《第一行代码 第二版》第五章

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) {
Toast.makeText(context, "network changes", Toast.LENGTH_SHORT).show();
}

}

}