What is Flutter?

Since the advent of the Apple App Store (and subsequently the Google Play Store), there has been a way for organizations to share programs with mobile users in a very controlled and managed way. Much like web pages on the internet, mobile apps have proliferated to encompass all aspects of our life. And much like web pages, over the years, developers have iteratively discovered and learned the best ways to create reliable, scalable, and intuitive mobile apps.

As developers have learned to work within the mobile ecosystem, they have followed similar design patterns and framework ideas as were created to deal with the web ecosystem. Much like the complications of developing code for multiple browsers, in the mobile ecosystem, there has been the challenge of developing code that can work on both iOS and Android devices, with the dream always being to have one code base that works on all devices and even the web.

Flutter is a framework that is the culmination of this learning. Like most other frameworks, developers use a programming language specified by the framework, and structure their code in a way that aligns with the needs of the framework so that ultimately, the developer creates the least amount of "boilerplate" code and can focus on their business needs. Examples of "boilerplate" code would be how to manage touch input, how to connect to the internet, and how to package the app code to work with the App Store, Play Store, or web hosting service.

Flutter is a very new framework, and this means that it does not have a big section of the mobile development market yet, but this is changing, and the outlook for the next few years is highly positive.

When choosing a new programming language or framework, it is hugely important to developers and software companies that what they have chosen has certain key aspects that will ensure it is easy to pick up and that it has a bright future. Investing time and money into learning a new solution, and then developing a code base and development processes around that language and framework, is incredibly expensive. If that solution becomes outdated after a short period of time, there is poor support and documentation, there are a lack of new developers available to take your product forward, or the solution has scaling issues or usability problems, then that investment is wasted. Let's look at some of the aspects that suggest Flutter may be a good long-term investment.

Backed by Google

Flutter, and the Dart programming language it depends on, were created by Google, and although they are open source, they continue to be backed by Google. This ensures the framework has all the tools it needs to succeed in the community, with support from the Google team, presence at big events such as Google I/O, and investment in continuous improvements in the code base.

From the launch of the first stable release during the Flutter Live Event at the end of 2018, the growth of Flutter is evident:

• More than 200 million users of Flutter apps
• More than 50,000 Flutter apps on the Play Store
• Nearly 500,000 developers
• The 18th most popular software repository on GitHub

Let's look at some of the reasons why Flutter has become so popular.

Fuchsia OS and Flutter

It's not a secret anymore that Google has been working on a new operating system called Fuchsia OS, which has been rumored to be a potential future replacement for the Android OS. One thing to pay attention to is that Fuchsia OS may be a universal Google operating system that runs on more than just mobile phones, and this would directly affect Flutter adoption. This is because Flutter will be the first method of developing mobile apps for the new Fuchsia OS, and, not only this, Fuchsia uses Flutter as its UI rendering engine. With the system targeting more devices than just smartphones, as seems to be the case, Flutter will certainly have a lot of improvements.

The growth of the framework's adoption is directly related to the new Fuchsia OS. As it gets closer to launch, it is important for Google to have mobile apps targeting the new system. For example, Google has announced that Android apps will be compatible with the new OS, making the transition to, and adoption of, Flutter significantly easier.

Dart

The Dart programming language was first unveiled by Google at the GOTO conference in 2011, and Dart 1.0 was released at the end of 2013. Initially viewed as a replacement for JavaScript (the main web programming language), the uptake of Dart by developers was relatively low. However, thanks to the emergence of Flutter and its reliance on Dart, the Dart programming language has seen a huge rise in usage.

So why did the Flutter project choose the Dart programming language? Since its inception, one of Flutter's main goals was to be a high-performance alternative to existing cross-platform frameworks. But not only that; to significantly improve the mobile developer's experience was one of the crucial points of the project.

With this in mind, Flutter needed a programming language that allowed it to accomplish these goals, and Dart seemed to be the perfect match for the following reasons:

