I have a service that downloads some data from internet and periodically sends progress to the indicator activity. In the end of processing service sends a result.
I have a question what is the best way to achieve persistence of the communication.
Messenger or ResultReceiver, I need to parcel them into Intent and store list of listeners in the service. But on configuration change activity destroys, and it's hard to maintain this list.
LocalBroadcastManager, I need to migrate from Messages to Intents, and also there is no sticky send in this class. So if I get result while my progress activity is in background result will be lost.
BroadcastManager is good, but I don't need to broadcast my progress system wide, and security issues.
Any ideas?
You may want to give Otto (http://square.github.io/otto/) a try.
In your service, whenever you want to communicate with the activity, post a new event using a shared Bus. You should do that on main thread with a handler or main looper since you are probably using IntentService. The service may act as a producer as well. When your activity is recreated, current known value will be posted.
Your activity just needs to register with the Bus and subscribes to the right event. When it is paused, just unregister with the Bus.
I belive the best way to achieve that persistance is:
After the service downloads, you should save your data in a database or a file.
The service then sends broadcast to update.
If the activity is "alive" all goes well and it goes to the database/file to get the updated content.
If the activity was killed or something, you just have to make sure the data is in the database/file so that when you start/restart the activity you can get the latest content from database/file.
While downloading keep a state and progress saved in the db/file the same way.
Check this Google I/O Session, it explains this really good.
Use static variables inside your Application class (extends Application). Inside Service you set this variables. Inside Activity you read periodically this variables.
You should use massenger to send download progress, because it is more secure and less expensive method then broadcast receiver.
Related
Am having an activity that will start service, service will download something from internet and i have to update activity UI.
but consider below cases
If my activity is alive i have to update activity UI without broadcast receiver, What is the best way to do this.?
If my activity is not alive and when it comes alive/foreground then i have to update UI with service result, How to do this.? (without saving into DB)
Let me drop my suggestions point by point,
Yes you can achieve it by EventBus (or) interface. Coming to eventbus, it is similar to broad cast receiver.
When the activity is not alive, you are fetching the data and when coming to alive you had to update the UI. In order to do that somehow you had to save the value somewhere either internally or externally (OR) you had to fetch the values from the internet whenever the activity comes to alive.
Lets say I have an activity that has a data object. It updates its gui component based on that object. Now lets say that it is paused ( OnPause is called) but not stopped or destoryed
Meanwhile , a push notification is received ( intentservice is started) and I need to update that object with the push notification object so I gui is updated when the app is resumed.
I thought about sending a broadcast to the activity so it can update its dataobject But I read somewhere that when activity is paused then broadcasts are not received.
What should I do in this case?
The hard but correct way to do this is to build your own custom ContentProvider for your app and update all data received from web services and push notifications to this ContentProvider. When the Activity comes back into the foreground, it updates itself with the new data provided by the ContentProvider.
It is hard because making a custom ContentProvider is a lot of work. It is the correct way because it is in conformance with the behavior of mobile applications and with the Android architecture: say a user activates a web-service or some computation-intensive task, and then dismisses the app; or say a push notification arrives and requires the app's data to be updated and displayed. In both cases, the app's Activitys may no longer be in the foreground, but a Service can be used to perform some non-UI operation. Now at the end of that operation, the Service makes changes to the data through the ContentProvider, and when the user activates the app again, the Activitys get their new data from the ContentProvider.
To quote the official tutorial:
Content providers are the standard interface that connects data in one
process with code running in another process.
As a developer, you should always assume that the user may invoke an app at any time and dismiss it at any time. Irrespective of whether an Activity is in the foreground or not, the app's data needs to be correctly updated and maintained.
Google's own apps use custom ContentProviders. The Gmail app in particular makes use of its ContentProvider to get new emails when network connectivity is available and display emails offline. The Facebook, WhatsApp & Twitter Android apps also make use of ContentProviders.
You pretty much treat it as if the Activity was destroyed. You can't really assume that it won't happen once onPause is called.
If the object represents something that's persistent, then simply update the persistent portion of the object and retrieve it when the Activity resumes. If it represents a state of a Service for example, then bind to the service and update. If it's a database, then update the database then refresh the Activity onResume.
