How often does an activity run?

by BobG » Mon, 19 Apr 2010 14:37:12 GMT

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 If we run a simple little hello world program that just puts some text
in a textview, I see the the onCreate runs, and I guess it calls
ondraw once, then it sort of returns to the os, and if we have
registered a sensor changed or an onclick listener, we can read the
sensor and call invalidate and the os will call ondraw again, and it
all is usually 'fast enough'. But my question is: Does ondraw ever get
called again? Or is this now a 'zombie process' that will just sit
there taking up memory until we kill it?


How often does an activity run?

by ~ TreKing » Mon, 19 Apr 2010 21:09:20 GMT


Sure, whenever the view needs to be re-drawn ...

I don't know what you're asking. Activities don't really "run" like threads
that have a definite function that gets executed to do work. They have
functions that are invoked in response to system events (onCreate, onPause,
onConfigurationChanged, etc).

TreKing - Chicago transit tracking app for Android-powered devices 


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How often does an activity run?

by BobG » Mon, 19 Apr 2010 22:27:22 GMT


Here is my 'model' that compares an embedded program to an android
embedded program: main gets called by os, main calls initstuff(),
falls into a while(1) loop that calls inputs(), process() and
outputs() forever. The os can kill it if it has to. In the android
program, the onCreate is the init, the os scheduler is the while(1)
loop, and the onSensorChanged events are like the input and process
functions, and the onDraw is like the output function. Sort of. Does
this model make sense to anyone else? Can it be explained more clearly
by another model?


How often does an activity run?

by Daniel Favela » Tue, 20 Apr 2010 14:05:32 GMT

 Well, this seems like a good exercise to test my learning and reading as a
newbie in the Android scene.

If I'm understanding BobG correctly, he means that the calls to onDraw
depend entirely on your application.  Your *Hello World!* sample renders the
text once and never has to render anything again -- the view stays put as
you left it.  It's a callback: when something happens (like user input),
then onDraw() might be called if something new has to be drawn.

A game, for example, might process game logic at each frame.  This results
in onDraw() being called for each frame, since game logic might place a game
object in a position different from that it was in during the last frame.

Hello World, as mentioned, doesn't need to be updated in that way.  That's
why onDraw() is not called anymore.

With regards to the "zombie" sentiment, onDraw() does not actually take up
any more memory than any other function might take; it is called when
needed, I suspect, much like onCreate(), onStart(), onResume, onDestroy() do
(these are methods involved in an activity's life cycle; check out the
Android Application fundamentals page
here< #lcycles>
see where I'm pulling these potentially wrong statements from).  From what
I've read, Android does things in a very, very, very modular manner;
everything is "there" and safe until it's needed.

I'm guessing that if you were to press "Home" or "Back" while *Hello
World!*was running, then you brought it back to the foreground, it
would call
onDraw() again.

If I'm wrong in anything I've said, please correct me!  I hope that helps.



How often does an activity run?

by Indicator Veritatis » Wed, 21 Apr 2010 06:40:37 GMT

 It is most definitely not a "zombie process". A zombie process, by
definition, is one not even the shell command 'kill' can kill. The
process you just described is still able to receive events -- and will
the next time the OS decides to call onDraw.

It is Android that decides when to call onDraw(). However, you can
tell it to do so by calling postInvalidate() or invalidate() as
described in 


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