• Dart compilation: Dart is flexible enough to provide different ways of running the code, so Flutter uses Dart ahead of time (AOT) compilation with performance in mind when compiling a release version of the application, and it uses just in time (JIT) compilation with sub-second compilation of code in development time, aiming for fast feedback for code changes. Dart JIT and AOT refer to when the compilation phase takes place. In AOT, code is compiled during the build process and before running the code; in JIT, code is compiled while running (check out the Dart introduction section in the next chapter).
• High performance: Due to Dart's support for AOT compilation, Flutter does not require a slow bridge between realms (for example, non-native Flutter code to native device code), which makes Flutter apps responsive and allows a fast startup.
• Garbage collection: Flutter uses a functional-style flow with short-lived objects, and this means a lot of short-lived allocations. Dart garbage collection works without locks, helping with fast allocation.
• Easy to learn: Dart is a flexible, robust, modern, and advanced language. The language has been adapted as Flutter has become more popular, with lots of syntactic sugar, and fundamental design changes, that really help with Flutter app creation. Although it is still evolving, the language has a well-defined object-oriented framework with familiar functionalities to dynamic and static languages, an active community, and very well-structured documentation.
• Declarative UI: In Flutter, you use a declarative style to lay out widgets, which means that widgets are immutable and are only lightweight "blueprints." To change the UI, a widget triggers a rebuild on itself and its subtree. In the opposite imperative style (the most common), we can change specific component properties after they are created.

  Declarative UI

  We will explore this a lot more throughout the book, but if you want to understand the concept of the Flutter declarative UI at this point, then take a look at the official introduction to declarative UI from Flutter: https://flutter.dev/docs/get-started/flutter-for/declarative.

• Dart syntax for layout: Different from many frameworks that have a separate syntax for layout, in Flutter, the layout is specified inline within the Dart code. This gives greater flexibility and reduces the developer's cognitive load. Flutter has great tools for debugging layout as well as rendering performance.

These are great reasons why Dart fits perfectly with Flutter. However, there is a key area of Flutter that is probably why you are learning and using it, and why it is a game-changer in the app development world, and that is a single code base for multiple platforms. Let's take a look at that now.

One code base to rule them all

The primary goal of the Flutter framework is to be a toolkit for building apps that are equivalent in performance, usability, and features to native apps (apps created directly for iOS or Android) while using only a single code base. You may have heard it stated often that there are big advantages to having a single code base. Let's see why that is the case:

• Multiple languages to learn: If a developer wants to develop for multiple platforms, they must learn how to do something in one OS and programming language, and later, the same thing in another OS and programming language. The developer then needs to decide whether to focus on one platform for a period of time, causing a mismatch of features/bug fixes between the apps, or constantly switch between platforms, impacting productivity and potentially introducing bugs.
• Long/more expensive development cycles: If you decide to create multiple development teams to avoid the previous issues, there are consequences in terms of cost, multiple deadlines, different capabilities of native frameworks, and disparate sets of bug reports.
• Inconsistency: Different native capabilities, or different development teams developing features in slightly different ways, may lead to inconsistencies between apps, annoying users and making bug reporting more complicated to diagnose.

Flutter is not the first attempt to create a single code base and there are existing frameworks available that have similar promises. However, they can suffer from some serious drawbacks:

• Performance: Some frameworks use workarounds to allow consistency of user experience across platforms. One of these is to effectively have a web page running inside a native app using a webview (a built-in web browser). This tends to have much worse performance than native apps, leading to a poor user experience.
• Design constraints: Some frameworks are based on languages that were designed before the mobile experience was created. This can mean they are not designed well for certain user interactions or certain device capabilities, leading to complicated or obscure code, and the inherent maintenance issues this can cause.
• Not quite one code base: Although some frameworks suggest a single code base approach to app development, once you get into the details, you find that you still need to write some platform-specific code, which causes code duplication and allows single platform bugs to creep in.

Now let's see how Flutter counters these problems.

High performance

Right now, it is hard to say that Flutter's performance is always better in practice than other frameworks, but it's safe to say that its aim is to be. For example, its rendering layer was developed with a high frame rate in mind. As we will see in the Flutter rendering section, some of the existing frameworks rely on JavaScript and HTML rendering, which might cause overheads in performance because everything is drawn in a webview (a visual component like a web browser).

Some use original equipment manufacturer (OEM) widgets but rely on a bridge to request the OS API to render the components, which creates a bottleneck in the application because it needs an extra step to render the user interface (UI). See the Flutter rendering section for more details of the Flutter rendering approach compared to others.

Some points that make Flutter's performance great are the following:

• Flutter owns the pixels: Flutter renders the application pixel by pixel (see the next section), interacting directly with the Skia graphics engine.
• No extra layers or additional OS API calls: As Flutter owns the app rendering, it does not need additional calls to use the OEM widgets, so no bottleneck.
• Flutter is compiled to native code: Flutter uses the Dart AOT compiler to produce native code. That means there's no overhead in setting up an environment to interpret Dart code on the fly, and it runs just like a native app, starting more quickly than frameworks that need some kind of interpreter.

