思路：创建一个类继承BroadcastReceiver类，在里面重写onReceive()方法，把接收到广播后需要添加的逻辑写在方法内public class MainActivity extends AppCompatActivity     private Broad broad;    private IntentFilter intentfilter;

protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        intentfilter=new IntentFilter();        intentfilter.addAction("android.net.conn.CONNECTIVITY_CHANGE");        broad=new Broad();        registerReceiver(broad,intentfilter);    }   class Broad extends BroadcastReceiver{       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 avaliable",Toast.LENGTH_SHORT).show();           }           else                Toast.makeText(context, "network is unavaliable", Toast.LENGTH_SHORT).show();       }   }}

注意：1.有些访问时需要权限的，所以要在AndroidManifest.xml中添加权限。

           2.动态注册的广播接收器一定要取消注册。