|Developer(s)||Google and community|
|Initial release||Alpha (v0.0.6) / May 2017|
1.5.4 / May 7, 2019
|Written in||C, C++, Dart and Skia Graphics Engine|
|Platform||Development: Windows, MacOS and Linux, Target: Android, iOS, Google Fuchsia, Web platform and Desktop|
|License||New BSD License|
Flutter is an open-source mobile application development framework created by Google. It is used to develop applications for Android and iOS, as well as being the primary method of creating applications for Google Fuchsia.
The first version of Flutter was known as "Sky" and ran on the Android operating system. It was unveiled at the 2015 Dart developer summit, with the stated intent of being able to render consistently at 120 frames per second. During the keynote of Google Developer Days in Shanghai, Google announced Flutter Release Preview 2 which is the last big release before Flutter 1.0. On December 4th, 2018, Flutter 1.0 was released at the Flutter Live event, denoting the first "stable" version of the Framework. 
The major components of Flutter include:
- Dart platform
- Flutter engine
- Foundation library
- Design-specific widgets
On Android, and on Windows, macOS and Linux via the semi-official Flutter Desktop Embedding project, Flutter runs in the Dart virtual machine which features a just-in-time execution engine. Due to App Store restrictions on dynamic code execution, Flutter apps use ahead-of-time (AOT) compilation on iOS.
A notable feature of the Dart platform is its support for "hot reload" where modifications to source files can be injected into a running application. Flutter extends this with support for stateful hot reload, where in most cases changes to source code can be reflected immediately in the running app without requiring a restart or any loss of state. This feature as implemented in Flutter has received widespread praise.
Flutter's engine, written primarily in C++, provides low-level rendering support using Google's Skia graphics library. Additionally, it interfaces with platform-specific SDKs such as those provided by Android and iOS. The Flutter Engine is a portable runtime for hosting Flutter applications. It implements Flutter's core libraries, including animation and graphics, file and network I/O, accessibility support, plugin architecture, and a Dart runtime and compile toolchain. Most developers will interact with Flutter via the Flutter Framework, which provides a modern, reactive framework, and a rich set of platform, layout and foundation widgets.
UI design in Flutter typically involves assembling and/or creating various widgets. A widget in Flutter represents an immutable description of part of the user interface; all graphics, including text, shapes, and animations are created using widgets. More complex widgets can be created by combining many simpler ones.
However, the use of widgets is not strictly required to build Flutter apps. An alternative option is to use the Foundation library's methods directly, interfacing with "canvas" commands to draw shapes, text, and imagery directly to the screen. This property of Flutter has been utilized in a few frameworks, such as the open-source Flame game engine.
The Flutter framework contains two sets of widgets which conform to specific design languages. Material Design widgets implement Google's design language of the same name, and Cupertino widgets imitate Apple's iOS design.
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