《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();
    }

}

}