Android Local Bound Services – A Kotlin Example

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As outlined in some detail in the previous chapters, bound services, unlike started services, provide a mechanism for implementing communication between an Android service and one or more client components. The objective of this chapter is to build on the overview of bound services provided in “An Overview of Android Started and Bound Services” before embarking on an example implementation of a local bound service in action.




Understanding Bound Services

In common with started services, bound services are provided to allow applications to perform tasks in the background. Unlike started services, however, multiple client components may bind to a bound service and, once bound, interact with that service using a variety of different mechanisms.

Bound services are created as sub-classes of the Android Service class and must, at a minimum, implement the onBind() method. Client components bind to a service via a call to the bindService() method. The first bind request to a bound service will result in a call to that service’s onBind() method (subsequent bind requests do not trigger an onBind() call). Clients wishing to bind to a service must also implement a ServiceConnection subclass containing onServiceConnected() and onServiceDisconnected() methods which will be called once the client-server connection has been established or disconnected, respectively. In the case of the onServiceConnected() method, this will be passed an IBinder object containing the information needed by the client to interact with the service.

Bound Service Interaction Options

There are two recommended mechanisms for implementing interaction between client components and a bound service. In the event that the bound service is local and private to the same application as the client component (in other words it runs within the same process and is not available to components in other applications), the recommended method is to create a subclass of the Binder class and extend it to provide an interface to the service. An instance of this Binder object is then returned by the onBind() method and subsequently used by the client component to directly access methods and data held within the service.

In situations where the bound service is not local to the application (in other words, it is running in a different process from the client component), interaction is best achieved using a Messenger/Handler implementation.

In the remainder of this chapter, an example will be created with the aim of demonstrating the steps involved in creating, starting and interacting with a local, private bound service.


An Android Studio Local Bound Service Example

The example application created in the remainder of this chapter will consist of a single activity and a bound service. The purpose of the bound service is to obtain the current time from the system and return that information to the activity where it will be displayed to the user. The bound service will be local and private to the same application as the activity.

Launch Android Studio and follow the usual steps to create a new project, entering LocalBound into the Application name field and ebookfrenzy.com as the Company Domain setting before clicking on the Next button.

On the form factors screen, enable the Phone and Tablet option and set the minimum SDK setting to API 14: Android 4.0 (IceCreamSandwich). Continue to proceed through the screens, requesting the creation of an Empty Activity named LocalBoundActivity with the remaining fields set to the default values.

Once the project has been created, the next step is to add a new class to act as the bound service.

Adding a Bound Service to the Project

To add a new class to the project, right-click on the package name (located under app -> java -> com.ebookfrenzy.localbound) within the Project tool window and select the New -> Service -> Service menu option. Specify BoundService as the class name and make sure that both the Exported and Enabled options are selected before clicking on Finish to create the class. By default, Android Studio will load the BoundService.kt file into the editor where it will read as follows:

package com.ebookfrenzy.localbound
 
import android.app.Service
import android.content.Intent
import android.os.IBinder
 
class BoundService : Service() {
 
    override fun onBind(intent: Intent): IBinder? {
        // TODO: Return the communication channel to the service.
        throw UnsupportedOperationException("Not yet implemented")
    }
}

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Implementing the Binder

As previously outlined, local bound services can communicate with bound clients by passing an appropriately configured Binder object to the client. This is achieved by creating a Binder subclass within the bound service class and extending it by adding one or more new methods that can be called by the client. In most cases, this simply involves implementing a method that returns a reference to the bound service instance. With a reference to this instance, the client can then access data and call methods within the bound service directly.

For the purposes of this example, therefore, some changes are needed to the template BoundService class created in the preceding section. In the first instance, a Binder subclass needs to be declared. This class will contain a single method named getService() which will simply return a reference to the current service object instance (represented by the this keyword). With these requirements in mind, edit the BoundService.kt file and modify it as follows:

package com.ebookfrenzy.localbound
 
import android.app.Service
import android.content.Intent
import android.os.IBinder
import android.os.Binder
 
class BoundService : Service() {
 
    private val myBinder = MyLocalBinder()
 
    override fun onBind(intent: Intent): IBinder? {
        // TODO: Return the communication channel to the service.
        throw UnsupportedOperationException("Not yet implemented")
    }
 
    inner class MyLocalBinder : Binder() {
        fun getService() : BoundService {
            return [email protected]
        }
    }
}

Having made the changes to the class, it is worth taking a moment to recap the steps performed here. First, a new subclass of Binder (named MyLocalBinder) is declared. This class contains a single method for the sole purpose of returning a reference to the current instance of the BoundService class. A new instance of the MyLocalBinder class is created and assigned to the myBinder IBinder reference (since Binder is a subclass of IBinder there is no type mismatch in this assignment).

Next, the onBind() method needs to be modified to return a reference to the myBinder object and a new public method implemented to return the current time when called by any clients that bind to the service:

package com.ebookfrenzy.localbound
 
import android.app.Service
import android.content.Intent
import android.os.Binder
import android.os.IBinder
import java.text.SimpleDateFormat
import java.util.*
 
class BoundService : Service() {
 
    private val myBinder = MyLocalBinder()
 
    override fun onBind(intent: Intent): IBinder? {
        return myBinder
    }
 
    fun getCurrentTime(): String {
        val dateformat = SimpleDateFormat("HH:mm:ss MM/dd/yyyy",
                Locale.US)
        return dateformat.format(Date())
    }
 
    inner class MyLocalBinder : Binder() {
        fun getService() : BoundService {
            return [email protected]
        }
 
    }
}

At this point, the bound service is complete and is ready to be added to the project manifest file. Locate and double-click on the AndroidManifest.xml file for the LocalBound project in the Project tool window and, once loaded into the Manifest Editor, verify that Android Studio has already added a <service> entry for the service as follows:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.ebookfrenzy.localbound.localbound" >
 
    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name=" .LocalBoundActivity" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
 
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
 
        <service
            android:name=".BoundService"
            android:enabled="true"
            android:exported="true" >
        </service>
    </application>
 
</manifest>

The next phase is to implement the necessary code within the activity to bind to the service and call the getCurrentTime() method.

