第一行代码5.2节接收系统广播

功能是在网络状态发生变化时，系统发送广播提示用户当前的网络状态。
MainActivity代码如下：

`package activitytest.example.com.broadcasttest;

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.net.ConnectivityManager;
import android.net.Network;
import android.net.NetworkInfo;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {
private IntentFilter intentFilter;
private NetworkChangeReceiver networkChangeReceiver;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
intentFilter=new IntentFilter();
intentFilter.addAction(“android.net.conn.CONNECTIVITY_CHANGE”);
networkChangeReceiver=new NetworkChangeReceiver();
registerReceiver(networkChangeReceiver,intentFilter);
}
@Override
protected void onDestroy(){
super.onDestroy();
unregisterReceiver(networkChangeReceiver);
}
class NetworkChangeReceiver extends BroadcastReceiver{
@Override
public void onReceive(Context context, Intent intent){
ConnectivityManager connectionManager=(ConnectivityManager)
getSystemService(Context.CONNECTIVITY_SERVICE);
NetworkInfo networkInfo=connectionManager.getActiveNetworkInfo();
if(networkInfo!=null&&networkInfo.isAvailable()){
Toast.makeText(context,“network is available”,
Toast.LENGTH_SHORT).show();
}else{
Toast.makeText(context,“network is unavailable”,
Toast.LENGTH_SHORT).show();
}
}

}

}`
AndroidManifest.xml代码如下：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="activitytest.example.com.broadcasttest">

    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
    <uses-permission android:name="android.permission.RECEIVE_BOOT_COMPLETED"/>

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

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

        <receiver
            android:name=".BootCompleteReceiver"
            android:enabled="true"
            android:exported="true">
            <intent-filter>
                <action android:name="android,intent.action.BOOT_COMPLETED"/>
            </intent-filter>
        </receiver>
    </application>

</manifest>

运行程序即可实现功能。