ShaderWarmUp class Null safety

Interface for drawing an image to warm up Skia shader compilations.

When Skia first sees a certain type of draw operation on the GPU, it needs to compile the corresponding shader. The compilation can be slow (20ms- 200ms). Having that time as startup latency is often better than having jank in the middle of an animation.

Therefore, we use this during the PaintingBinding.initInstances call to move common shader compilations from animation time to startup time. If needed, app developers can create a custom ShaderWarmUp subclass and hand it to PaintingBinding.shaderWarmUp before PaintingBinding.initInstances is called. Usually, that can be done before calling runApp.

To determine whether a draw operation is useful for warming up shaders, check whether it improves the slowest frame rasterization time. Also, tracing with flutter run --profile --trace-skia may reveal whether there is shader-compilation-related jank. If there is such jank, some long GrGLProgramBuilder::finalize calls would appear in the middle of an animation. Their parent calls, which look like XyzOp (e.g., FillRecOp, CircularRRectOp) would suggest Xyz draw operations are causing the shaders to be compiled. A useful shader warm-up draw operation would eliminate such long compilation calls in the animation. To double-check the warm-up, trace with flutter run --profile --trace-skia --start-paused. The GrGLProgramBuilder with the associated XyzOp should appear during startup rather than in the middle of a later animation.

This warm-up needs to be run on each individual device because the shader compilation depends on the specific GPU hardware and driver a device has. It can't be pre-computed during the Flutter engine compilation as the engine is device-agnostic.

If no warm-up is desired (e.g., when the startup latency is crucial), set PaintingBinding.shaderWarmUp either to a custom ShaderWarmUp with an empty warmUpOnCanvas or null.

See also:

• PaintingBinding.shaderWarmUp, the actual instance of ShaderWarmUp that's used to warm up the shaders.
• flutter.dev/docs/perf/rendering/shader