Flutter vs Kotlin: Which One to Choose

Mobile Developer Droids On Roids compared two cross-platform frameworks, Kotlin and Flutter, and explained which projects each of them is suitable for.

I am Flutter Tech Lead at Friflex Yura Petrov, translated this article and invite the community to discuss the features of frameworks.

Let's compare

Flutter was recently named the best framework for developing cross-platform applications. Later, Kotlin Multiplatform (KMP) appeared. It attracted a lot of attention and became a serious competitor to Flutter.

Should we now abandon Flutter projects? Not at all! Flutter still holds a strong position. Both technologies have their pros and cons. And you need to choose a framework for a specific project.

However, it is easy to get confused in this choice. We are here to help you understand which technology is best for the project, given your resources and objectives.

Kotlin Multiplatform and Flutter are relatively new cross-platform development tools. They approach their task differently.

Flutter uses its own UI rendering tool to create everything the user sees on the screen. This tool is called the Flutter engine. The framework draws all the elements itself and ensures that the app looks and works the same on any device and platform.

Kotlin Multiplatform uses the built-in tools of each platform, iOS or Android, to create visual elements. This approach allows the application to look and work natively on each platform according to its requirements.

KMP extends native apps, while Flutter replaces them. Many people don’t know this, but both technologies are native from a code perspective.

We suggest comparing the main functions of the frameworks.

The table below highlights the key differences that are important to consider when choosing between Flutter and Kotlin Multiplatform. The final choice depends on the project goals and the team’s capabilities.

Let's look at the details

Let's talk about each technology separately.

What is Flutter?

It is an open-source framework for developing user interfaces that Google introduced at the Google I/O conference in December 2018.

With Flutter, developers can create natively compiled applications from a single code base for different platforms: iOS, Android, Windows, macOS, Linux, Google Fuchsia. Flutter can also be used to develop web applications and interfaces for embedded devices.

Flutter uses the Dart programming language, which is known for its efficiency and performance.

Advantages of Flutter

• Single code base. With Flutter, you write code once and run it on multiple platforms. The framework reduces development time and requires fewer resources.

• Hot Reboot. Changes in the code are reflected instantly. The developer does not need to restart the application. Development becomes faster and more efficient.

• Rich set of widgetsFlutter offers a large library of widgets that can be customized to create visually appealing and responsive interfaces.

• Performance. Compiling Dart and native code provides performance that is as good as native apps.

• Open source and community supportedThere is a large and active community of Flutter developers, whose members are constantly publishing new plugins and packages. They also help with support and releases.

• Access to native functions. Integrating device-specific features is easy. Flutter plugins provide access to native device features and APIs.

Disadvantages of Flutter

• Application sizeFlutter apps are usually large in size. They require more space and time to load compared to native apps. The Flutter engine is loaded along with the app, which adds 3-4 MB to the overall size of the app.

• Performance issuesFlutter's performance has improved a lot since the first release, but it's still inferior to native. This is due to the middleware layer that is needed to interact with native functions.

• Limited access to native features. Flutter relies heavily on third-party plugins to access native mobile device capabilities. Developers now have access to a wide range of powerful plugins. But they are still not enough, especially for complex features.

• Limited access to the latest system functions. Native apps get instant access to new Android and iOS features. Flutter products get it with a delay. And sometimes they don’t get it at all, because updates are needed to support new features.

• Features of the Dart language. Compared to other programming languages, Dart is used less frequently. The Dart developer community is smaller, but it is growing along with the popularity of the language.

The need to rewrite an existing native applicationTo switch to Flutter, the existing native app will have to be completely rewritten.

What is Kotlin Multiplatform?

To understand what Kotlin Multiplatform is, you need to get acquainted with the Kotlin programming language, which was developed by JetBrains.

Kotlin is now associated with the Android platform, but it was created as a general-purpose language that could interact effectively with other languages ​​and be used for cross-platform development.

Kotlin Multiplatform (KMP) is a set of tools that simplifies and organizes work with the Kotlin language on several platforms at the same time, Android, iOS, web. It can also be used for desktop and server applications.

Most code can be implemented once in Kotlin and reused in native apps. Developing fully functional native apps becomes faster.

Advantages of Kotlin Multiplatform

• High performanceNative KMP applications have performance that is not inferior to their counterparts in terms of efficiency and speed.

• Full access to platform APIs and libraries. You can use any native technologies and combine them with Kotlin Multiplatform. This is especially convenient for developing applications on native solutions, such as AR.

• Common code base. You can write common code for different platforms and save resources.

• Easy migration. Developers can gradually migrate a finished native application to KMP. They do not need to rewrite the entire application. This approach allows for changes to be made without affecting the user interface.

• Native UI. With KMP, developers can implement platform UI. SwiftUI and Jetpack Compose frameworks help them with this.

• Native design. The app can be customized to look and behave in a way that matches the platform's recommendations. The app will look like a native app.

• Access to all Kotlin features. Developers can use all the features of Kotlin, including strong typing, nullability, and coroutines.