If it's a temporary object that's only valid through the life of an Activity, then you need to make something persistent that the Activity can check when it resumes. Something simple like a boolean variable in the "sharepreferences". In onResume, check for the object, if it exists, then retrieve object, then clear the object.
I thought about sending a broadcast to the activity so it can update
its dataobject But I read somewhere that when activity is paused then
broadcasts are not received.
What should I do in this case?
I think the resource of that information is incorrect, if you declare a receiver in onCreate and remove it at onDestroy you wont have any problems. What I mean is, first parse the push notification with your service and then send localBroadcast to the activity. It is as simple as you thought. then update your GUI at onRecieve method of your reciever.
You should bind to the Service when the Activity starts. Then in onResume you can request updates that may have occurred while the activity was paused.
You may consider passing data to the activity through a callback to update variables in the activity while it is paused, but then you will need to update the UI in onResume.
Also consider that when your Activity is paused, it may get destroyed and re-created. You don't seem concerned with that scenario based on your question, but I thought it important to explain it. to handle any changes for this scenario, you would need to persist the data for the change. If you are persisting the data, then you don't need to bind to the Service you simply need to check the persistent data store in onResume.
EDIT:
In your comment you mention an IntentService which you cannot "bind" to. You have several options in this case (basically listed in order of preference). First, you can create a Service when your activity is created, then have the IntentService forward intents to that service. Second, you can have a Static variable in your Activity to allow the IntentService to access a shared data store. As mentioned previously, you can also persist the data (put in SharedPreferences, a file or a database. Last, you can use the Application class to store references to the data so that while your app is active, you can pass data.
I'm developing an app with a service that forwards calls to a web-service, and a few activities that place those calls. The activities need to process the results of those calls. For example, I have a writeComment method on the service, that accesses the web-service and returns some information about the newly written comment.
Right now I let the Activity take care of all the threading. The Activity binds the service, and then uses an AsyncTask that calls the bound service's writeComment method.
All works well as long as the Activity isn't stopped while the AsyncTask is running. If it does (easily happens when flipping the phone), the AsyncTask dies a violent death when trying to update the UI in onPostExecute. I'm not entirely sure how to fix this - I do need to let the user know the server has been updated.
If I go the other way around, and register a callback with the Service, I'm still a bit stump, because I need to notify the Service the Activity has changed - I need to tell it not to notify me in the first Activity's onDestory, and reregister in the second Activity's onCreate. And I need to handle the case where the asynchronous task completes after onDestroy and before onCreate.
What is considered Best Practice in this case?
Thanks,
Itay.
My intuition tells me to let the service handle the threading. Services are far less transient (although still transient to some degree) than activities and therefore you'll have less issues of threads trying to interact with a Context (be it an Activity or a Service) that's no longer there. Have you looked at the IntentService class? It handles a lot of the threading for you.
In my app, I have a long-running service and Activities that need to render data in the service. The service also pings the Activities when there is a change but the Activity can also query the service. The way I approached this was two-fold.
Firstly, I bind my activity to the Service in order to send messages from Activity to service.
Secondly, the Service sends notifications with Broadcasts and the Activity listens for those broadcasts. I set that up in the Activity onResume and tear it down in the onPause. I think this is the part that you're missing.
I have a service running in the background. Based on some condition it has to start some activity. Activity has to send back the response.
I did google search and found out we have to use Notification mechanism. But I am not clear how to send the response back from activity to the service running.
Also service is collecting sensor data(acclerometer, gps). So should activity be started in separate thread so that collecting sensor data is not affected.
Please clarify.
Activity would be started in main UI thread, instead, your long-running service should run in and manage its own thread, since according to the document, service is also created in main thread.
Basically the best way to communicate is to use Intent. This allows loose couple of sender/receiver (i.e., activity/service in your case). Intent is a large topic in terms of android, and yet it is one of the most fundamental one, I think you should look for tutorials online about it.
If your activity is opened and return the result after completing the task of Activity then you can use startActivityForResult and then return the result.
Using of notification is simple, just create a interface class, and implement the class in your service. When you need to send data back to service, you can just call the appropriate method with data.
I have the following framework for my application:
1. a Network thread that runs in the background (a queue) for issuing request and get async responses. The thread is started and stopped in the Application Object so it's leaving through out the whole application.
