Activity Architecture §

When an application starts for the first time, the AcitivtyManagerService creates a special kind of window token (see Binder IPC) called an application window token. This uniquely identifies the application’s top-level container window.

The ActivityManager gives this token to both the application and the WindowManager. Each time the application wants to add a new screen, it must send this token to the WindowManager again.

This ensures that it’s impossible for one application to draw over another. It also makes it easier for the ActivityManager to request to close all of a token’s windows, as the WindowManager will be able to correctly identify all of the affected windows.