• Growing Ecosystem. The KMP community is growing and support for libraries is growing.

Disadvantages of Kotlin Multiplatform

• Libraries are in their early stages of development.In terms of architecture and testing, KMP does not yet have as many software libraries as Flutter.

• Limited online resources. KMP developers have fewer resources (documentation, training materials) compared to more established tools. It is more difficult for developers to find the information and support they need. The situation may improve as the KMP ecosystem grows. As the tool matures, the number of available resources will increase.

• UI sharing is in its early stages. KMP enables collaborative UI development with Compose Multiplatform. But the feature is still in its infancy and can't quite match the capabilities of Flutter.

• Objective-C Limitations. Translating Kotlin code to Objective-C for iOS apps can lead to limitations and other complications, such as issues with default method parameters.

Main differences

Now let's look at what makes Flutter and Kotlin Multiplatform different, and why one technology might be better than the other in different circumstances.

Technical approach

Flutter and Kotlin Multiplatform are very different in their technical approach.

Flutter uses the Dart programming language and compiles code into a native binary that includes Flutter add-on engine. This engine is written in C++. It controls every pixel on the screen. This is how Flutter apps can work across devices via intermediate layer.

KMP compiles Kotlin code into native code for each platform without additional layers. The application works on KMP as usual native. Kotlin Multiplatform integrates easily with native code and provides direct access to platform APIs.

Integration with the system

In KMP, the common code can use native code directlyspecific to each platform.

Flutter has shared code gets access to native code through Platform Channels mechanisms that serialize and deserialize data for interaction between the Flutter engine and the system.

Platform Channels are generally efficient, but they can have a performance impact. Most Flutter features are handled by packages that use Platform Channels. If there is no suitable package or it does not meet the requirements, developers are forced to implement the abstraction layer themselves.

Migration Strategy

Flutter and KMP offer different migration strategies.

For migrations to Flutter the entire application has to be rewritten in Dart. Some mature applications require several months for full migration.

Flutter also allows you to write individual functions in Dart and integrate them into native applications.

KMP has a different approach. Instead of rewriting the code, it can be gradually moved from the Android app to the multiplatform module. And then integrated into the iOS app.

Migration starts with abstract elements such as data models, then moves to business logic and finally to the presentation layer, all while preserving the native user interface.

This approach allows migration to take place without interrupting business operations, as well as ensuring a high quality user experience.

4 Key Criteria to Help You Choose a Framework

KMP helps reduce development costs while maintaining native performance and user experience. Migration can be done in iterations to avoid disruption to business processes.

Flutter works well in cross-platform projects thanks to a single code base. But migrating large, existing apps can be difficult and time-consuming.

Let's look at a few other cases where frameworks are suitable.

Application Focus

Kotlin Multiplatform is suitable for applications where:

• functions related to hardware capabilities: virtual fitting rooms (AR), navigation, image processing, phone calls or video conferencing;

• It is necessary to use Android and iOS features immediately after their release;

• The user interface must comply with platform requirements, such as Google Material Design or Apple Human Interface Guidelines.

Flutter is suitable for applications where:

• the main focus is on UI/UX, and native features like barcode scanning are a secondary concern;

• a unified user experience is required on Android and iOS without strict adherence to guides;

• The priority is a unique and customizable UI with possible expansion to web platforms in the future.

Organizational structure

Kotlin Multiplatform is suitable if:

• you already have a mobile app and a team of developers on Android/iOS, and you want to work with a technical partner;

• You plan to integrate new components into an existing application or simplify development with a single code base.

Flutter is suitable if:

• you are new to the industry and need a technical partner to build your first app;

• You don't have a development team and you need a partner to manage the development.

Priorities and Market Environment

Kotlin Multiplatform is suitable for companies that:

• are entering a highly competitive market where it is critical to provide a good native experience for each platform;

• prioritize native user interface and user experience across platforms.

Flutter is suitable for companies that:

• want to quickly enter the market to test business hypotheses or launch MVPs in innovative and little-studied industries;

• benefit from rapid development and features like hot reloading.

Summary

Choosing between Kotlin Multiplatform and Flutter is choosing between two powerful tools, each with its own advantages. We recommend choosing a framework based on your priorities and conditions.

Kotlin Multiplatform is better suited for applications:

• functions and business tasks of which are related to hardware capabilities: virtual fitting rooms for clothes or furniture configurators in AR, navigation, image processing, phone calls or video conferencing;

• who require constant access to the latest Android and iOS features;

• where UI/UX should be as consistent as possible with platform recommendations.

Kotlin Multiplatform applications are developed for highly competitive markets where every detail matters.

Flutter is suitable for applications where:

• Native functions are used as additional ones. For example, for online store applications, for which the most important thing is to show products and organize a convenient process of registration and payment for a purchase;

• UI/UX should be the same across all platforms;

• UI/UX should be customized and not follow Google or Apple guidelines.

Remember that you don't have to choose just one path. Both tools offer their own ways to develop and grow an application. The choice depends on the business objectives and project conditions.