2. a DataManager which is also a member of Application and has different DataManagers for the data types i retrieve from the network. the data manager itself is the listener for the responses from the network so it's safe until the application itself dies.
3. this is the problematic part. Some of my Adapters and part of my Activities are DataListeners for my DataManagers, that means that the data manager keeps a reference to them.
When a phone call or some other phone event occurs i've noticed that the activity is usually in paused and not destroyed and so receives my events, which is ok. the problem starts when landscape\portrait is changed. since i keep a referenced to the activity in an Application bound object, the activity can't be destroyed on one hand, BUT the event is still getting to the listener, only the wrong one...
Basically i can fix that issue by removing the listener in onDestroy and retaining configuration boolean to tell me that request was allready issues and i just need to put a listener and try to retrieve the data from the data manager.
However :-) i was wondering how android handles this cases usually, if for example this was a Service running. or if the Service is a local Service that used Bound and passed on the Activity as a Listener to the network Event, the same things happen, untill the listener is not removed the Activity is leaked and lives on, but without it, no way to get callbacks from the network...
an Intent requires serilaztion and deserilazation of data which can be heavy (Bitmaps for example?)
And anywa, asuming i send an intent on each respose i get, how do i get the intent to the Activity (i know of getIntent, but if i get another one , not related, do i get it as an 'event' ?)
From what I gather it's customary on Android to remove yourself from listener lists when the activity is destroyed. It's kinda error-prone, but I think it's the generally accepted way to do it.
You could imagine your service accepts only one listener, which may or may not fit your case, and when the activity restarts its registering with the DataManager would overwrite the old activity which would in turn be garbage collected. The drawback is, you don't free the activity memory if it is destroyed but the service lives on, so it's probably better to just remove the activity from listeners.
Android development is rather different from other platforms (e.g. BlackBerry). I'm not able to give you a quick silver bullet solution, however here are my thoughts on this:
Some of my Adapters and part of my
Activities are DataListeners for my
DataManagers, that means that the data
manager keeps a reference to them.
OS kills Activities according to their lifecycle. So you should avoid keeping a handle to an Activity in another object which is supposed to live after the Activity is destroyed by OS. Otherwise you'll get memory leak.
Also keep in mind Application sublass instance does not always live for the whole application session (a session from a user perspective). If your app goes in the background, for example, due to an incoming phone call, then your entire process can be killed. See details here. As soon as you Application sublass contains some state which is not persisted if process is killed you may mistakenly expect your handles to point to some non-null entities. However after going to foreground (and process restore) those may just be nulls because a new instance of Application sublass has been created by OS.
Ok, so let me describe the problem and the solution i found in more details.
The problem:
I have a Service\Network Thread that needs to notify Activities that sent requests through it that either request or Error has arrived in an Async way. Using Listener Pattern requires me to set listener before or when i send a request like so:
mNetService.setRequest(request, this);
where this is Activity that implements my listener Interface.
But doing it this way requires me to remove the listener from the service in onDestroy and returning the listener, if i ever sent a request back in onCreate\onResume, but the response can also arrive exactly when the activity is not listening (landscape\portrait event) which requires me to keep the Error\Response in the service until some1 picks it up and resets it.
The solution i found:
using Broadcasts and BroadcastReciever.
this is only part of the solution but it let you have a listener to broadcasts (that can be specific for a certain class type meaning Activity) and action.
Since all of my Activities inherit a base Activity class i've made they all have a BroadcaseReciever inner class that listens on certain action in it's filter.
is i enable the listening in the C'tor of my Activity the listener will be registered in onResume and deregister in onPause.
If the listener gets onRecieved event it will call a method in the Activity (which i can override in my specific activty) and pass it the Intent i got which can contain all the data from the response.
The only missing part is what happens if the Activity dies for a second and only then the broadcast arrives ? ah, that's a problem, so android intorduces Sticky Broadcasts that stays there untill you remove them with removeStickyBroadcast(Intent), so when is ent broadcast from my service i send Sticky broadcast, when the Activity gets my Broadcast it removes it so it wont stay around and mislead the activity about new response that arrived.
The only problem with it is if i send a request, don't wait for the response and goes to the next Activity right away, in this case when i'll go back to that Activity it will think it got the response. Didn't find a proper solution to that just yet. But it's better then my previous solution.