65. An Overview of Android Services

The Android Service class is designed specifically to allow applications to initiate and perform background tasks. Unlike broadcast receivers, which are intended to perform a task quickly and then exit, services are designed to perform tasks that take a long time to complete (such as downloading a file over an internet connection or streaming music to the user) but do not require a user interface.

In this chapter, an overview of the different types of services available will be covered, including started services, bound services and intent services. Once these basics have been covered, subsequent chapters will work through a number of examples of services in action.

65.1 Started 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.

By default, a service will run within the same main thread as the application process from which it was launched (referred to as a local service). It is important, therefore, that any CPU intensive tasks be performed in a new thread within the service. Instructing a service to run within a separate process (and therefore known as a remote service) requires a configuration change within the manifest file.

Unless a service is specifically configured to be private (once again via a setting in the manifest...

65.2 Intent Service

As previously outlined, services run by default within the same main thread as the component from which they are launched. As such, any CPU intensive tasks that need to be performed by the service should take place within a new thread, thereby avoiding impacting the performance of the calling application.

The JobIntentService class is a convenience class (subclassed from the Service class) that sets up a worker thread for handling background tasks and handles each request in an asynchronous manner. Once the service has handled all queued requests, it simply exits. All that is required when using the JobIntentService class is that the onHandleWork() method be implemented containing the code to be executed for each request.

For services that do not require synchronous processing of requests, JobIntentService is the recommended option. Services requiring synchronous handling of requests will, however, need to subclass from the Service class and manually implement...

65.3 Bound Service

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. Through the implementation of interprocess communication (IPC), this interaction can also take place across process boundaries. An activity might, for example, start a service to handle audio playback. The activity will, in all probability, include a user interface providing controls to the user for the purpose of pausing playback or skipping to the next track. Similarly, the service will quite likely need to communicate information to the calling activity to indicate that the current audio track has completed and to provide details of the next track that is about to start playing.

A component (also referred to in this context as a client) starts and binds to...

65.4 The Anatomy of a Service

A service must, as has already been mentioned, be created as a subclass of the Android Service class (more specifically android.app.Service) or a sub-class thereof (such as android.app.IntentService). As part of the subclassing procedure, one or more of the following superclass callback methods must be overridden, depending on the exact nature of the service being created:

•onStartCommand() – This is the method that is called when the service is started by another component via a call to the startService() method. This method does not need to be implemented for bound services.

•onBind() – Called when a component binds to the service via a call to the bindService() method. When implementing a bound service, this method must return an IBinder object facilitating communication with the client. In the case of started services, this method must be implemented to return a NULL value.

•onCreate() – Intended...

65.5 Controlling Destroyed Service Restart Options

The onStartCommand() callback method is required to return an integer value to define what should happen with regard to the service in the event that it is destroyed by the Android runtime system. Possible return values for these methods are as follows:

•START_NOT_STICKY – Indicates to the system that the service should not be restarted in the event that it is destroyed unless there are pending intents awaiting delivery.

•START_STICKY – Indicates that the service should be restarted as soon as possible after it has been destroyed if the destruction occurred after the onStartCommand() method returned. In the event that no pending intents are waiting to be delivered, the onStartCommand() callback method is called with a NULL intent value. The intent being processed at the time that the service was destroyed is discarded.

•START_REDELIVER_INTENT – Indicates that, if the service was destroyed...

65.6 Declaring a Service in the Manifest File

In order for a service to be useable, it must first be declared within a manifest file. This involves embedding an appropriately configured <service> element into an existing <application> entry. At a minimum, the <service> element must contain a property declaring the class name of the service as illustrated in the following XML fragment:

.

.

    <application

        android:icon="@mipmap/ic_launcher"

        android:label="@string/app_name" >

        <activity

            android:label="@string/app_name"

            android:name=".MainActivity" >

      &...

65.7 Starting a Service Running on System Startup

Given the background nature of services, it is not uncommon for a service to need to be started when an Android-based system first boots up. This can be achieved by creating a broadcast receiver with an intent filter configured to listen for the system android.intent.action.BOOT_COMPLETED intent. When such an intent is detected, the broadcast receiver would simply invoke the necessary service and then return. Note that, in order to function, such a broadcast receiver will need to request the android.permission.RECEIVE_BOOT_COMPLETED permission.

65.8 Summary

Android services are a powerful mechanism that allows applications to perform tasks in the background. A service, once launched, will continue to run regardless of whether the calling application is the foreground task or not, and even in the event that the component that initiated the service is destroyed.

Services are subclassed from the Android Service class and fall into the category of either started services or bound services. Started services run until they are stopped or destroyed and do not inherently provide a mechanism for interaction or data exchange with other components. Bound services, on the other hand, provide a communication interface to other client components and generally run until the last client unbinds from the service.

By default, services run locally within the same process and main thread as the calling application. A new thread should, therefore, be created within the service for the purpose of handling CPU intensive tasks. Remote services...