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Add more warm up frame docs (#150464)

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Updated the wiki to call out the warm up frame. This contains details from the following pages:

1. b98d48549c/packages/flutter/lib/src/scheduler/binding.dart (L994-L1016)
2. https://api.flutter.dev/flutter/dart-ui/PlatformDispatcher/scheduleWarmUpFrame.html
3. https://api.flutter.dev/flutter/dart-ui/FlutterView/physicalSize.html

Part of https://github.com/flutter/flutter/issues/149974
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Loïc Sharma 2024-06-25 13:36:05 -07:00 committed by GitHub
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docs/engine/Life-of-a-Flutter-Frame.md
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@ -18,7 +18,6 @@ Flutter does some minimal housekeeping when a frame is requested, primarily to i
Once a frame is scheduled, Flutter [waits for a vsync][vsyncWaiter] from the operating system to proceed.
## Building the frame
At the heart of Flutter's graphics workflow is the frame [pipeline][pipeline]. The pipeline is responsible for coordinating work between the UI thread, where the application code runs, and the Raster thread, where rasterization and compositing is performed. See the [threading section][engineArchThreading] of the Engine Architecture wiki for more details on threading in the engine.
@ -41,6 +40,28 @@ Once a surface is acquired, the [LayerTree][layerTree] is rasterized to the surf
The above process is repeated until the pipeline is empty.
## Warm-up frame
Normally, the Flutter framework begins producing a frame when it receives
a vsync event from the operating system. However, this may not happen for
several milliseconds after the app starts (or after a hot reload). To make
use of the time between when the widget tree is first configured and when
the engine requests an update, the framework schedules a _warm-up frame_
using [PlatformDispatcher.scheduleWarmUpFrame][scheduleWarmUpFrame].
A warm-up frame may never actually render (as it invokes
[FlutterView.render][flutterViewRender] outside of the scope of
[PlatformDispatcher.onBeginFrame][onBeginFrame] or
[PlatformDispatcher.onDrawFrame][onDrawFrame]), but it will cause the framework
to go through the steps of building, laying out, and painting, which can
together take several milliseconds. Thus, when the engine requests a real frame,
much of the work will already have been completed, and the framework can
generate the frame with minimal additional effort.
At startup, a warm-up frame can be produced before the Flutter engine has reported the
initial view metrics using [PlatformDispatcher.onMetricsChanged][onMetricsChanged].
As a result, the first frame can be produced with a size of zero.
## Cleaning up frame resources
TODO(cbracken): write this up using [this patch](https://github.com/flutter/engine/pull/38038) as a reminder.
@ -53,10 +74,13 @@ TODO(cbracken): write this up using [this patch](https://github.com/flutter/engi
[handleBeginFrame]: https://api.flutter.dev/flutter/scheduler/SchedulerBinding/handleBeginFrame.html
[layerTree]: https://github.com/flutter/engine/blob/main/flow/layers/layer_tree.h
[onBeginFrame]: https://api.flutter.dev/flutter/dart-ui/PlatformDispatcher/onBeginFrame.html
[onDrawFrame]: https://api.flutter.dev/flutter/dart-ui/PlatformDispatcher/onDrawFrame.html
[onMetricsChanged]: https://api.flutter.dev/flutter/dart-ui/PlatformDispatcher/onMetricsChanged.html
[pipeline]: https://github.com/flutter/engine/blob/main/shell/common/pipeline.h
[rasterizer]: https://github.com/flutter/engine/blob/main/shell/common/rasterizer.h
[renderingPipelineTalk]: https://www.youtube.com/watch?v=UUfXWzp0-DU
[scene]: https://api.flutter.dev/flutter/dart-ui/Scene-class.html
[scheduleWarmUpFrame]: https://api.flutter.dev/flutter/dart-ui/PlatformDispatcher/scheduleWarmUpFrame.html
[surface]: https://github.com/flutter/engine/blob/main/flow/surface.h
[scheduleFrame]: https://api.flutter.dev/flutter/dart-ui/PlatformDispatcher/scheduleFrame.html
[vsyncWaiter]: https://github.com/flutter/engine/blob/main/shell/common/vsync_waiter.h