前言

本文讲述如何开发一个Flutter插件，用于监听手机网络状态的改变。需要注意的是，此插件目前只支持安卓平台。

使用

每当网络状态改变时，NetworkListener的builder方法都会被调用，通过status回调网络状态。

NetworkListener(
 builder: (_,status){
 var networkStatuds = "";
 if (status == ConnectivityResult.none) {
 networkStatuds = "网络不可用";
 }
 if (status == ConnectivityResult.has) {
 networkStatuds = "网络可用";
 }
 return  Text(networkStatuds);
 },
) 

原理

使用StreamBuilder

NetworkListener内部封装了一个StreamBuilder，接收native端返回的Stream，便可回调网络状态。代码如下：

class NetworkListener extends StatelessWidget {
 const NetworkListener({
 required this.builder,
 Key? key}) : super(key: key);

 final  Widget Function(BuildContext context, ConnectivityResult connectivityResult) builder;

 @override
 Widget build(BuildContext context) {
 return StreamBuilder(
 stream: NetworkStatusNotifier.getNetworkStatus(),
 builder: (BuildContext context, AsyncSnapshot<dynamic> snapshot) {
 if(snapshot.connectionState == ConnectionState.active && snapshot.hasData){
 return builder(context,snapshot.data);
 }

 return builder(context,ConnectivityResult.none);
 },
 );
 }
} 

如何让native端返回Stream

如何让native端返回的Strea呢？首先，需要在native端FlutterPlugin的 onAttachedToEngine中创建一个EventChannel,通过其setStreamHandler方法设置好回调什么内容给dart端。代码如下,注意network_status_notifier_ec,其需要与dart端对应。

// ...
private EventChannel eventChannel;

public void onAttachedToEngine(@NonNull FlutterPluginBinding flutterPluginBinding) {
 eventChannel = new EventChannel(flutterPluginBinding.getBinaryMessenger(),"network_status_notifier_ec");
 ConnectivityBroadcastReceiver connectivityBroadcastReceiver = new ConnectivityBroadcastReceiver(flutterPluginBinding.getApplicationContext());
 eventChannel.setStreamHandler(connectivityBroadcastReceiver);
}

// ... 

在dart端，同样创建一个EventChannel，用于接收native返回的stream

/// Connection status check result.
enum ConnectivityResult {
 /// has network.
 has,
 /// None: Device not connected to any network
 none
}

class NetworkStatusNotifier {

 static const EventChannel _eventChannel = EventChannel("network_status_notifier_ec");

 static Stream<ConnectivityResult>? _networkStatus;

 static Stream<ConnectivityResult> getNetworkStatus(){
 _networkStatus ??= _eventChannel.receiveBroadcastStream().map((satues) => parseResult(satues));
 return _networkStatus!;
 }

 static ConnectivityResult parseResult(int i){
 switch(i){
 case 0:
 return ConnectivityResult.none;
 case 1:
 return ConnectivityResult.has;
 default:
 return ConnectivityResult.none;
 }
 }
} </pre>

## 如何监听网络

监听网络在不同的安卓版本有不同的实现，此项目在安卓N以下采用监听广播的方式监听网络状态，而大于安卓N则采用注册回调的方式。代码如下：

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="" cid="n26" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;"> public class ConnectivityBroadcastReceiver  extends BroadcastReceiver
 implements EventChannel.StreamHandler{

 public ConnectivityBroadcastReceiver(Context context) {
 this.context = context;
 connectivityManager = (ConnectivityManager)context.getSystemService(Context.CONNECTIVITY_SERVICE);
 }

 private Context context;
 private ConnectivityManager connectivityManager;
 // 回调结果
 private EventChannel.EventSink events;
 public static final String CONNECTIVITY_CHANGE = "android.net.conn.CONNECTIVITY_CHANGE";
 private Handler mainHandler = new Handler(Looper.getMainLooper());
 private ConnectivityManager.NetworkCallback networkCallback;

 @Override
 public void onReceive(Context context, Intent intent) {
 if(android.os.Build.VERSION.SDK_INT < Build.VERSION_CODES.N) {
 NetworkInfo networkInfo = connectivityManager.getActiveNetworkInfo();
 //
 if (networkInfo != null && networkInfo.isAvailable()) {
 // network is available.
 callbackNetworkStatus(1);
 } else {
 // network is unavailable.
 callbackNetworkStatus(0);
 }
 }
 }

 @Override
 public void onListen(Object arguments, EventChannel.EventSink events) {
 this.events = events;

 if (android.os.Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
 networkCallback =
 new ConnectivityManager.NetworkCallback() {
 @Override
 public void onAvailable(Network network) {
 sendEvent(1);
 }

 @Override
 public void onUnavailable() {
 sendEvent(0);
 }

 @Override
 public void onLost(Network network) {
 sendEvent(0);
 }
 };
 connectivityManager.registerDefaultNetworkCallback(networkCallback);
 } else {
 context.registerReceiver(this, new IntentFilter(CONNECTIVITY_CHANGE));
 }
 }

 @Override
 public void onCancel(Object arguments) {
 if (android.os.Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
 if (networkCallback != null) {
 connectivityManager.unregisterNetworkCallback(networkCallback);
 }
 } else {
 context.unregisterReceiver(this);
 }
 }

 private void callbackNetworkStatus(int status){
 if(events != null){
 events.success(status);
 }
 }

 private void sendEvent(final int status) {
 Runnable runnable =
 new Runnable() {
 @Override
 public void run() {
 events.success(status);
 }
 };
 mainHandler.post(runnable);
 }
} 

完整代码

github.com/obweix/flut…