FAQ

Introduction

What is Flutter?

Flutter is a mobile app SDK, complete with framework, widgets, and tools, that gives developers an easy and productive way to build and deploy beautiful mobile apps on both Android and iOS.

What does Flutter do?

For users, Flutter makes beautiful app UIs come to life.

For developers, Flutter lowers the bar to entry for building mobile apps. It speeds up development of mobile apps and reduces the cost and complexity of app production across iOS and Android.

For designers, Flutter helps deliver the original design vision, without loss of fidelity or compromises. It also acts as a productive prototyping tool.

Who is Flutter for?

Flutter is for mobile app developers wanting a faster or easier way to build beautiful mobile apps.

Flutter is also for designers looking for a high-fidelity, fast prototyping tool. Prototypes built with Flutter can be handed to developers and shipped to users as real apps.

Fundamentally, Flutter is for users that want beautiful apps, with delightful motion and animation, and UIs with character and an identity all their own.

What kinds of apps can I build with Flutter?

Flutter is optimized for 2D mobile apps that want to run on both Android and iOS.

Apps that need to deliver brand-first designs are particularly well suited for Flutter. However, apps that need to look like stock platform apps can also be built with Flutter.

You can build full-featured apps with Flutter, including camera, geolocation, network, storage, 3rd-party SDKs, and more.

Who uses Flutter?

Google uses Flutter to build business-critical apps for iOS and Android. For example, Google’s mobile sales tool app is built with Flutter, along with a Store Manager app for Google Shopping Express. Other Flutter apps are in the works, or “inside the firewall”.

Teams outside of Google are also using Flutter. For example, Newsvoice shipped to both iOS and Android.

What makes Flutter unique?

Flutter is different than most other options for building mobile apps because Flutter uses neither WebView nor the OEM widgets that shipped with the device. Instead, Flutter uses its own high-performance rendering engine to draw widgets.

In addition, Flutter is different because it only has a thin layer of C/C++ code. Flutter implements most of its system (compositing, gestures, animation, framework, widgets, etc) in Dart (a modern, terse, object-oriented language) that developers can easily approach read, change, replace, or remove. This gives developers tremendous control over the system, as well as significantly lowers the bar to approachability for the majority of the system.

Should I build my next production app with Flutter?

Flutter is still being developed and is not yet at 1.0.

Our APIs are stabilizing, and we continue to improve parts of the system based on user feedback. We email flutter-dev@googlegroups.com when we make a change that might impact our users.

Flutter is used inside of Google and apps built with Flutter are deployed to users.

Some key features, such as accessibility, are not yet ready for broad deployment.

So really, it is up to you. The features you need may be available today. Please let us know if you released an app built with Flutter to users. We’d love to hear what you’re building!

What does Flutter provide?

What is inside the Flutter SDK?

  • Heavily optimized, mobile-first 2D rendering engine with excellent support for text
  • Functional-reactive framework
  • Rich set of widgets for Android and iOS
  • APIs for unit and integration tests
  • Interop and plugin APIs to connect to the system and 3rd-party SDKs
  • Headless test runner for running tests on Windows, Linux, and Mac
  • Command-line tools for creating, building, testing, and compiling your apps

Does Flutter work with any editors or IDEs?

We are building plugins for IntelliJ IDEA, enabling fully integrated development experience for Flutter. The plugins work in both the Ultimate and the free Community editions.

Alternatively, you can use a combination of the flutter command in a terminal and one of the many editors that support editing Dart.

Does Flutter come with a framework?

Yes! Flutter ships with a functional-reactive style framework, inspired by React. Flutter’s framework is designed to be layered and customizable (and optional). Developers can choose to use only parts of the framework, or a different framework.

Does Flutter come with widgets?

Yes! Flutter ships with a set of high quality Material and Cupertino (iOS-style) widgets, layouts, and themes. Of course, these widgets are only a starting point. Flutter is designed to make it easy to create your own widgets, or customize the existing widgets.

Does Flutter come with a testing framework?

Yes, Flutter provides APIs for writing unit and integration tests. Learn more about testing with Flutter.

We use our own testing capabilities to test our SDK. We measure our test coverage on every commit.

Does Flutter come with a dependency injection framework or solution?

