管理網絡 · Android Training官方培訓課程中文版

> 編寫:[kesenhoo](https://github.com/kesenhoo) - 原文:[http://developer.android.com/training/basics/network-ops/managing.html](http://developer.android.com/training/basics/network-ops/managing.html) 這一課會介紹如何細化管理使用的網絡資源。如果你的程序需要執行很多網絡操作，你應該提供用戶設置選項來允許用戶控制程序的數據偏好。例如，同步數據的頻率，是否只在連接到WiFi才進行下載與上傳操作，是否在漫游時使用套餐數據流量等等。這樣用戶才不大可能在快到達流量上限時關閉你的程序獲取數據功能，因為他們可以精確控制您的app使用多少數據流量。關于如何編寫一個最小化下載與網絡操作對電量影響的程序，請參考： - [Optimizing Battery Life](http://developer.android.com/training/monitoring-device-state/index.html): - [Transferring Data Without Draining the Battery](http://developer.android.com/training/efficient-downloads/index.html): **示例**：[NetworkUsage.zip](http://developer.android.com/shareables/training/NetworkUsage.zip) ### 1)檢查設備的網絡連接設備可以有許多種網絡連接。這節課主要關注使用Wi-Fi或移動網絡連接的情況。關于所有可能的網絡連接類型，請看[ConnectivityManager](http://developer.android.com/reference/android/net/ConnectivityManager.html). 通常Wi-Fi是比較快的。移動數據通常都是需要按流量計費，會比較貴。通常我們會選擇讓app在連接到WiFi時去獲取大量的數據。在執行網絡操作之前檢查設備當前連接的網絡連接信息是個好習慣。這樣可以防止你的程序在無意間連接使用了非意向的網絡頻道。如果網絡連接不可用，你的應用應該優雅的做出響應。為了檢測網絡連接，我們需要使用到下面兩個類： - [ConnectivityManager](http://developer.android.com/reference/android/net/ConnectivityManager.html): 它會回答關于網絡連接狀態的查詢，并在網絡連接改變時通知應用程序。 - [NetworkInfo](http://developer.android.com/reference/android/net/NetworkInfo.html): 描述一個給定網絡類型(就本節而言是移動網絡或Wi-Fi)的網絡接口的狀態。這段代碼檢查了Wi-Fi與移動網絡(Mobile)的網絡連接。它檢查了這些網絡接口是否可用(available,也就是說網絡連接存在)及是否已連接(connected,也就是說網絡連接存在,并且可以建立套接字(socket)來傳輸數據)： ~~~ 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); ~~~ 請注意你不應該僅僅靠網絡是否可用(available)做出決策。由于[isConnected()](http://developer.android.com/reference/android/net/NetworkInfo.html#isConnected())能夠處理不可靠的移動網絡(flaky mobile networks)，飛行模式(airplane mode)，受限制的后臺數據(restricted background data)等情況，你應該總是在執行網絡操作前檢查 [isConnected()](http://developer.android.com/reference/android/net/NetworkInfo.html#isConnected())。一個更簡潔的檢查網絡是否可用的示例如下。[getActiveNetworkInfo()](http://developer.android.com/reference/android/net/ConnectivityManager.html#getActiveNetworkInfo())方法返回一個[NetworkInfo](http://developer.android.com/reference/android/net/NetworkInfo.html)實例，它表示可以找到的第一個已連接的網絡接口，如果返回null，則表示沒有已連接的網絡接口(意味著網絡連接不可用)： ~~~ public boolean isOnline() { ConnectivityManager connMgr = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE); NetworkInfo networkInfo = connMgr.getActiveNetworkInfo(); return (networkInfo != null && networkInfo.isConnected()); } ~~~ 你可以使用[NetworkInfo.DetailedState](http://developer.android.com/reference/android/net/NetworkInfo.DetailedState.html), 來獲取更加詳細的網絡信息，但很少有這樣的必要。 ### 2)管理網絡的使用你可以實現一個偏好設置的[activity](# "An activity represents a single screen with a user interface.") ，使用戶能直接設置你的程序對網絡資源的使用。例如: - 你可以允許用戶在僅僅連接到Wi-Fi時上傳視頻。 - 你可以根據諸如網絡可用，時間間隔等條件來選擇是否做同步的操作。寫一個支持連接網絡和管理網絡使用的app，你的manifest需要有正確的權限(permission)和意圖過濾器(intent filter)： - [android.permission.INTERNET](http://developer.android.com/reference/android/Manifest.permission.html#INTERNET)—允許應用程序打開網絡套接字. - [android.permission.ACCESS_NETWORK_STATE](http://developer.android.com/reference/android/Manifest.permission.html#ACCESS_NETWORK_STATE)—允許應用程序訪問網絡連接信息. 你可以為你的[activity](# "An activity represents a single screen with a user interface.")聲明[ACTION_MANAGE_NETWORK_USAGE](http://developer.android.com/reference/android/content/Intent.html#ACTION_MANAGE_NETWORK_USAGE)動作(action)(Android 4.0中引入)的intent filter，這樣你的[activity](# "An activity represents a single screen with a user interface.")