I have a service, I need to communicate to with it (one service - many fragments/activities). There are two options for this:
Have a singleton that controls the service - starts it and then binds to it (using the app context)
Have a singleton that controls the service - starts it, the service when ready registers back as a delegate to the singleton (in a WeakReference)
Solution no 2 seems simpler to me, but whenever I read about communication with services there is the concept of the bound service.
Is there any benefit of having a bound service instead of the service registering itself as a delegate (and unregistering with onDestroy)?
Edit 1: The service is to keep the communication alive, it's expensive to set up a new communication channel. Even if no one requested any data it should keep the channel alive (heartbeat).
The service is foreground, it should run even if the activity that requested the data gets killed by the system. The next time it is created the data will be there.
The data requested by one screen might be useful for some other (therefore has to be stored in a singleton).
Bound and unbound services are both usable patterns and you should pick whatever pattern is better for you use case.
You should pick bound service if you want your service to have the same lifecycle as the components that bind to it. If you need an independent service use an unbound version.
The only benefit of one approach versus another is the simplicity of implementation.
In you case, I think you need the service only while there are running activities and fragments, then the easiest way, in my opinion, would be to make a bound service and make every activity bind to it. With that, you'll get a simple communication interface between you activities (and fragments, since they have access to containing activity) and your service.
The benefits of this approach are:
the service will stop itself if all activities unbind and start itself when first activity binds to it.
you won't need to track all running activities in the singleton and manually unbind
you won't need to maintain a singleton manager, less code -> less bugs
sometimes onDestroy can be skipped by the system and you can leak the service with the 2 approach.
Since you need your service to be running the correct option will be to use a started service and make each activity bind to it when needed. It's a common pattern.
Started service will run until you explicitly stop it or it stops itself, you can have a singleton manager that will be responsible for that.
But at the same time you can communicate with the service from your activity using binding.
So basically comparing with the first suggested approach, you'll need some instance that will start and stop the service, but the communication between activities and service will be the same - using binding.
Yes, using a bound service in Android is a much better option when communicating with Views like Activities/Fragments. This is because of the following reasons,
It runs synchronously.
You can have more control on the service data when to show on UI thread of the view. You can choose when to call it in async/sync way.
LocalBroadcastManager only runs Asynchronously.
Related
I am curious to know why would you use Bound Service for 2 way interaction between Activity and Service when you can do the same kind of interaction with Started Service using local broadcasts and receivers defined in both Activity and Service
It will be helpful to know the pros and cons of each implementation.
I couldn't find any clear answer to this anywhere.
Using a bound Service is much more flexible. You can define methods on the Service (using AIDL) that return immediate results (synchronously), which you cannot do using LocalBroadcastManager. Using LocalBroadcastManager requires that you use your Service in a completely asynchronous way. This decouples the initiation of a service action with the return of the results (callback) which can make your code more complicated and more difficult to understand. Asynchronous use has some benefits, and there are places where you should use it, but if you have a bound Service you can choose exactly when to use synchronous calls and when to use asynchronous callbacks.
Also, the use of AIDL allows you to exactly describe the signatures of the services's method calls. If you use startService(Intent), you cannot guarantee that the caller will provide the correct arguments in the passed Intent, so you need to rely on the caller to "do the right thing" and/or you need to add a lot of additional argument verification.
Don't forget the comment from #CommonsWare about how LocalBroadcastManager only works if the Service is running in the same OS process as the rest of your app (which makes it unsuitable for programming things like system services, which are not running in your OS process).
A service is a component that runs in the background. It works to perform long-running operations without needing to interact with the user For example, a service might retrieve data over the network without stopping or blocking user interaction with an activity of an app or user may play music in the background while the user is in a different application. A service can be of two types or states:
Started: Once started, a service can run in the background indefinitely, even if the component which is started, is destroyed. A service is started when an app module or component, such as an activity, starts it by calling startService(). For example, retrieving contacts from phone book using service when Splash Screen starts.
Bound: A bound service offers a client-server interface that allows components to interact with the service, get results, send requests and even do so across processes with interprocess communication (IPC). A bound service is bound, when an app component binds to it by calling bindService().
I am trying to dive deep into service architecture.
