我的在線狀態更改時，Android Broadcastreceiver崩潰

Question

我擴展了Broadcastreceiver，以便我的應用程序知道何時更改了在線狀態。 關鍵是：每當在線狀態發生變化時，我的廣播接收器似乎就會重建。 因為我在構造函數之后添加了偵聽器，所以我的偵聽器為空，並且得到了NullPointerException：

02-07 11:11:45.804: E/AndroidRuntime(21416): FATAL EXCEPTION: main
02-07 11:11:45.804: E/AndroidRuntime(21416): java.lang.RuntimeException: Unable to start receiver nl.raakict.android.dnd.util.NetworkStateReceiver: java.lang.NullPointerException
02-07 11:11:45.804: E/AndroidRuntime(21416):    at android.app.ActivityThread.handleReceiver(ActivityThread.java:2699)
02-07 11:11:45.804: E/AndroidRuntime(21416):    at android.app.ActivityThread.access$1500(ActivityThread.java:162)
02-07 11:11:45.804: E/AndroidRuntime(21416):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1453)
02-07 11:11:45.804: E/AndroidRuntime(21416):    at android.os.Handler.dispatchMessage(Handler.java:99)
02-07 11:11:45.804: E/AndroidRuntime(21416):    at android.os.Looper.loop(Looper.java:158)
02-07 11:11:45.804: E/AndroidRuntime(21416):    at android.app.ActivityThread.main(ActivityThread.java:5751)
02-07 11:11:45.804: E/AndroidRuntime(21416):    at java.lang.reflect.Method.invokeNative(Native Method)
02-07 11:11:45.804: E/AndroidRuntime(21416):    at java.lang.reflect.Method.invoke(Method.java:511)
02-07 11:11:45.804: E/AndroidRuntime(21416):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1083)
02-07 11:11:45.804: E/AndroidRuntime(21416):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:850)
02-07 11:11:45.804: E/AndroidRuntime(21416):    at dalvik.system.NativeStart.main(Native Method)
02-07 11:11:45.804: E/AndroidRuntime(21416): Caused by: java.lang.NullPointerException
02-07 11:11:45.804: E/AndroidRuntime(21416):    at nl.raakict.android.dnd.util.NetworkStateReceiver.onReceive(NetworkStateReceiver.java:38)
02-07 11:11:45.804: E/AndroidRuntime(21416):    at android.app.ActivityThread.handleReceiver(ActivityThread.java:2687)
02-07 11:11:45.804: E/AndroidRuntime(21416):    ... 10 more

我的自定義廣播接收器如下所示：

package nl.raakict.android.dnd.util;

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.net.ConnectivityManager;
import android.net.NetworkInfo;
import android.util.Log;

public class NetworkStateReceiver extends BroadcastReceiver{ 
    private Listener mListener = null;
    private static NetworkStateReceiver _instance;

    public static NetworkStateReceiver getInstance()
    {
        if (_instance == null)
        {
            _instance = new NetworkStateReceiver();
        }
        return _instance;
    }

    public NetworkStateReceiver(){
    }

    @Override
    public void onReceive(Context context, Intent intent) {
     Log.d("app","Network connectivity change");
     if(intent.getExtras()!=null) {
        NetworkInfo ni=(NetworkInfo) intent.getExtras().get(ConnectivityManager.EXTRA_NETWORK_INFO);
        if(ni!=null && ni.getState()==NetworkInfo.State.CONNECTED) {
            Log.i("app","Network "+ni.getTypeName()+" connected");
            mListener.onStateChange(true);
        }
     }
     if(intent.getExtras().getBoolean(ConnectivityManager.EXTRA_NO_CONNECTIVITY,Boolean.FALSE)) {
            Log.d("app","There's no network connectivity");
            mListener.onStateChange(false);
     }
   }

    public interface Listener {
        public void onStateChange(boolean state);
    }

    public void registerListener (Listener listener) {
        mListener = listener;
    }

    public boolean isOnline(Context context) {
        ConnectivityManager cm =
            (ConnectivityManager) context.getSystemService(Context.CONNECTIVITY_SERVICE);
        NetworkInfo netInfo = cm.getActiveNetworkInfo();
        if (netInfo != null && netInfo.isConnectedOrConnecting()) {
            return true;
        }
        return false;
    }

}

和我的清單：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="nl.raakict.android.dnd"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="14"
        android:targetSdkVersion="19" />
    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
    <uses-permission android:name="android.permission.INTERNET" />

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name="nl.raakict.android.dnd.HomeActivity"
            android:configChanges="orientation|keyboardHidden|screenSize"
            android:label="@string/app_name"
            android:theme="@style/FullscreenTheme" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
            <receiver android:name="nl.raakict.android.dnd.util.NetworkStateReceiver">
           <intent-filter>
              <action android:name="android.net.conn.CONNECTIVITY_CHANGE" />
           </intent-filter>
        </receiver>
    </application>
</manifest>

這是我的偵聽器類：

public class RequestHandler implements NetworkStateReceiver.Listener {

    private static RequestHandler _instance;
    private NetworkStateReceiver networkStateReveiver;
    private ArrayList<CustomJsonRequest> delayedRequestQueue;
    private boolean currentlyOnline;
    private Context applicationContext;


    public static RequestHandler getInstance(Context applicationContext)
    {
        if (_instance == null)
        {
            _instance = new RequestHandler(applicationContext);
        }
        return _instance;
    }

    private RequestHandler(Context applicationContext){
        delayedRequestQueue= new ArrayList<CustomJsonRequest>();
        this.applicationContext = applicationContext;
        networkStateReveiver = NetworkStateReceiver.getInstance();
        networkStateReveiver.registerListener(this);
        currentlyOnline = networkStateReveiver.isOnline(applicationContext);
    }

    @Override
    public void onStateChange(boolean online) {
        // TODO Auto-generated method stub
        currentlyOnline = online;
        if(online){
            sendRequestQueue();
        }
    }

    public void newPostRequest(CustomJsonRequest request) {
        delayedRequestQueue.add(request);
        if(currentlyOnline) 
            sendRequestQueue();
    }

    public void sendRequestQueue(){
        //code logic here
    }
}

我做錯了什么？

Answer 1

問題在於，完成網絡狀態后，Android將創建您的NetworkStateReceiver類實例，調用其onReceive ，然后銷毀您的接收器實例。 這就是為什么您的偵聽器始終為null的原因，如果您想通知網絡更改的其他某些組件，則應例如在應用程序中發送本地廣播。

您可能還會注冊本地BroadcastReceiver：

“以編程方式注冊您的BroadcastReceiver”： http : //www.grokkingandroid.com/android-tutorial-broadcastreceiver/