Not at this time. Please share your ideas at flutter-dev@googlegroups.com.

Technology

What technology is Flutter built with?

Flutter is built with C, C++, Dart, and Skia (a 2D rendering engine). See this architecture diagram for a better picture of the main components.

How does Flutter run my code on Android?

The engine’s C/C++ code is compiled with Android’s NDK, and the majority of the framework and application code is running as native code compiled by the Dart compiler.

How does Flutter run my code on iOS?

The engine’s C/C++ code is compiled with LLVM, and any Dart code is AOT-compiled into native code. The app runs using the native instruction set (no interpreter is involved).

Does Flutter use my system’s OEM widgets?

No. Instead, Flutter provides a set of Material Design and Cupertino (iOS-styled) widgets, managed and rendered by Flutter’s framework and engine. You can browse a catalog of Flutter’s widgets.

We are hoping the end-result will be higher quality apps. If we reused the OEM widgets, the quality and performance of Flutter apps would be limited by the quality of those widgets.

In Android, for example, there’s a hard-coded set of gestures and fixed rules for disambiguating them. In Flutter, you can write your own gesture recognizer that is a first-class participant in the gesture system. Moreover, two widgets authored by different people can coordinate to disambiguate gestures.

Modern app design trends point towards designers and users wanting more motion-rich UIs and brand-first designs. In order to achieve that level of customized, beautiful design, Flutter is architectured to drive pixels instead of the OEM widgets.

What operating systems can I use to build a Flutter app?

Flutter supports development on Linux, Mac, and, Windows.

What language is Flutter written in?

We looked at a lot of languages and runtimes, and ultimately adopted Dart for the framework and widgets. The underlying graphics framework and the Dart virtual machine are implemented in C/C++.

Why did Flutter choose to use Dart?

The primary criteria we used to pick a programming language were the following:

  • Developer productivity. One of Flutter’s main value propositions is that it saves engineering resources by letting developers create apps for both iOS and Android with the same codebase. Using a highly productive language accelerates developers further and makes Flutter more attractive.

  • Object-orientation. For Flutter, we want a language that’s suited to Flutter’s problem domain: creating visual user experiences. The industry has multiple decades of experience building user interface frameworks in object-oriented languages. While we could use a non-object-oriented language, this would mean reinventing the wheel to solve several hard problems.

  • Predictable, high performance. With Flutter, we want to empower developers to create fast, fluid user experiences. In order to achieve that, we need to be able to run a significant amount of end-developer code during every animation frame. That means we need a language that both delivers high performance and delivers predictable performance, without periodic pauses that would cause dropped frames.

  • Fast allocation. The Flutter framework uses a functional-reactive style programming model, whose performance depends heavily on the underlying memory allocator efficiently handling small, short-lived allocations. The functional-reactive style was developed in languages with this property and does not work efficiently in languages that lack this facility.

Dart scores highly on all four dimensions. In addition, we have the opportunity to work closely with the Dart community, which is actively investing resources in improving Dart for use in Flutter. For example, when we adopted Dart, the language did not have an ahead-of-time toolchain for producing native binaries, which is instrumental in achieving predictable, high performance, but now the language does because the Dart team built it for Flutter. Similarly, the Dart VM has previously been optimized for throughput but the team is now optimizing the VM for latency, which is more important for Flutter’s workload.

Can Flutter run any Dart code?

Flutter should be able to run most Dart code that does not import (transitively, or directly) dart:mirrors or dart:html.

How big is the Flutter engine?

In November 2015, we measured the size of a minimal Flutter app, bundled as an APK, to be approximately 8MB. For this simple app that used Material Design widgets, the core engine is approximately 5MB, the framework + app code is approximately 400kb, necessary Java code is 330k, and there is approximately 2.5MB of ICU data.

Capabilities

What kind of app performance can I expect?

You can expect excellent performance. Flutter is designed to help developers easily achieve a constant 60fps. Flutter apps run via natively compiled code – no interpreters are involved. This means Flutter apps start quickly.

What kind of developer cycles can I expect? How long between edit and refresh?

Flutter implements a hot reload developer cycle. You can expect sub-second reload times, on a device or an emulator/simulator.

