Hitch?

The Render Loop

The Render Loop is a continuous process by which touch events are handed to an app, and then changes to the UI are sent to the OS where the frame is finalized.
It happens at the device's refresh rate.

Refresh rate: 60Hz
Frame duration: 16.67ms

This is 60 frames per second. which means a new frame can be displayed every 16.67ms.

Beginning
- the hardware emits an event called a VSYNC.
  
  The VSYNC denotes when a new frame must be ready.
The Render Loop is timed to VSYNCs.
Three Stages
- App
- Render Server
  - This stage is where your UI is actually rendered.
- On the display
This 3 stages must complete before the next VSYNC.
Double Buffering
- App + Render Server stages
- To avoid hitches, the system may switch to triple buffering.
  - render server stage is given one extra frame duration to complete its work.
The Real Render Loop (5 phrases)
- App
  - Event (Layer Tree)
    - your app handles touch events and decides if a change is needed in the UI.
  - Commit (Layer Tree)
    - your app updates the UI, and submits that to the render server for rendering.
- Render Server
  - Render prepare (Image)
    - the Render Server takes the submission, and prepares it for drawing on the GPU.
  - Render execute
    - the GPU draws your UI into a final image.
- On the display
  - Display
    - the frame can be displayed to the user.

App takes too long to commit or process an event.
happen within the app's process.
the render server has nothing to process and must wait for the next VSYNC to begin rendering
16.67 ms, we call this delay duration "hitch time".
more info about commit hitches

Rendering on the server does not complete in time.
happend within the Render Server's process.
These happen when the render server is unable to prepare or execute our layer tree on time.

it's difficult to compare unless each scroll or animation takes the exact same amout of time and therefore the exact same number of frames.
iOS device does not always update their screen, if there are no commits sent to the Render Server.

It is the total hitch time in an interval divided by its duration.
- Because it's normalized to total time, we can compare it across experiences.
The Way of Using it
To track hitch time ration in the test suite