Introduction
The Async Pan/Zoom module (APZ) (formerly called "Async Pan/Zoom Controller" or "APZC") is a platform component that allows panning and zooming to be performed asynchronously (on the compositor thread rather than the main thead).
For zooming, this means that the APZ reacts to a pinch gesture immediately and instructs the compositor to scale the already-rendered layers at whatever resolution they have been rendered (so e.g. text becomes more blurry as you zoom in), and meanwhile sends a request to Gecko to re-render the content at a new resolution (with sharp text and all).
For panning, this means that the APZ asks Gecko to render a portion of a scrollable layer, called the "display port", that's larger than the visible portion. It then reacts to a pan gesture immediately, asking the compositor to render a different portion of the displayport (or, if the displayport is not large enough to cover the new visible region, then nothing in the portions it doesn't cover), and meanwhile sends a request to Gecko to render a new displayport. (The displayport can also be used when zooming out causes more content of a scrollable layer to be shown than before.)
Main APZ projects on the horizon
- Make APZ work well for Metro - bug 886321
- Implement support for Fennec-like dynamic toolbar on B2G - bug 860812
- Improve hit detection for the different touch-sensitive regions - bug 928833
- Switch Fennec over from the Java version to the C++ APZ - bug 776030