I want to build a "smart downloader" that can download images in background (using AsyncTask).
There are two Activity, A and B. In activity A, I can choose the image list I want to view, (first download if not have been download once). Then enter activity B, here I launch a asynctask to download the image in background and in the UI thread, show the image to the user.
there are some large list that may take minutes to finish downloading. One tricky problem is when the asynctask is still downloading the list, the user may exit activity B (come back to activity A. (I know the asyntask will still continue working even if activity B is destroyed).
But if at this time user choose another list to view (then enter activity B). I want to stop the previous task for a while, began to downloading the new list first, and then the old list.
My thoughts to do that is retrieve the previous asynctask and modify the downloading order. But I don't know how to retrieve the asynctask, I have search some questions about recreate asynctask, but they are all about after re-configuration (like rotate the screen). is there a way to retrieve the background working asynctask, after i destroy the activity and recreate it.
Thanks!
Once you destroy an Activity, the AsyncTask is gone. For this reason, AsyncTasks are not the optimum solution if you want to avoid running the background operation from scratch.
I think you should structure your app to download images using an IntentService. In activity A, choose the image list. Once it's chosen, fire off an IntentService to download the images. Once the IntentService is done, it can notify the user that the operation is complete or send a local broadcast message back to the activity. In either case, even if the user has navigated away from the activity, the work hasn't disappeared. If the activity was destroyed, you can figure out if the download is done, and if so, display the images. If it's not done, you can put up an activity indicator.
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I have 2 Activities.
First one is a list view and when i choose one, detail information Activity is opened.
Second one requires complicated loading process along with ViewPager.
The problem is that there are too many data to load & progress(includes Google Map data) and it makes my beginning animation's FPS terribly laggy, which is not good at all.
I decided to wait in the first Activity screen with central progress circle until second one is fully loaded.
How can I request Android to 'pre-parse' another Activity before being displayed?
I think the best way is after clicking on an item in the listview (first activity) start an asynctask to fetch your data. Then this task is complete, start your second activity passing a bundle with the data previouly fetched
The question is more conceptual than coding-related.
I have an app with Activity A and B.
I call an AsyncTask from Activity A, while the call is being made, I don't want to block the user showing the progressdialog, so my user can freely move around the application without getting bored of waiting.
Now, the query is AsyncTask or lets say a Service is being called from Activity A which is responsible for downloading some kind of data from the server. While the call is being made, the user has changed the Activity and Activity A has gone to background.
Now, while the user is freely moving around the application, he just wants to come back to Activity A to check the download status,(which is something lets say I set some data to my TextView).
Now the question is, once the download is over , while my Activity A is still in background, my UI should be updated while Activity is still in background. This way the user feels he gets the data before he switches to Activity A.
Is this possible, if so how?
Summarizing my question, can I update the UI of an Activity while it is still in background with the Asynctask or Service which the Activity invoked to fetch data from server.
One post suggested that ideally I need to update the **UI in onResume(). My question is, is this the only approach?
once the download is over , while my Activity A is still in background, my UI should be updated while Activity is still in background. This way the user feels he gets the data before he switches to Activity A.
It isn't possible. You see, the Activity has to pass through the onCreate() and onResume() callbacks for the UI to be changed.
Also, you should be using a bound Service to update the Activity when it returns to the foreground.
onResume() would be the best approach. You may save the changes in a SharedPreferenes or Pass the data using Intent and show the changes before the UI is visible.
Another approach would be running a service and checking if the activity is visible. If its visible immediately update the UI or wait until user visits the activity. To check if the activity is currently visible see here,
How to check if activity is in foreground or in visible background?
I've got the following problem. In my app I'm loading data in an AsyncTask. The problem is, when the user now clicks on the icon to open the Navigation Drawer and opens up another fragment the app crashes. When the AsyncTask is finished the app doesn't crash. The problem that is encountering is, that when I switch the fragment (The fragments are always the same, just with another content dependent on the NavigationDrawer Item click) the app crashes.