Flutter’s hot reload is stateful, which means the app state is retained after a reload. This means you can quickly iterate on a screen deeply nested in your app, without starting from the home screen after every reload.

Where can I deploy my Flutter app?

You can compile and deploy your Flutter app to iOS and Android.

What devices and OS versions does Flutter run on?

Mobile operating systems: Android Jelly Bean, v16, 4.1.x or newer, and iOS 8 or newer.

Mobile hardware: 64-bit iOS devices (starting with iPhone 5S and newer iPhone models), and ARM Android devices.

We support developing Flutter apps with Android and iOS devices, as well as with Android emulators and the iOS simulator.

We test on a variety of low-end to high-end phones (excluding tablets) but we don’t yet have an official device compatibility guarantee. We do not offer support for tablets or have tablet-aware layouts.

Does Flutter run on the web?

No. We are not working to provide a web version of Flutter.

Can I use Flutter to build desktop apps?

We are focused on mobile-first use cases. However, Flutter is open source and we encourage the community to use Flutter in a variety of interesting ways.

Can I use Flutter inside of my existing native app?

Yes, you can embed a Flutter view in your existing Android or iOS app. You can learn more about this at [[docs coming soon]]. If you want to do this, we encourage you to email our mailing list: flutter-dev@googlegroups.com.

Please follow this issue to be notified when docs for this feature are available.

Can I access platform services and APIs like sensors and local storage?

Yes. Flutter gives developers out-of-the-box access to some platform-specific services and APIs from the operating system. However, we want to avoid the “lowest common denominator” problem with most cross-platform APIs, so we do not intend to build cross-platform APIs for all native services and APIs.

We encourage developers to use Flutter’s asynchronous message passing system to create your own integrations with platform and third-party APIs. Developers can expose as much or as little of the platform APIs as they need, and build layers of abstractions that are a best fit for their project.

Can I extend and customize the bundled widgets?

Absolutely. Flutter’s widget system was designed to be easily customizable.

Rather than having each widget provide a large number of parameters, Flutter embraces composition. Widgets are built out of smaller widgets that you can reuse and combine in novel ways to make custom widgets. For example, rather than subclassing a generic button widget, RaisedButton combines a Material widget with a GestureDetector widget. The Material widget provides the visual design and the GestureDetector widget provides the interaction design.

To create a button with a custom visual design, you can combine widgets that implement your visual design with a GestureDetector, which provides the interaction design. For example, CupertinoButton follows this approach and combines a GestureDetector with several other widgets that implement its visual design.

Composition gives you maximum control over the visual and interaction design of your widgets while also allowing a large amount of code reuse. In the framework, we’ve decomposed complex widgets to pieces that separately implement the visual, interaction, and motion design. You can remix these widgets however you like to make your own custom widgets that have full range of expression.

Can I interop with my mobile platform’s default programming language?

Yes, Flutter supports calling into the platform, including integrating with Java code on Android, and ObjectiveC or Swift code on iOS. This is enabled via a flexible message passing style where a Flutter app may send and receive messages to the mobile platform using a BasicMessageChannel.

Learn more about accessing platform and third-party services in Flutter.

Here is an example project that shows how to use a platform channel to access battery state information on iOS and Android.

Does Flutter come with a reflection/mirrors system?

Not at this time. Because Flutter apps are pre-compiled for iOS, and binary size is always a concern with mobile apps, we disabled dart:mirrors. We are curious what you might need reflection/mirrors for – please let us know at flutter-dev@googlegroups.com.

How do I do internationalization (i18n), localization (l10n), and accessibility (a11y) in Flutter?

Flutter has basic support for accessibility on iOS and Android, though this feature is a work in progress.

Flutter developers are encouraged to use the intl package for internationalization and localization.

We encourage you to email flutter-dev@googlegroups.com with your questions regarding these features.

How do I write parallel and/or concurrent apps for Flutter?

Flutter supports isolates. Isolates are separate heaps in Flutter’s VM, and they are able to run in parallel (usually implemented as separate threads). Isolates communicate by sending and receiving asynchronous messages. Flutter does not currently have a shared-memory parallelism solution, although we are evaluating solutions for this.

Check out an example of using isolates with Flutter.

Can I use JSON/XML/protobuffers/etc with Flutter?

