How does iOS UIKit rendering pipeline work from touch event to screen display?

UIKit rendering flows through event handling, Core Animation tree updates, and GPU composition via Metal.

Touch events are processed by UIKit on the main run loop. Views update their state, which modifies the Core Animation layer tree. The render server (backed by SpringBoard and RenderServer) commits layer transactions. These are converted into GPU commands via Metal. The GPU composites layers into final framebuffer which is displayed during VSync.