I guess the problem is, that the async task isn't finished, I'm calling the same fragment again want to display different data.
So what would be my approach to handle this? Use for every different view a different fragment? I thought using the same fragment every time is much easier, since it's just displaying different data but the structure, layout etc. is all the same. Just the data that it gets is different.
I also thought about somehow "blocking" the user from doing any other actions while the asynctask but still show him that the app is processing. But that would be not the definition of an AsyncTask.
How would you approach it? Use different fragments for every different display? Or how? Block somehow? If a user clicks on an item of the navigation drawer the asynctask needs to stop all its actions (if some are done) and then restart doing all the actions. Is there a way to do it?
Please note that the fragment where the async is executed and the activity where the fragments are called are in two different files
You can either block the screen with a loading screen (not that good UX wise...) or you could cancel the asynctask when you change the fragment, in the destroy or detach method.
You didnt show the errors, but I would guess that the app crashes because you are trying to acess something in the asynctask onPostExecute method and it is no longer available...
I guess that it crashes because your AsyncTask is sending data to a class instance that doens't exist.You should change the Class that receives callbacks from asynctask. Anyway i can't give you a better answer till i will see your real code of AsyncTask ( at least onPostExecute() and onProgressUpdate())
use intent service to do that ask task means call ask task in a intent service that one is capable to handle background task without hang UI
I have 2 activity.
Activiy A will list links to download.
Activity B has a listview of download item.
When I click the link in activity A, how to send the link to activity B to download without change activity A (while activity B still downloading on background) ?
You need to understand that Activities Dont need any result to work, activities need data. And Activities can use these data to load contents of its. And there is no use of this data into activity untill its into background. SO Here is solution:
Create some Data ArrayList or Flag, Global to the application.
Changes this Data into Second activity, which is in forground, and want to notify about some result.
When First Activity which is in background, and want to listen results, check for changes into data in onResume method, and on change load new contents.
My approach would be:
Activity A: Shows a list of items to download.
Service B: Downloads the item in the background. Maybe you can queue
multiple download items.
File C: When you finish downloading something you should save that
fact in a file.
Activity D: When this activity starts it should read 'File C' and
display its contents.
You can't really send data to an Activity without starting it. When an Activity is not visible it is pretty much asleep.
You should read this: Application Fundamentals, read it many many times ... then read it again. I still go back and read it after years of working with Android.
I've got an app that uses ListActivity to give users a list of actions. When they click one I use an Intent to launch a separate activity.
My problem is that the actions that the app performs take about 20 seconds to finish, and since I don't want the user to receive that nasty ANR dialog, I tried to use AsyncTask to present them with a loading screen in the mean time. I tried using setContentView(R.layout.loading); on onPreExecute(), but it throws a NullPointerException which as far as I have figured out is due to the fact that loading.xml is not "a ListView whose ID is android.R.id.list".
So what can I do now? How can I show that loading screen? Is there a way around this pretty annoying situation? Any help would be greatly appreciated. Thanks!
I am not sure exactly what your use case is; you have a list of items that are populated immediately, and upon selecting one an action is taken? The action that is taken is to launch another Activity which performs background processing?
Or does it take that long to populate the list of actions?
If the former, you can use an AsyncTask for the long-running activity instead of an Intent to launch another Activity: in the callback you get for the click on the item in question, you would create the AsyncTask, and in doInBackground you would perform the long-running activity, with onPostExecute refreshing or manipulating your list as necessary.
Another thing to consider is using a dialog box to show a loading screen, if the loading is required to happen before you launch a new Activity.
If you can further describe your use case, I can help you more.
It's not the loading screen you need to have on the AsyncTask, it's that 20-second Activity initialization. I would look for a way to do all the setup in a background thread in a Service while the user is free to merrily bop around in other Activities. I'd try hard to find a way not to just stall the user for 20 seconds. Maybe take them to the target Activity and show them data cached from their last visit until the new set is ready.
Fire up and display your loading dialogs in your onCreate() of the Activity being called, then call Dialog.dismiss() in your AsyncTask's onPostExecute().