When a parcel version is retracted, the pub client no longer resolves to that version in pub get or pub upgrade. If whatever developers take already resolved to the retracted version (and it's thus in their pubspec.lock file), they'll see a alarm the next time they run pub:

          $ dart pub get
Resolving dependencies…
mypkg 0.0.181-buggy (retracted, 0.0.182-stock-still available)
Got dependencies!

Security analysis for detecting bidirectional Unicode characters (CVE-2021–22567)

Recently a general programming language vulnerability was discovered involving bidirectional Unicode characters (CVE-2021–42574). This vulnerability affects almost mod programming languages that support Unicode. The following Dart source code illustrates the problem:

          main() {
            final accessLevel = 'user';                      if (accessLevel == 'user‮ .⁦// Check if admin⁩ ⁦') {
            print('You are a regular user.');
            } else {
            print('Yous are an admin.');
            }
}

You'd probably think that this program prints You are a regular user., only information technology might in fact print You are an admin.! This exploit is possible by using a string containing bidirectional Unicode characters. These are characters that change the direction of the text from left-to-right to right-to-left and dorsum, all within a single line. With bidirectional characters, the text can render onscreen quite differently from the actual text contents. You can see an example of this in this GitHub code gist.

Mitigations against this vulnerability include using tools (editors, lawmaking review tools, etc.) that detect bidirectional Unicode characters, so that a developer can exist made enlightened of them, and knowingly have their apply. The GitHub gist file viewer linked to to a higher place is one example of a tool that reveals these characters.

Dart ii.fifteen introduces one farther mitigation (Sprint security advisory CVE-2021–22567): the Dart analyzer now scans for bidirectional Unicode characters, and flags whatsoever use of them:

          $ dart analyze
Analyzing cvetest...                   ii.6s          info • bin/cvetest.dart:four:27 • The Unicode code point 'U+202E'
            changes the appearance of text from how information technology'due south interpreted
            past the compiler. Try removing the code point or using the            
            Unicode escape sequence '\u202E'. •
            text_direction_code_point_in_literal

We recommend replacing these characters by Unicode escape sequences, and so that they're visible in any text editor or viewer. Alternatively, if you do have a legitimate apply of these characters, you can disable the alarm by calculation an override in the line preceding the use:

          // ignore: text_direction_code_point_in_literal

Pub.dev credentials vulnerability when using third-political party pub servers (CVE-2021–22568)

We're besides publishing a 2nd pub.dev-related Sprint security advisory: CVE-2021–22568. This informational is targeted at packet publishers who may have published packages to third-party pub package servers, such as individual or company-internal package servers. Developers who publish only to the public pub.dev repository (the standard configuration) are non affected by this vulnerability.

If you've published to a 3rd-party repository, the vulnerability is that the OAuth2 temporary (one-hour) access token presented for authentication at that third-party repository can be misused to authenticate against the public pub.dev repository. Thus a malicious tertiary-political party pub server might employ an access token to impersonate you on pub.dev and publish packages in that location. If you've published a package to an untrusted third-party package repository, consider doing an audit of all your account activity on the pub.dev public parcel repository. You can use the pub.dev activity log for this purpose.

Closing comments

Nosotros promise that yous'll bask the new features in Dart ii.15, available today. This is our last release of the year, and we'd like to take the run a risk to express our gratitude for the wonderful Dart ecosystem. Thanks for all the peachy feedback, for your continued support as witnessed by our continued growth, and for extending our ecosystem with thousands of packages published on pub.dev over the by year. Nosotros can't wait to get back to it next year, and we have lots of exciting things planned for 2022. Until then, savour the holidays!