检查设备的网络连接
设备可以有许多种网络连接。这节课主要关注使用 Wi-Fi 或移动网络连接的情况。关于所有可能的网络连接类型，请看
通常 Wi-Fi 是比较快的。移动数据通常都是需要按流量计费，会比较贵。通常我们会选择让 app 在连接到 WiFi 时去获取大量的数据。
在执行网络操作之前，检查设备当前连接的网络连接信息是个好习惯。这样可以防止我们的程序在无意间连接使用了非意向的网络频道。如果网络连接不可用，那么我们的应用应该优雅地做出响应。为了检测网络连接，我们需要使用到下面两个类：
- ConnectivityManager：它会回答关于网络连接的查询结果，并在网络连接改变时通知应用程序。
- NetworkInfo：描述一个给定类型（就本节而言是移动网络或 Wi-Fi）的网络接口状态。
这段代码检查了 Wi-Fi 与移动网络的网络连接。它检查了这些网络接口是否可用（也就是说网络是通的）及是否已连接（也就是说网络连接存在，并且可以建立 socket 来传输数据）：
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);
请注意我们不应该仅仅靠网络是否可用来做出决策。由于
能够处理片状移动网络（flaky mobile networks），飞行模式和受限制的后台数据等情况，所以我们应该总是在执行网络操作前检查
一个更简洁的检查网络是否可用的示例如下。getActiveNetworkInfo()
方法返回一个
实例，它表示可以找到的第一个已连接的网络接口，如果返回 null，则表示没有已连接的网络接口(意味着网络连接不可用)：
public boolean isOnline() {
ConnectivityManager connMgr = (ConnectivityManager)
getSystemService(Context.CONNECTIVITY_SERVICE);
NetworkInfo networkInfo = connMgr.getActiveNetworkInfo();
return (networkInfo != null && networkInfo.isConnected());
}
我们可以使用
NetworkInfo.DetailedState，来获取更加详细的网络信息，但很少有这样的必要。
管理网络的使用情况
我们可以实现一个偏好设置的 activity ，使用户能直接设置程序对网络资源的使用情况。例如:
- 可以允许用户仅在连接到 Wi-Fi 时上传视频。
- 可以根据诸如网络可用，时间间隔等条件来选择是否做同步的操作。
写一个支持连接网络和管理网络使用的 app，manifest 里需要有正确的权限和 intent filter。
-
manifest 文件里包括下面的权限：
-
android.permission.INTERNET——允许应用程序打开网络套接字。
-
android.permission.ACCESS_NETWORK_STATE——允许应用程序访问网络连接信息。
-
-
我们可以为
action（Android 4.0中引入）声明 intent filter，表示我们的应用定义了一个提供控制数据使用情况选项的 activity。ACTION_MANAGE_NETWORK_USAGE显示管理指定应用程序网络数据使用情况的设置。当我们的 app 有一个允许用户控制网络使用情况的设置 activity 时，我们应该为 activity 声明这个 intent filter。在章节概览提供的示例应用中，这个 action 被
SettingsActivity类处理，它提供了偏好设置 UI 来让用户决定何时进行下载。
<?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>
实现一个首选项 Activity
正如上面 manifest 片段中看到的那样，SettingsActivity
有一个
ACTION_MANAGE_NETWORK_USAGEaction 的 intent filter。SettingsActivity
是
的子类，它展示一个偏好设置页面（如下两张图）让用户指定以下内容:
- 是否显示每个 XML 提要条目的总结，或者只是每个条目的一个链接。
- 是否在网络连接可用时下载 XML 提要，或者仅仅在 Wi-Fi 下下载。
network-settings1.png
network-settings2.png
Figure 1.
首选项 activity
下面是
SettingsActivity。请注意它实现了
OnSharedPreferenceChangeListener。当用户改变了他的偏好，就会触发
onSharedPreferenceChanged()
这个方法会设置
refreshDisplay
为 true（这里的变量存在于自己定义的 activity，见下一部分的代码示例）。这会使得当用户返回到 main activity 的时候进行刷新：
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;
}
}
响应偏好设置的改变
当用户在设置界面改变了偏好，它通常都会对 app 的行为产生影响。在下面的代码示例中，app 会在
onStart()
方法中检查偏好设置。如果设置的类型与当前设备的网络连接类型相一致，那么程序就会下载数据并刷新显示。（例如, 如果设置是"Wi-Fi" 并且设备连接了 Wi-Fi）。
（这是一个很好的代码示例，如何选择合适的网络类型进行下载操作）
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();
}
}
...
