There are two ways to register for broadcast:

• Dynamic registration: registering in code
• Static registration: register in Android manifest.xml

Idea: create a new class, let it inherit the BroadcastReceiver, and override the onReceiver() method of the parent class. When a broadcast arrives, the onReceive() method will be executed, and the specific logic can be processed in this method.

MainActivity code is as follows

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();
        //When the network status changes, the system issues a value of android.net.conn.connectivity'change broadcast
        intentFilter.addAction("android.net.conn.CONNECTIVITY_CHANGE");
        networkChangeReceiver = new NetworkChangeReceiver();
        registerReceiver(networkChangeReceiver,intentFilter);
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        //Dynamic registered broadcast receivers must be deregistered
        unregisterReceiver(networkChangeReceiver);
    }


    class NetworkChangeReceiver extends BroadcastReceiver {
        @Override
        public void onReceive(Context context, Intent intent) {
            ConnectivityManager connectivityManager=(ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
            NetworkInfo networkInfo =connectivityManager.getActiveNetworkInfo();
            if (networkInfo != null && networkInfo.isAvailable()) {
                Toast.makeText(context, "network is available", Toast.LENGTH_SHORT).show();
            } else {
                Toast.makeText(context,"newwork is unavailable",Toast.LENGTH_SHORT).show();
            }
        }
    }


}

To access the network state of the system, you need to declare permissions
Add the following code to Android manifest.xml

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

Run the code, press the Home key to return to the main interface (can't press the BACK key, otherwise the ondestore() method will execute), then open the Settings program - > data usage to enter the data usage details interface, and then switch the Cellular data button to start and disable the network, Toast will give a prompt.