Android Services

Android Services

A service is a component that runs in the background and performs long-running tasks that do not require user interaction. A pplications can still work even if they are destroyed. The service basically consists of two states -

A service has a lifecycle approach that enables monitoring changes in service status and can perform work at the right stage. The following image on the left shows the declaration cycle when a service is created through startService(), and the figure on the right shows the life cycle when a service is created through bindService():

To create a service, you need to create a Java class that inherits from the service base class or its known sub-classes. T he service base class defines different callback methods and most important methods. Y ou don't need to implement all the callback methods. N evertheless, it is important to understand all methods. Implementing these callbacks ensures that your app is implemented the way users expect it to.

The following main service demonstrates the lifecycle of each method -

package cn.uprogrammer.androidservices;

import android.app.Service;
import android.os.IBinder;
import android.content.Intent;
import android.os.Bundle;

public class HelloService extends Service {

    /** 标识服务如果被杀死之后的行为 */
    int mStartMode;

    /** 绑定的客户端接口 */
    IBinder mBinder;

    /** 标识是否可以使用onRebind */
    boolean mAllowRebind;

    /** 当服务被创建时调用. */
    @Override
    public void onCreate() {

    }

    /** 调用startService()启动服务时回调 */
    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        return mStartMode;
    }

    /** 通过bindService()绑定到服务的客户端 */
    @Override
    public IBinder onBind(Intent intent) {
        return mBinder;
    }

    /** 通过unbindService()解除所有客户端绑定时调用 */
    @Override
    public boolean onUnbind(Intent intent) {
        return mAllowRebind;
    }

    /** 通过bindService()将客户端绑定到服务时调用*/
    @Override
    public void onRebind(Intent intent) {

    }

    /** 服务不再有用且将要被销毁时调用 */
    @Override
    public void onDestroy() {

    }
}

This example will show you how to create your own Android service in a simple step. Follow these steps to modify the Android application you created earlier in the Hello World Instances section:

The following is the main activity file src/cn.uprogrammer.androidservices/mainActivity.java the main event file. T his file contains all the basic lifecycle methods. We've added startservice() and stop service methods.

package cn.uprogrammer.androidservices;

import android.app.Activity;
import android.os.Bundle;
import android.view.Menu;


import android.content.Intent;
import android.view.View;

public class MainActivity extends Activity {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    // Method to start the service
    public void startService(View view) {
        startService(new Intent(getBaseContext(), MyService.class));
    }

    // Method to stop the service
    public void stopService(View view) {
        stopService(new Intent(getBaseContext(), MyService.class));
    }
}

Here's what src/cn.uprogrammer.androidservices/MyService .java content. T his file can implement one or more service association methods based on requirements. For newcomers, we only implement onStartCommand() and onDestroy() -

package cn.uprogrammer.androidservices;

import android.app.Service;
import android.content.Intent;
import android.os.IBinder;
import android.widget.Toast;

public class MyService extends Service {

    @Override
    public IBinder onBind(Intent arg0) {
        return null;
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        // Let it continue running until it is stopped.
        Toast.makeText(this, "服务已经启动", Toast.LENGTH_LONG).show();
        return START_STICKY;
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        Toast.makeText(this, "服务已经停止", Toast.LENGTH_LONG).show();
    }
}

The Android Manifest file will .xml below. Add the hashtag here to include our services:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="cn.uprogrammer.androidservices"
    android:versionCode="1"
    android:versionName="1.0">

    <uses-sdk
        android:minSdkVersion="13"
        android:targetSdkVersion="22" />

    <application
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >

        <activity
            android:name=".MainActivity"
            android:label="@string/title_activity_main" >

            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER"/>
            </intent-filter>

        </activity>

        <service android:name=".MyService" />

    </application>

</manifest>

Here's what the res/layout/activity_main.xml file contains two buttons:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
    android:layout_height="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".MainActivity">

    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Android 服务实例"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:textSize="30dp" />

    <TextView
        android:id="@+id/textView2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="www.uprogrammer.cn"
        android:textColor="#ff87ff09"
        android:textSize="30dp"
        android:layout_above="@+id/imageButton"
        android:layout_centerHorizontal="true"
        android:layout_marginBottom="40dp" />

    <ImageButton
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/imageButton"
        android:src="@drawable/ic_launcher"
        android:layout_centerVertical="true"
        android:layout_centerHorizontal="true" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/button2"
        android:text="启动服务"
        android:onClick="startService"
        android:layout_below="@+id/imageButton"
        android:layout_centerHorizontal="true" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="停止服务"
        android:id="@+id/button"
        android:onClick="stopService"
        android:layout_below="@+id/button2"
        android:layout_alignLeft="@+id/button2"
        android:layout_alignStart="@+id/button2"
        android:layout_alignRight="@+id/button2"
        android:layout_alignEnd="@+id/button2" />

</RelativeLayout>

Here's what res/values/strings .xml to define two new constants:

<?xml version="1.0" encoding="utf-8"?>
<resources>

    <string name="app_name">Android Services</string>
    <string name="title_activity_main">MainActivity</string>
    <string name="menu_settings">Settings</string>
    <string name="action_settings">Settings</string>

</resources>

Let's run the My Application application that we just modified. L et me assume that you created the AVD when you installed the environment. O pen the active file in your project and click the icon in the toolbar to run the app in Android Studio. A ndroid Studio installs the app on the AVD and launches it. If all goes well, it will appear on the emulator window as follows:

Now click the "Start Service" button to start the service, which will execute the onStartCommand() method we wrote, and a "Service started" message appears at the bottom of the emulator, as follows:

You can stop the service by clicking the Stop Service button at the bottom.