User:VladVukicevic/Android Notes

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16 Nov 2009

  • Android apps/activies are all separate processes
    • Therefore, they communicate with system services using IPC (binder)
    • Therefore, we should be able to duplicate that communication
  • Apps are all launched from a forked Zygote process, which is basically a pre-initialized dalvik instance
    • the zygote process is executed at system boot; it does initialization, and then runs a select() loop listening to requests
    • the zygote process is sent a message, which includes the start args, which will include the class name for main launch
    • the zygote process reads connection args in ZygoteConnection.runOnce
    • the zygote process forks in ZygoteConnection.runOnce (Zygote.forkAndSpecialize, native)
    • ZygoteConnection.handleChildProc is called, which does some cleanup and eventually throws a MethodAndArgsCaller (via ZygoteInit.invokeStaticMain)
    • ... which takes us all the way back to ZygoteInit.main, which catches that trampoline exception, and calls run()
  • Stack on onCreate, generally the initial entry point for a normal Android activity:
TestApp.onCreate(Bundle) line: 10	
Instrumentation.callActivityOnCreate(Activity, Bundle) line: 1123	
ActivityThread.performLaunchActivity(ActivityThread$ActivityRecord, Intent) line: 2364	
ActivityThread.handleLaunchActivity(ActivityThread$ActivityRecord, Intent) line: 2417	
ActivityThread.access$2100(ActivityThread, ActivityThread$ActivityRecord, Intent) line: 116	
ActivityThread$H.handleMessage(Message) line: 1794	
ActivityThread$H(Handler).dispatchMessage(Message) line: 99	
Looper.loop() line: 123	
ActivityThread.main(String[]) line: 4203	
Method.invokeNative(Object, Object[], Class, Class[], Class, int, boolean) line: not available [native method]	
Method.invoke(Object, Object...) line: 521	
ZygoteInit$ line: 791	
   This is the run() that was called after the fork, in our new process
ZygoteInit.main(String[]) line: 549	
NativeStart.main(String[]) line: not available [native method]	
  • ActivityThread.main() is effectively the "main" for an app; an app launch involves:
    • telling the ActivityManagerService that an app launch is desired, with info about which app
    • spinning up a new process via zygote that executes ActivityThread.main()
    • ActivityThread.main() calls Looper.prepareMainLooper, to initialize the main event loop for the new process
    • ActivityThread.main() connects to the ActivityManagerService and asks it for the next app that's waiting in the launch queue
    • Looper.loop() is called, which reads and dispatches messages
  • Instrumenting ActivityThread's message dispatch code, at activity startup, the main (only?) message that's sent is a LAUNCH_ACTIVITY:
    • LAUNCH_ACTIVITY (ActivityRecord token=android.os.BinderProxy {acitivy class})

1 Sep 2009

Rendering is easy
frameworks/base/cmds/bootanimation has a simple native command that shows how to connect to SurfaceComposer, create a Surface (which has move/etc. functions on ISurface), and draw to it.
Events are all java side
the core pieces are IWindowManager and IWindow. See IWindowManager.aidl, IWindow.aidl, IWindowSession.aidl in frameworks/base/core/java/android/view. Also core is WindowManagerService in frameworks/base/services/java/com/android/server/ . The service seems to be the piece that we need to interact with; we need to have an IWindow and call addWindow on the service. It dispatches events to windows that it knows about, as best I can tell, using the IWindow interface.
For exploration
create a java shim that would basically bridge all the java pieces back to native code; essentially, in the end, part of widget/src/android would be implemented in java (and a simple java launcher). We should create that shim and some native rendering/event handling code first (ignoring gecko entirely) to see what that would look like.