在Activity中定义继承Broadcast的内部类，实现接受系统网络状态的广播。

public class MainActivity extends AppCompatActivity {

    private NetworkChangeReceiver networkChangeReceiver;
    private IntentFilter intentFilter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_another);

        networkChangeReceiver = new NetworkChangeReceiver();

        //过滤出系统网络变化的ACTION
        intentFilter = new IntentFilter("android.net.conn.CONNECTIVITY_CHANGE");

        //动态注册广播接收器
        registerReceiver(networkChangeReceiver,intentFilter);
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        //活动销毁时解除注册广播
        unregisterReceiver(networkChangeReceiver);
    }

    class NetworkChangeReceiver extends BroadcastReceiver {

        public NetworkChangeReceiver() {
        }

        @Override
        public void onReceive(Context context, Intent intent) {

            //获取ConnectivityManager的对象，以便获取系统网络状态
            ConnectivityManager connectivityManager = (ConnectivityManager)
                    getSystemService(Context.CONNECTIVITY_SERVICE);

            //调用isAvailable(),获取网络状态
            NetworkInfo activeNetworkInfo = connectivityManager.getActiveNetworkInfo();
            if (activeNetworkInfo != null && activeNetworkInfo.isAvailable()) {
                Toast.makeText(context, "Network is available", Toast.LENGTH_SHORT).show();
            } else {
                Toast.makeText(context, "Network is unavailable", Toast.LENGTH_SHORT).show();
            }
        }
    }
}