Android Training - 网络操作(二) - 管理网络用途

Android Training - 网络操作(2) - 管理网络用途
如果程序需要执行很多网络操作，那么你应该让用户能够设置网络连接和管理数据，比如，多久同步一次数据，是否只有Wi-Fi连接时才上传和下载数据，漫游情况下是否使用数据等。

检查设备的网络连接

设备可以使用多种方式连接网络，下面我们只关注Wi-Fi和移动网络。

Wi-Fi比较快，移动网络一般是计费的，通常的做法是在Wi-Fi状态下获取大数据。

在使用网络前，我们应该检查网络连接状态，然后根据状态执行操作或者返回提示，检查网络需要下面这些类：

ConnectivityManager：返回网络状态，网络连接改变时发出通知。
NetworkInfo：网络接口状态的类型。

下面的代码判断什么网络类型可用，并且已连接：

private static final String DEBUG_TAG = "NetworkStatusExample";
...      
ConnectivityManager connMgr = (ConnectivityManager) 
        getSystemService(Context.CONNECTIVITY_SERVICE);
NetworkInfo networkInfo = connMgr.getNetworkInfo(ConnectivityManager.TYPE_WIFI); 
boolean isWifiConn = networkInfo.isConnected();
networkInfo = connMgr.getNetworkInfo(ConnectivityManager.TYPE_MOBILE);
boolean isMobileConn = networkInfo.isConnected();
Log.d(DEBUG_TAG, "Wifi connected: " + isWifiConn);
Log.d(DEBUG_TAG, "Mobile connected: " + isMobileConn);

在执行网络操作前，你应该使用isConnected()来检查网络是否连接。

还有一张方法可以检查到一个网络是否可用，getActiveNetworkInfo()返回一个NetworkInfo的实例表示第一个连接的网络，null表示没有网络连接：

public boolean isOnline() {
    ConnectivityManager connMgr = (ConnectivityManager) 
            getSystemService(Context.CONNECTIVITY_SERVICE);
    NetworkInfo networkInfo = connMgr.getActiveNetworkInfo();
    return (networkInfo != null && networkInfo.isConnected());
}

管理网络用途

你可以使用一个activity来让用户显式的控制网络资源，例如：

你可以允许用户在有Wi-Fi网络的时候上传视频。
可以根据网络可用性，时间间隔来同步数据等。

开发支持网络权限和管理网络用途的程序，你需要有下面这些权限：

清单文件中要有下面的权限：
android.permission.INTERNET - 允许程序打开网络。
android.permission.ACCESS_NETWORK_STATE - 允许程序获取网络信息。
你可以在intent filter中添加ACTION_MANAGE_NETWORK_USAGE action（4.0开始使用）来声明程序定义一个activity来控制网络用途。

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.android.networkusage"
    ...>

    <uses-sdk android:minSdkVersion="4" 
           android:targetSdkVersion="14" />
        
    <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

    <application
        ...>
        ...
        <activity android:label="SettingsActivity" android:name=".SettingsActivity">
             <intent-filter>
                <action android:name="android.intent.action.MANAGE_NETWORK_USAGE" />
                <category android:name="android.intent.category.DEFAULT" />
          </intent-filter>
        </activity>
    </application>
</manifest>

实现一个网络选择Activity

上面的SettingsActivity是PreferenceActivity的子类，它显示一个选择窗口让用户选择：

为每个XML feed entry展示概要，还是只显示链接。
有任何网络连接就下载XML，还是只是有Wi-Fi时才下载。

下面是SettingsActivity的代码。实现了onSharedPreferenceChangeListener接口。当用户改变选项时，触发onSharedPreferenceChanged()方法，设置refreshDisplay为true。当用户返回主activity时刷新页面：

public class SettingsActivity extends PreferenceActivity implements OnSharedPreferenceChangeListener {
    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        
        // Loads the XML preferences file
        addPreferencesFromResource(R.xml.preferences);
    }
  
    @Override
    protected void onResume() {
        super.onResume();

        // Registers a listener whenever a key changes            
        getPreferenceScreen().getSharedPreferences().registerOnSharedPreferenceChangeListener(this);
    }
  
    @Override
    protected void onPause() {
        super.onPause();

       // Unregisters the listener set in onResume().
       // It's best practice to unregister listeners when your app isn't using them to cut down on 
       // unnecessary system overhead. You do this in onPause().            
       getPreferenceScreen().getSharedPreferences().unregisterOnSharedPreferenceChangeListener(this);    
    }
  
    // When the user changes the preferences selection, 
    // onSharedPreferenceChanged() restarts the main activity as a new
    // task. Sets the the refreshDisplay flag to "true" to indicate that 
    // the main activity should update its display.
    // The main activity queries the PreferenceManager to get the latest settings.
    