Full control of the UI

The Flutter framework chooses to do all the UI by itself, rendering the visual components directly to the canvas, as we have seen previously, requiring nothing more than the canvas from the platform so it's not limited by rules and conventions. Most of the time, frameworks just reproduce what the platform offers in another way. For example, other webview-based cross-platform frameworks reproduce visual components using HTML elements with CSS styling. Other frameworks emulate the creation of the visual components and pass them to the device platform, which will render the OEM widgets like a natively developed app. We are not talking about performance here, so what else does Flutter offer by not using the OEM widgets and doing the job all by itself?

Let's see:

• Ruling all the pixels on the device: Frameworks limited by OEM widgets will reproduce at most what a native developed app would, as they use only the platform's available components. On the other hand, frameworks based on web technologies may reproduce more than platform-specific components, but may also be limited by the mobile web engine available on the device. By getting control of the UI rendering, Flutter allows the developer to create the UI in their own way by exposing an extensible and rich Widgets API, which provides tools that can be used to create a unique UI with no drawbacks in performance and no limits in design.
• Platform UI kits: By not using OEM widgets, Flutter can break the platform design, but it does not. Flutter is equipped with packages that provide platform design widgets, the Material set in Android, and Cupertino in iOS. We will see more on platform UI kits in Chapter 5, Widgets – Building Layouts in Flutter.
• Achievable UI design requirements: Flutter provides a clean and robust API with the ability to reproduce layouts that are faithful to the design requirements. Unlike web-based frameworks that rely on CSS layout rules that can be large and complicated and even conflicting, Flutter simplifies this by adding semantic rules that can be used to make complex but efficient and beautiful layouts.
• Smoother look and feel: In addition to native widget kits, Flutter seeks to provide a native platform experience where the application is running, so fonts, gestures, and interactions are implemented in a platform-specific way, bringing a natural feel to the user, like a native application.

We refer to visual components as widgets, which we will go into more detail on that in the Widgets, widgets, everywhere section in this chapter.

Open source framework

Having a big company such as Google behind it is fundamental to a framework such as Flutter (see React, for example, which is maintained by Facebook). In addition, community support becomes even more important as it becomes more popular.

By being open source, the community and Google can work together to do the following:

• Help with bug fixes and documentation through code collaboration
• Create new educational content about the framework
• Support documentation and usage
• Make improvement decisions based on real feedback

Improving the developer experience is one of the main goals of the framework. Therefore, in addition to being close to the community, the framework provides great tools and resources for developers. Let's see them.

Developer resources and tooling

The focus on developers in the Flutter framework goes from documentation and learning resources to providing tools to helping with productivity:

• Documentation and learning resources: Flutter websites are rich for developers coming from other platforms, including many examples and use cases, for example, the famous Google Codelabs (https://codelabs.developers.google.com/?cat=Flutter).
• IDE integration: Flutter, and Dart, have a completed, integrated IDE experience with Android Studio, IntelliJ, and Visual Studio Code. Within this book, we will show examples from Visual Studio Code, but these examples will work very similarly in Android Studio and IntelliJ.
• Command-line tools: Dart has tools that help with analyzing, running, and managing dependencies and these are also part of Flutter. In addition, Flutter has commands to help with debugging, deploying, inspecting layout rendering, and integration with IDEs through Dart plugins. Here's a list of the various commands:

Figure 1.1 – Available commands in Flutter

• Quick setup: Flutter has the create command shown in the preceding list that allows you to create a new and fully functional Flutter project with minimal input. IDEs also offer a Flutter project creation menu option, replicating the command-line functionality.
• Environment issue diagnostics: Flutter comes with the flutter doctor tool, which is a command-line tool that guides the developer through the system setup by indicating what is needed in order to be ready to set up a Flutter environment. We will see this tool in action when we set up your environment very soon. The flutter doctor command also identifies connected devices and whether there are any upgrades available.
• Hot reload: This is a huge benefit to developers and a feature that is getting a lot of attention. By combining the capabilities of the Dart language (such as JIT compilation) and the power of Flutter, it is possible for the developer to instantly see design changes made to code in the simulator or device. In Flutter, there is no specific tool for layout preview. Hot reload makes it unnecessary.

Now that we have learned about the benefits of Flutter, let's start looking at the software's compilation process.