动态注册广播接收器，监听网络变化

public class NetChangeReceiver extends BroadcastReceiver{

    @Override
    public void onReceive(Context context, Intent intent) {
        ConnectivityManager cm = (ConnectivityManager)
                context.getSystemService(Context.CONNECTIVITY_SERVICE);
        NetworkInfo netInfo = cm.getActiveNetworkInfo();
        if(netInfo!=null && netInfo.isAvailable()){
            Toast.makeText(context, "网络已链接", Toast.LENGTH_SHORT).show();
        }else{
            Toast.makeText(context, "网络未链接", Toast.LENGTH_SHORT).show();
        }
    }
    
}

public class MainActivity extends Activity {
    IntentFilter intentFilter;
    NetChangeReceiver netChangeReceiver;
    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        intentFilter = new IntentFilter();
        //当前网络发生变化后，系统会发出一条值为android.net.conn.CONNECTIVITY_CHANGE的广播，所以要监听它
        intentFilter.addAction("android.net.conn.CONNECTIVITY_CHANGE");
        netChangeReceiver = new NetChangeReceiver();
        //进行注册
        registerReceiver(netChangeReceiver, intentFilter);
    }
    
    
    @Override
    protected void onDestroy() {
        super.onDestroy();
        //动态注册的广播接收器一定要进行取消注册
        unregisterReceiver(netChangeReceiver);
    }
}

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