    @Override
    public void onSharedPreferenceChanged(SharedPreferences sharedPreferences, String key) {    
        // Sets refreshDisplay to true so that when the user returns to the main
        // activity, the display refreshes to reflect the new settings.
        NetworkActivity.refreshDisplay = true;
    }
}

响应选项的改变

在onStart()中检查选项设置，这里有一个设置和网络连接的匹配，比如，设置为“Wi-Fi”，而网络连接也是Wi-Fi，那么程序下载feed。

public class NetworkActivity extends Activity {
    public static final String WIFI = "Wi-Fi";
    public static final String ANY = "Any";
    private static final String URL = "http://stackoverflow.com/feeds/tag?tagnames=android&sort=newest";
   
    // Whether there is a Wi-Fi connection.
    private static boolean wifiConnected = false; 
    // Whether there is a mobile connection.
    private static boolean mobileConnected = false;
    // Whether the display should be refreshed.
    public static boolean refreshDisplay = true;
    
    // The user's current network preference setting.
    public static String sPref = null;
    
    // The BroadcastReceiver that tracks network connectivity changes.
    private NetworkReceiver receiver = new NetworkReceiver();
    
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        
        // Registers BroadcastReceiver to track network connection changes.
        IntentFilter filter = new IntentFilter(ConnectivityManager.CONNECTIVITY_ACTION);
        receiver = new NetworkReceiver();
        this.registerReceiver(receiver, filter);
    }
    
    @Override 
    public void onDestroy() {
        super.onDestroy();
        // Unregisters BroadcastReceiver when app is destroyed.
        if (receiver != null) {
            this.unregisterReceiver(receiver);
        }
    }
    
    // Refreshes the display if the network connection and the
    // pref settings allow it.
    
    @Override
    public void onStart () {
        super.onStart();  
        
        // Gets the user's network preference settings
        SharedPreferences sharedPrefs = PreferenceManager.getDefaultSharedPreferences(this);
        
        // Retrieves a string value for the preferences. The second parameter
        // is the default value to use if a preference value is not found.
        sPref = sharedPrefs.getString("listPref", "Wi-Fi");

        updateConnectedFlags(); 
       
        if(refreshDisplay){
            loadPage();    
        }
    }
    
    // Checks the network connection and sets the wifiConnected and mobileConnected
    // variables accordingly. 
    public void updateConnectedFlags() {
        ConnectivityManager connMgr = (ConnectivityManager) 
                getSystemService(Context.CONNECTIVITY_SERVICE);
        
        NetworkInfo activeInfo = connMgr.getActiveNetworkInfo();
        if (activeInfo != null && activeInfo.isConnected()) {
            wifiConnected = activeInfo.getType() == ConnectivityManager.TYPE_WIFI;
            mobileConnected = activeInfo.getType() == ConnectivityManager.TYPE_MOBILE;
        } else {
            wifiConnected = false;
            mobileConnected = false;
        }  
    }
      
    // Uses AsyncTask subclass to download the XML feed from stackoverflow.com.
    public void loadPage() {
        if (((sPref.equals(ANY)) && (wifiConnected || mobileConnected))
                || ((sPref.equals(WIFI)) && (wifiConnected))) {
            // AsyncTask subclass
            new DownloadXmlTask().execute(URL);
        } else {
            showErrorPage();
        }
    }
...
    
}

检测连接转换

最后的难题是BroadcastReceiver子类，NetworkReceiver。当网络连接方式改变时，NetworkReceiver拦截CONNECTIVITY_ACTION动作，判断网络连接状态，然后设置wifiConnected和mobileConnected为true或者false。结果就是当用户下一次返回程序时，如果NetworkActivity.refreshDisplay为true就下载最新的feed，更新显示。

程序在onCreate()中注册BroadcastReceiver，前提是需要在清单文件中声明<receiver>:

public class NetworkReceiver extends BroadcastReceiver {   
      
@Override
public void onReceive(Context context, Intent intent) {
    ConnectivityManager conn =  (ConnectivityManager)
        context.getSystemService(Context.CONNECTIVITY_SERVICE);
    NetworkInfo networkInfo = conn.getActiveNetworkInfo();
       
    // Checks the user prefs and the network connection. Based on the result, decides whether
    // to refresh the display or keep the current display.
    // If the userpref is Wi-Fi only, checks to see if the device has a Wi-Fi connection.
    if (WIFI.equals(sPref) && networkInfo != null && networkInfo.getType() == ConnectivityManager.TYPE_WIFI) {
        // If device has its Wi-Fi connection, sets refreshDisplay
        // to true. This causes the display to be refreshed when the user
        // returns to the app.
        refreshDisplay = true;
        Toast.makeText(context, R.string.wifi_connected, Toast.LENGTH_SHORT).show();

    // If the setting is ANY network and there is a network connection
    // (which by process of elimination would be mobile), sets refreshDisplay to true.
    } else if (ANY.equals(sPref) && networkInfo != null) {
        refreshDisplay = true;
                 
    // Otherwise, the app can't download content--either because there is no network
    // connection (mobile or Wi-Fi), or because the pref setting is WIFI, and there 
    // is no Wi-Fi connection.
    // Sets refreshDisplay to false.
    } else {
        refreshDisplay = false;
        Toast.makeText(context, R.string.lost_connection, Toast.LENGTH_SHORT).show();
    }
}

Android Training - 网络操作(二) - 管理网络用途

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