Absolutely. There are libraries in pub.dartlang.org for JSON, XML, protobufs, and many other utilities and formats.

Can I build 3D (OpenGL) apps with Flutter?

Today we don’t support for 3D via OpenGL ES or similar. We have long-term plans to expose an optimized 3D API, but right now we’re focused on 2D.

Framework

Why is the build() method on State, not StatefulWidget?

Putting a Widget build(BuildContext context) method on State rather putting a Widget build(BuildContext context, State state) method on StatefulWidget gives developers more flexibility when subclassing StatefulWidget. You can read a more detailed discussion on the API docs for State.build.

Where is Flutter’s markup language? Why doesn’t Flutter have a markup syntax?

Flutter UIs are built with an imperative, object-oriented language (Dart, the same language used to build Flutter’s framework). Flutter does not ship with a declarative markup.

We found that UIs dynamically built with code allow for more flexibility. For example, we have found it difficult for a rigid markup system to express and produce customized widgets with bespoke behaviours.

We have also found that our “code-first” better allows for features like hot reload and dynamic environment adaptations.

It is possible to create a custom language that is then converted to widgets on the fly. Since build methods are “just code”, they can do anything, including interpreting markup and turning it into widgets.

My app has a Slow Mode banner/ribbon in the upper right. Why am I seeing that?

By default flutter run command uses the debug build configuration.

The debug configuration runs your Dart code in a VM (Virtual Machine) enabling a fast development cycle with hot reload (release builds are compiled using the standard Android and iOS toolchains).

The debug configuration also checks all asserts, which helps you catch errors early during development, but imposes a runtime cost. The “slow mode” banner indicates that these checks are enabled. You can run your app without these checks by using either the --profile or --release flag to flutter run.

Project

Where can I get support?

If you think you’ve encountered a bug, please file it in our issue tracker. We encourage you to use Stack Overflow for “HOWTO” type questions. For discussions, please join our mailing list at flutter-dev@googlegroups.com.

How do I get involved?

Flutter is open source, and we encourage you to contribute. You can start by simply filing issues for feature requests and bugs in our issue tracker.

We recommend that you join our mailing list at flutter-dev@googlegroups.com and let us know how you’re using Flutter and what you’d like to do with it.

If you’re interested in contributing code, you can start by reading our Contributing Guide and check out our list of easy starter issues.

Is Flutter open source?

Yes, Flutter is open source technology. You can find the project on GitHub.

Which software license(s) apply to Flutter and its dependencies?

Flutter includes two components: an engine that ships as a dynamically linked binary, and the Dart framework as a separate binary that the engine loads. The engine uses multiple software components with many dependencies; view the complete list here.

The framework is entirely self-contained and requires only one license.

In addition, any Dart packages you use may have their own license requirements.

How can I determine the licenses my Flutter application needs to show?

There’s an API to find the list of licenses you need to show:

Who works on Flutter?

Flutter is an open source project. Currently, the bulk of the development is done by engineers at Google. If you’re excited about Flutter, we encourage you to join the community and contribute to Flutter!

What are Flutter’s guiding principles?

We believe that:

  • In order to reach every potential user, developers need to target multiple mobile platforms.
  • HTML and WebViews as they exist today make it challenging to consistently hit high frame rates and deliver high-fidelity experiences, due to automatic behavior (scrolling, layout) and legacy support.
  • Today, it’s too costly to build the same app multiple times: it requires different teams, different code bases, different workflows, different tools, etc.
  • Developers want an easier, better way to use a single codebase to build mobile apps for multiple target platforms, and they don’t want to sacrifice quality, control, or performance.

We are focused on three things:

  • Control - Developers deserve access to, and control over, all layers of the system. Which leads to:
  • Performance - Users deserve perfectly fluid, responsive, jank-free apps. Which leads to:
  • Fidelity - Everyone deserves precise, beautiful, delightful mobile app experiences.

Will Apple reject my Flutter app?

We can’t speak for Apple, but Apple’s policies have changed over the years, and they have allowed apps built with systems like Flutter. We are aware of apps built with Flutter that have been reviewed and released via the App Store.

Of course, Apple is ultimately in charge of their ecosystem, but our goal is to continue to do everything we can to ensure Flutter apps can be deployed into Apple’s App Store.