[android-developers] Re: Async pattern required (professionals welcome)

Keep a reference to the current Context inside your AsyncTask. Then in your onCreate(), either start up new tasks if they aren't running, or set the new Context in them if they are.

On Wednesday, August 29, 2012 3:25:31 PM UTC-4, itai wrote:

I'm struggling to figure out a straight forward design for the
following scenario:

A user clicks a button which results in sending an http request
asynchronously to fetch a list of image URL's (e.g some json
collection response).
Upon completion, a ListView would need to be populated with the images
referenced by the URL's (e.g each list view item would issue its own
async http requests to download and display the image). The image
being downloaded will always be the most recent for the proper
recycled image.

I'm concerned about three situations:

1. Configuration change can happen at any stage which will replace the
Activity instance receiving the async results with a new instance,
hiding already displayed images – My understanding is that the
solution for this is to maintain a some list of all active async tasks
on the initiating instance and transfer them all together in
onRetainNonConfigurationInstance(), However; my concern here is from
the doc's: "This function is called purely as an optimization, and you
must not rely on it being called", Does this mean that a configuring
change can happen and function won't be called ?@!

2. A different activity comes into the foreground, causing the
executing activity to be in "pause state" while still getting updates
from the in-progress async operations – Here I'm not sure if there is
any violation (e.g updating an Activity UI in pause state).

3. A different activity comes into the foreground, causing the
executing activity to be paused and subsequently killed (the entire
process is killed) due to memory pressure - In this case I would want
to:

a. Know that this is the case before it happens.

b. Save to persistent storage all the *new* images if and only If all
the downloads have completed, if not, I would like to save all
existing images that where present before the whole download process
kicked in (if present from a past download).
That way if the user navigates back to Activity and the process is
recreated, I can load the persisted information and have the user see
a snapshot of images either from a previous request or a new request
(no interleaving)

Initially I was inclined to use AsyncTask's (1 AsyncTask for getting
the Url's and X others for the images) and use
onRetainNonConfigurationInstance, to correct instance binding in case
of a configuration change… )I have yet to figure out how to detect the
process recycling case from configuration change before hand as there
is no defined order between onSaveInstanceState /
onRetainNonConfigurationInstance and I don't want to persist the
information at every configuration change). However, after watching
Virgil's google i/o presentation "Building Rest Clients" it seems
there is a recommended approach, to perform all the network calls
through a service which is supposed to decrease the likelihood of the
process from being killed. However, after further investigating this
it seems like a major overkill with many pitfalls of its own as it
requires implementing a 2 way messaging protocol possible involving
queuing messages, with no obvious way on how to handle callbacks in a
way that address the concerns outlined above, so I prefer to avoid
this risk unless absolutely required.

So to sum up, I would really appreciate a professionals feedback on
this, as I'm kind of new to android... so if you feel like sharing an
*explanation* on how to do this right, great!!

If you feel this is work and you can provide a concise and *correct*
sample that address all the concerns above, I would be happy
compensate, pypal works :)

-Itai

e: itaitai2003 … the at sign … yahoo .. com

p. s

No need for actual communication/imaging or persistence logic, simple
mock will do, without Fragments – I don't use them, I don't need them
in my project.

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