加入权限

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

创建了一个IntentFilter的实例,并给它添加一个值,"android.net.conn.CONNECTIVITY_CHANGE"的action
当网络状态发生变化时,系统发出的证书一条值为android.net.CONNECTIVITY_CHANGE的广播,广播接收器想要监听什么广播,就在这里添加相应的action.

 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);

    }

动态注册的广播接收器一定都要取消注册

 @Override
    protected void onDestroy() {
        super.onDestroy();
        unregisterReceiver(networkChangeReceiver);
    }

在MainActivity 中定义一个内部类NetworkChangeReceiver,继承BroadcastReceiver,并重写onReceive方法,每当网络发生变化时
onReceive就会执行

   class NetworkChangeReceiver extends BroadcastReceiver{
        @Override
        public void onReceive(Context context, Intent intent) {
            ConnectivityManager connectivityManager = (ConnectivityManager) getSystemService(context.CONNECTIVITY_SERVICE);
            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();
            }
        }
    }