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  • Writer's pictureMATTHEW NALLEY

Flutter’s Rendering Engine Unveiled: A Technical Walkthrough

Updated: Jul 16, 2023




Under the Hood of Flutter's Rendering Engine

Unlike traditional UI frameworks that utilize platform-provided widgets, Flutter creates its own widgets. This fundamental difference makes Flutter independent of the platform it runs on and allows it to create an identical UI on any platform.

The secret behind this capability lies in Flutter's rendering engine, which is powered by Skia, Google's open-source 2D graphics library. This engine paints every widget onto a 'canvas', leading to each pixel on the screen being controlled by the application.

Flutter's rendering process is initiated when a 'Frame' is triggered. Each Frame corresponds to the on-screen application UI at a single point in time. The Frame goes through the 'Pipeline', a series of stages, each adding more detail to the Frame.

The Flutter Rendering Pipeline

Let's break down the stages of Flutter's rendering pipeline:

  • Build: In this phase, Flutter takes your widget tree and builds the corresponding 'Element Tree'. Each widget has a corresponding Element that saves its current configuration and state.

  • Layout: The Element Tree undergoes the layout phase, where each Element calculates its size and position based on the constraints it receives from its parent.

  • Paint: After layout, each Element 'paints' itself onto a canvas. This painting process results in a 'RenderObject' for each Element, and collectively they form the 'RenderObject Tree'.

  • Composite: This final stage takes the RenderObject Tree and transforms it into a Layer Tree. The Layer Tree is then rasterized by the Skia engine into a single image that is presented on the screen.

Achieving High-Performance Rendering

Flutter achieves high performance through the following mechanisms:

  • Efficient Repaints: When the state of a widget changes, only the individual widget and its descendants are repainted, instead of repainting the entire widget tree. This process, known as 'Diffing', significantly enhances rendering efficiency.

  • GPU Acceleration: Flutter leverages the power of the device's GPU to accelerate the rendering process, ensuring smooth animations and transitions, even in complex applications.

  • Ahead-Of-Time (AOT) Compilation: Flutter's Dart language is compiled Ahead-Of-Time into native code, enhancing execution speed and startup times, resulting in smoother animations and responsive interfaces.

  • Thread Decoupling: Flutter uses multiple threads to perform different operations concurrently. Notably, UI operations and rendering are done on separate threads, allowing smooth UI performance even under heavy operations.

How Flutter's Rendering Differs from Other Frameworks

Most other frameworks rely on native components provided by the underlying platform, leading to discrepancies in look and feel across platforms. Flutter, on the other hand, paints its own UI, ensuring consistent rendering across platforms.

Moreover, many frameworks use Just-In-Time (JIT) compilation, which can be slower than Flutter's AOT compilation. Lastly, not all frameworks use a diffing algorithm or GPU acceleration as effectively as Flutter, leading to slower rendering.

Conclusion

In essence, Flutter's rendering engine, underpinned by the Skia library, the efficient use of threads, and AOT compilation, creates a powerhouse for high-performance, cross-platform applications. By painting its own widgets, Flutter ensures that its apps look and feel the same

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Are you inspired by Flutter’s robust rendering engine and looking to harness its power for your next project? Whether you're launching a cutting-edge mobile app or innovating with a web platform, our team is eager to help you navigate Flutter's rich ecosystem.




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