What is the difference between React Native and Flutter?
Architecturally, React Native (RN) and Flutter are very similar.
What Flutter calls a "Widget" is the strict equivalent to React's "Component". That's what Flutter means when it says its inspired from React.
The difference between them lies in the other aspects of the frameworks:
Interpreted Javascript VS Compiled Dart
Flutter uses Dart, a typed language that offers both "Just in time" (JIT) and "Ahead of time" (AOT) compilation (with tree-shaking included)
In development, Flutter uses JIT compilation to empower hot-reload. And for production builds, it uses AOT compilation for better performances.
React-Native uses Javascript enhanced by some syntax sugar called JSX.
JSX being a different language, it compiles to JS, then evaluated at runtime.
Bridge to native VS Complete rewrite
React native is built on the top of native.
When using a button or a text in React Native, you are manipulating the official object used for native Android/iOS apps.
We can consider React as a common language between Android/iOS to declare layouts – but fundamentally the applications are different with potential inconsistencies.
It's is not true cross-platform. But at the same time, it allows better interoperability with native elements.
Flutter is the opposite. The goal of Flutter is to use as few native elements as possible.
Flutter requests to the OS a window, and then entirely manage its content using Dart and Skia (it's c++ graphics engine).
It has a few implications:
- All the UI logic had to be reimplemented by Flutter. Be it scroll, touch events, animations, ...
- The application is written entirely in Dart, even deep into the lower layers. It means that whatever the platform is, it's always the same code that is executed.
- Potentially anything that can run Dart code and create a window can run Flutter and apps should work with little to no change. As such, web is in progress ( Hummingbird) and basic support of desktops is available.
To some extent, we can compare Flutter to a webview/game engine, but optimized for casual applications.
Remi already has a couple good points. I have one more.
Interpreted with bridges - vs. native and no bridges
Despite what the name might imply, React Native apps are not compiled to native code. React Native apps interpret Javascript code during runtime, and component updates in a React Native app go through a bridge to the native view counterpart. This might slow things down a little bit and be a bottleneck.
On the contrary, Flutter apps (on release mode) are compiled to native code and don't require a bridge for manipulating the UI. This in turn, at least in theory, will be more performant - there's no need to do roundtrips to the native land to make simple UI changes. Not to mention that release Flutter code is natively compiled and there are no interpreters involved.
Dance monkey, dance
Now that we know that release mode Flutter apps don't have an interpreter or the need for bridges for UI manipulations, let's see what those two things actually are in the first place.
We'll do this with a highly hypothetical example app. Our React Native app has a button that makes a monkey dance on the screen. In React Native, our button and the dancing monkey components are written in Javascript and React.
Interpreters
Since Javascript is not a first class language on Android or iOS, your React Native app includes a Javascript interpreter that interprets your Javascript code in runtime. Without the interpreter, you wouldn't be able to write apps with Javascript at all - even a simple console.log('Hello World!') wouldn't work.
According to the React Native docs, in "most cases", the Javascript code will be interpreted with JavascriptCore.
Bridges
Under the hood, React Native uses the native Android Views and iOS UIViews for displaying UI components (such as dancing monkeys) on the screen. But since the UI parts of the Android and iOS SDKs don't use Javascript, you can't make the monkey dance by using Javascript alone.
This is where a bridge comes into play. On the other side of the bridge are your React Native components and logic, written in Javascript. On the opposite side, we have the host Android/iOS app that renders native views into the screen. From now on, let's call the two sides of the bridge as the Javascript land and the native land.
So, what happens when the user clicks on our "dance, monkey, dance!" button?
- The native Android/iOS view dispatches an onclick event, which goes over the bridge to the Javascript land.
- Our onclick listener written in Javascript gets invoked. It is a simple call that toggles a boolean inside the component. Something along the lines of
setState(() {isMonkeyDancing = true})or similar. - React sees that something has changed. It comes up with an updated representation of UI elements that has a dancing monkey. The representation is just a tree of plain Javascript objects that describes the updated state of the UI.
- The Javascript object tree gets serialized and sent over the bridge to the native land.
- The host app receives the serialized object tree and deserializes it. Now it can update the native Android/iOS view to match the deserialized UI representation. Our monkey is now dancing and our user is eternally happy.
So in this example, one button click required going over the bridge two times.
Actually, it's three - just simply rendering a button initially is a call across the bridge itself. In an app that is more than just one button and a dancing monkey, you're likely to go over the bridge a lot more. And every time you do, it requires serializing data and sending it over from one side to the another.
This is slower than just coming up with the UI representation and updating the UI with that directly. Additionally, there's a cost of interpreting Javascript in runtime compared to having the code compiled ahead of time.
The bottom line
Since Flutter is essentially a portable rendering engine, Flutter doesn't need a bridge to do an UI update. And because of that, UI updates, at least in theory, are faster. That's one reason why building apps with complex animations or things like Flare, SpriteWidget, or even games would be more lucrative with Flutter compared to React Native.
And because Flutter on release mode is AOT compiled, Flutter doesn't need an interpreter either. That's the difference between Flutter and React Native.