就能提供控制數據使用的選項了.[ACTION_MANAGE_NETWORK_USAGE](http://developer.android.com/reference/android/content/Intent.html#ACTION_MANAGE_NETWORK_USAGE)顯示管理指定應用程序網絡數據使用所需的設置。當你的app有一個允許用戶控制網絡使用的settings [activity](# "An activity represents a single screen with a user interface.")時，你應該為你的[activity](# "An activity represents a single screen with a user interface.")聲明這個intent filter。在章節概覽提供的Sample中，這個action被SettingsActivity類處理, 它提供了偏好設置UI來讓用戶決定何時進行下載。 ~~~ <?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> ~~~ ### 3)實現一個Preferences [Activity](# "An activity represents a single screen with a user interface.") 正如上面manifest片段中看到的那樣，SettingsActivity有一個[ACTION_MANAGE_NETWORK_USAGE](http://developer.android.com/reference/android/content/Intent.html#ACTION_MANAGE_NETWORK_USAGE)的intent filter。SettingsActivity是[PreferenceActivity](http://developer.android.com/reference/android/preference/PreferenceActivity.html)的子類，它展示一個偏好設置頁面(如下兩張圖)讓用戶指定以下內容: - 是否顯示每個XML提要條目的總結，或者只是每個條目一個鏈接。 - 是否在網絡連接可用時下載XML提要，或者僅僅在Wi-Fi下下載。 ![network-settings1.png](https://box.kancloud.cn/2015-07-28_55b724715696f.png) ![network-settings2.png](https://box.kancloud.cn/2015-07-28_55b72471614f7.png) 下面是 SettingsActivity. 請注意它實現了[OnSharedPreferenceChangeListener](http://developer.android.com/reference/android/content/SharedPreferences.OnSharedPreferenceChangeListener.html). 當用戶改變了他的偏好，就會觸發 [onSharedPreferenceChanged()](http://developer.android.com/reference/android/content/SharedPreferences.OnSharedPreferenceChangeListener.html#onSharedPreferenceChanged(android.content.SharedPreferences, java.lang.String)), 這個方法會設置refreshDisplay 為true(這里的變量存在于自己定義的[activity](# "An activity represents a single screen with a user interface.")，見下一部分的代碼示例). 這會使的當用戶返回到main [activity](# "An activity represents a single screen with a user interface.")的時候進行refresh。(請注意，代碼中的注釋，不得不說，Googler寫的Code看起來就是舒服) ~~~ 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; } } ~~~ ### 4)響應Preference Changes 當用戶在設置界面改變了偏好，它通常都會對app的行為產生影響。在下面的代碼示例中，app會在onStart()方法中檢查偏好設置。如果設置的類型與當前設備的網絡連接類型相一致，那么程序就會下載數據并刷新顯示。(例如, 如果設置是"Wi-Fi" 并且設備連接了Wi-Fi)。(這是一個很好的代碼示例，如何選擇合適的網絡類型進行下載操作) ~~~ 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(); } } ... } ~~~ ### 5)檢測網絡連接變化最后一部分是關于 [BroadcastReceiver](http://developer.android.com/reference/android/content/BroadcastReceiver.html) 的子類： NetworkReceiver. 當設備網絡連接改變時，NetworkReceiver會監聽到 [CONNECTIVITY_ACTION](http://developer.android.com/reference/android/net/ConnectivityManager.html#CONNECTIVITY_ACTION), 這時需要判斷當前網絡連接類型并相應的設置好 wifiConnected 與 mobileConnected .這樣做的結果是下次用戶回到app時，app只會下載最新的feed，如果NetworkActivity.refreshDisplay被設置為true，app會更新顯示. 我們需要控制好[BroadcastReceiver](http://developer.android.com/reference/android/content/BroadcastReceiver.html)的使用，不必要的聲明注冊會浪費系統資源。通常是在 onCreate() 去registers 這個BroadcastReceiver ，在onDestroy()時unregisters它。這樣做會比直接在manifest里面直接注冊更輕量. 當你在manifest里面注冊了一個，你的程序可以在任何時候被喚醒, 即使你已經好幾個星期沒有使用這個程序了。而通過前面的辦法進行注冊，可以確保用戶離開你的程序之后，不會因為那個Broadcast而被喚起。如果你確實要在manifest中注冊，且確保知道何時需要使用到它，你可以在合適的地方使用 [setComponentEnabledSetting()](http://developer.android.com/reference/android/content/pm/PackageManager.html#setComponentEnabledSetting(android.content.ComponentName, int, int)) 來開啟或者關閉它。下面是NetworkReceiver的代碼: ~~~ 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(); } } ~~~