Binding the Client to the Service

For the purposes of this tutorial, the client is the LocalBoundActivity instance of the running application. As previously noted, in order to successfully bind to a service and receive the IBinder object returned by the service’s onBind() method, it is necessary to create a ServiceConnection subclass and implement onServiceConnected() and onServiceDisconnected() callback methods. Edit the LocalBoundActivity.kt file and modify it as follows:

package com.ebookfrenzy.localbound
 
import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.content.ComponentName
import android.content.Context
import android.content.ServiceConnection
import android.os.IBinder
import android.content.Intent
 
class LocalBoundActivity : AppCompatActivity() {
 
    var myService: BoundService? = null
    var isBound = false
 
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_local_bound)
    }
 
    private val myConnection = object : ServiceConnection {
        override fun onServiceConnected(className: ComponentName,
                                        service: IBinder) {
            val binder = service as BoundService.MyLocalBinder
            myService = binder.getService()
            isBound = true
        }
 
        override fun onServiceDisconnected(name: ComponentName) {
            isBound = false
        }
    }
}

The onServiceConnected() method will be called when the client binds successfully to the service. The method is passed as an argument the IBinder object returned by the onBind() method of the service. This argument is cast to an object of type MyLocalBinder and then the getService() method of the binder object is called to obtain a reference to the service instance, which, in turn, is assigned to myService. A Boolean flag is used to indicate that the connection has been successfully established.

You are reading a sample chapter from the Android Studio 3.0 / Android 8 Edition book.

Purchase the fully updated Android Studio 3.2 / Android 9 / Jetpack Edition of this publication in eBook ($29.99) or Print ($45.99) format

Android Studio 3.2 Development Essentials - Kotlin Edition Print and eBook (ePub/PDF/Kindle) editions contain 96 chapters and over 800 pages

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The onServiceDisconnected() method is called when the connection ends and simply sets the Boolean flag to false.

Having established the connection, the next step is to modify the activity to bind to the service. This involves the creation of an intent and a call to the bindService() method, which can be performed in the onCreate() method of the activity:

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_local_bound)
    val intent = Intent(this, BoundService::class.java)
    bindService(intent, myConnection, Context.BIND_AUTO_CREATE)
}

Completing the Example

All that remains is to implement a mechanism for calling the getCurrentTime() method and displaying the result to the user. As is now customary, Android Studio will have created a template activity_local_bound.xml file for the activity containing only a TextView. Load this file into the Layout Editor tool and, using Design mode, select the TextView component and change the ID to myTextView. Add a Button view beneath the TextView and change the text on the button to read “Show Time”, extracting the text to a string resource named show_time. On completion of these changes, the layout should resemble that illustrated in Figure 56-1. If any constraints are missing, click on the Infer Constraints button in the Layout Editor toolbar.

As3.0 local bound ui.png 

Complete the user interface design by selecting the Button and configuring the onClick property to call a method named showTime.

Finally, edit the code in the LocalBoundActivity.kt file to implement the showTime() method. This method simply calls the getCurrentTime() method of the service (which, thanks to the onServiceConnected() method, is now available from within the activity via the myService reference) and assigns the resulting string to the TextView:

package com.ebookfrenzy.localbound
 
import android.content.ComponentName
import android.content.Context
import android.content.ServiceConnection
import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.os.IBinder
import android.content.Intent
import android.view.View
import kotlinx.android.synthetic.main.activity_local_bound.*
 

class LocalBoundActivity : AppCompatActivity() {
 
    var myService: BoundService? = null
    var isBound = false
 
    fun showTime(view: View) {
        val currentTime = myService?.getCurrentTime()
        myTextView.text = currentTime
    }
.
.
}

Testing the Application

With the code changes complete, perform a test run of the application. Once visible, touch the button and note that the text view changes to display the current date and time. The example has successfully started and bound to a service and then called a method of that service to cause a task to be performed and results returned to the activity.

Summary

When a bound service is local and private to an application, components within that application can interact with the service without the need to resort to inter-process communication (IPC). In general terms, the service’s onBind() method returns an IBinder object containing a reference to the instance of the running service. The client component implements a ServiceConnection subclass containing callback methods that are called when the service is connected and disconnected. The former method is passed the IBinder object returned by the onBind() method allowing public methods within the service to be called.

Having covered the implementation of local bound services, the next chapter will focus on using IPC to interact with remote bound services.


You are reading a sample chapter from the Android Studio 3.0 / Android 8 Edition book.

Purchase the fully updated Android Studio 3.2 / Android 9 / Jetpack Edition of this publication in eBook ($29.99) or Print ($45.99) format

Android Studio 3.2 Development Essentials - Kotlin Edition Print and eBook (ePub/PDF/Kindle) editions contain 96 chapters and over 800 pages

Buy Print Preview Book



PreviousTable of ContentsNext
Android Started Service – A Kotlin ExampleAndroid Remote Bound Services – A Kotlin Example