Just for testing purpose I am creating music app.
Without doubts music should be played in service, but what kind of communication to use ?
As far as I know service can be bound or started (or both at the same time)
Firstly, I need to play/stop/pause/set source...
Secondly, I need to notify UI if music player is opened about events like progress, buffering...
Here my thoughts about this.
I think about mixing bound and started service.
As far as song can be set only from UI as well as paused/played/stopped/seek I choose communication with service from UI using binder. (Bound service inside activity and get binder back to communicate with service)
Notifications like track completed, next track , current track position coming from the service. I decided to use Broadcast receiver to send such notifications, because it can be multiple interested components.
So my question are
What is the best choice of communication (bound or broadcast) for music player service ?
How does System knows what kind of service is ? I mean that even if the service is bound I need to call startService method at first and than bind it. Does it mean that if even one bindService method was called it is considered as bound service and it would be killed when unbind method is called ?
Does LocalBroadcastManager make sense in communication with service ? As far as LocalBroadcastManager is local for each app, why not to use bound service than ? Global broadcast makes sense in case other apps are interested in events.
Please help to understand this mechanism.
Best choice of communication is using both LocalBroadcasts as well as Binder methods as per your requirement. If you want to do something in service from your bound component like activity then you should use binders. If you need to send result back to application you should use LocalBroadcast.
There is one more option available to use messaging. In this case both activity and service use same ibinder hence two way communication is easy. You can send message from activity to service and service to your activity
In this point you are wrong about starting bound service. You don't need to call startservice in this case. Just a call of bindservice method is required.
bindService(new Intent(this, MessengerService.class), mConnection,
Context.BIND_AUTO_CREATE);
you can bind multiple components to this service. When all of them are unbound then service will be stopped.
Yes LocalBroadcastmanagers make sense. Lets say you want to broadcast something in your app. There are 4-5 components that you want to update.How will you achieve this. Hence the use of localbroadcasts is a good feature.
For example lets say there are two services starting from different activities and second service starts its work when first service has done its work. So, only way to achieve this is send a broadcast and second service will be registering for this broadcast. Hence it will receive it.
Yeah. Services are pretty hard to understand.
There are some things that are easy. A bound service is always started with a bindService method. A started service is always started with a startService method. You do not need to start a bound service, or bind a started service.
A lot of what follows is gross generalization...
Started Service
A started service is nearly useless, unless it is an IntentService. IntentServices are pretty good tools for running asynchronous tasks. You send them a small bundle of parameters and they go off and do whatever those parameters indicate. They are like void methods in that one expects to use their side-effects, not a return value.
Bound Service
A bound service is harder to explain. Although the metaphor breaks down on careful examination, a bound service something like a singleton factory. It is, for instance, a way of providing a single object, with is single state, to all of the Activities in a application. Among its interesting features is that, as long as the service that provided the singleton object is bound, the hosting process is less likely to be killed off. Note that, the "singleton" object that the bound service provides has little to do with the service that provides it. Unbinding the service does not invalidate it.
Bound services are also the main means of doing inter-process communication, in Android.
What should you do?
Well, that's a pretty general question. Here's a thought. Putting your music player in a service makes a lot of sense. If the communication to it is mostly one way -- commands to the service -- there is a chance that you can do it with an IntentService. Until there's a specific reason to do something more complex, an IntentService has the advantage of being simpler.
I am new to android development and having trouble understanding how I should use service's and more specifically which kind. I am developing an simple system that only will do to things. One of those to is to continuously ask a server simple telnet questions. The answer to these questions should be prompted on the screen.
So to simplify my question. What kind of service should I prefer? bound, intent service etc?
I presume it has to run a own thread since it is suppose to do network comm, so how should I do that.
Finally and most importantly, how do I supply/give the MainActivity the information the service has gathered and post it on the screen?
What kind of service should I prefere? bound, intentservice etc?
A bound service runs only as long as another application component is bound to it. With other words, if an activity bounds to that service, and later that activity gets finished, the service also gets destroyed.
So, first decide the behaviour of the service you want to have. Do you want it to be destroyed when the activity bound to it gets destroyed? If yes, then perhaps a bound service is the right thing to do, if not then use a started service instead which can run in the background indefinitely, even if the component that started it is destroyed.
I presume it has to run a own thread since it is suppose to do network
comm, so how should I do that.
Yes, you are right. You can use the Service class and create a thread inside it that will do the heavy work, or, you could simplify things by using an IntentService which provides its own worker thread.
Finally and most importantly, how do I supply/give the MainActivity
the information the service has gathered?
If you decide to go with a bound Service, then you'll be able to communicate with the service through a so called binder object. On the other hand if you go with IntentService then you could use a ResultReceiver, or a BroadcastReceiver to send the result back.
Suggested readings:
http://developer.android.com/guide/components/services.html
http://developer.android.com/guide/components/bound-services.html
Here is a quick summary on services in Android, hopefully this will help in deciding what approach to go. Reading Android Services is highly recommended
Android Service
"A Service is an application component that can perform long-running
operations in the background and does not provide a user interface"
runs on the main (UI) application thread.
it is given preference over activities in termination when resources
are low
it is not intended to interact directly with an activity/fragment (an activity may be destroyed at any time) no simple callback ability due to above... however there are some ways to address this using Intents, Handlers and Messages
an activity can be bound to a service which
basically gives an instance of the service to call methods, the methods will run on the main thread, a suggested way to use a separate thread, is to use Executors
I want to use a started (foreground) service to manage a network connection that should persist when the user leaves the application for a short time, and that the user should be aware of (so he can return to the app and maybe disconnect). This service will only ever be used locally by activities in the same process.
Maybe it's just because I am new to Android, but I find it unnecessarily difficult to bind to this service in every activity that uses it - in particular, the asynchronous nature of binding, which only really seems to be necessary for accessing services in a different process. Is there any indication against just accessing the started service through a static variable instead?
Maybe I'm understanding your question wrong, but there is no need to bind to the started Service from every Activity. Instead, you could simply start the Service from wherever you need to interact with it. This calls the onStartCommand() if the Service is already started. You could include an extra with the Intent that starts the Service to distinguish between the first start and subsequent ones.
Of course - this addresses the use case where you do not want to have a client-server mode of interaction between your activities and the Service - that scenario requires binding and if you really need binding, then you need to bind from every component that needs to be served by the Service.
I'm creating an application that connects to an XMPP server on Android. I want to keep the connection on till the user logs out.
Should I use a regular Service or a Bound Service to keep the connection on?
Any tips, advice and helpful information are welcomed.
I like this explanation:
Started services are easy to program for simple one way interactions
from an activity to a service, however, they require more complex and
ad hoc programming for extended two-way conversations with their
clients.
In contrast, bound services may be a better choice for more
complex two-way interactions between activities and services. For
example, they support two-way conversations.
So, as you said, If you want to interact with the service use bound service. With started services (or intent services) you could do it, only it would require more complex programming.
(by Douglas Schmidt: https://www.youtube.com/watch?v=cRFw7xaZ_Mg (11'10'')):
Here is a summary that helped me understand (thanks Doug):
Finally, one last link that helped me also:
http://www.techotopia.com/index.php/An_Overview_of_Android_Started_and_Bound_Services
Started services are launched by other application components (such as an activity or even a broadcast receiver) and potentially run indefinitely in the background until the service is stopped, or is destroyed by the Android runtime system in order to free up resources. A service will continue to run if the application that started it is no longer in the foreground, and even in the event that the component that originally started the service is destroyed
A bound service is similar to a started service with the exception that a started service does not generally return results or permit interaction with the component that launched it. A bound service, on the other hand, allows the launching component to interact with, and receive results from, the service.
A bound service is the server in a client-server interface. A bound service allows components (such as activities) to bind to the service, send requests, receive responses, and even perform interprocess communication (IPC). A bound service typically lives only while it serves another application component and does not run in the background indefinitely.
If all the code exists in one activity from user connected to user logout then go for bound service
But if it is code exists in multiple activities try with service
I found out the difference between the two and when to use it. If you want to interact with the service (for example send arguments etc), use bound service and it return the service object in the onServiceConnected method (where you can call methods in the service). You cannot interact with a regular service (from another class)