}
检测网络连接变化
这部分是关于
的子类：NetworkReceiver。 当设备网络连接改变时，NetworkReceiver会监听到CONNECTIVITY_ACTION，这时需要判断当前网络连接类型并相应的设置好wifiConnected与mobileConnected。这样做的结果是下次用户回到 app 时，app 只会下载最新返回的结果。如果NetworkActivity.refreshDisplay被设置为true，app 会更新显示。
我们需要控制好BroadcastReceiver的使用，不必要的声明注册会浪费系统资源。示例应用在onCreate()中注册BroadcastReceiver``NetworkReceiver，在onDestroy()中销毁它。这样做会比在 manifest 里面声明<receiver>更轻巧。当我们在 manifest 里面声明一个<receiver>，我们的程序可以在任何时候被唤醒，即使我们已经好几个星期没有运行这个程序了。而通过前面的办法注册NetworkReceiver，可以确保用户离开我们的应用之后，应用不会被唤起。如果我们确实要在 manifest 中声明<receiver>，且确保知道何时需要使用到它，那么可以在合适的地方使用setComponentEnabledSetting()来开启或者关闭它。
下面是NetworkReceiver的代码:
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();
}
}
通过AsyncTask来进行网络下载操作
例子中，是下载www.baidu.com的网页xml内容
device-2017-12-21-152305.png
device-2017-12-21-152134.png
xml：
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/container"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.evan.http.MainActivity"
tools:ignore="MergeRootFrame" >
<ProgressBar
android:id="@+id/progressBar"
style="?android:attr/progressBarStyleHorizontal"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:max="100"
android:progress="0" />
<ScrollView
android:layout_below="@+id/progressBar"
android:layout_width="fill_parent"
android:layout_height="wrap_content" >
<TextView
android:id="@+id/tv_netinfo"
android:layout_width="fill_parent"
android:layout_height="wrap_content" />
</ScrollView>
</RelativeLayout>
Main
package com.evan.http;
import java.io.ByteArrayOutputStream;
import java.io.InputStream;
import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.HttpStatus;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
import android.app.Activity;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.content.SharedPreferences;
import android.net.ConnectivityManager;
import android.net.NetworkInfo;
import android.os.AsyncTask;
import android.os.Bundle;
import android.preference.PreferenceManager;
import android.util.Log;
import android.widget.ProgressBar;
import android.widget.TextView;
import android.widget.Toast;
public class MainActivity extends Activity {
public static final String WIFI = "Wi-Fi";
public static final String ANY = "Any";
private static final String URL = "https://www.jd.com/";
// 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);
setContentView(R.layout.activity_main);
initView();
// Registers BroadcastReceiver to track network connection changes.
IntentFilter filter = new IntentFilter(ConnectivityManager.CONNECTIVITY_ACTION);
receiver = new NetworkReceiver();
this.registerReceiver(receiver, filter);
}
TextView tv_netinfo;
ProgressBar progressBar;
private void initView() {
tv_netinfo=(TextView) findViewById(R.id.tv_netinfo);
progressBar=(ProgressBar)findViewById(R.id.progressBar);
}
@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();
initView();
// 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 baidu.com.
public void loadPage() {
if (((sPref.equals(ANY)) && (wifiConnected || mobileConnected))
|| ((sPref.equals(WIFI)) && (wifiConnected))) {
new MyTask().execute(URL);
} else {
showErrorPage();
}
}
private void showErrorPage() {
Toast.makeText(this, "Error", 1).show();
}
String TAG = "md";
private class MyTask extends AsyncTask<String, Integer, String> {
//onPreExecute方法用于在执行后台任务前做一些UI操作
@Override
protected void onPreExecute() {
Log.i(TAG, "onPreExecute() called");
tv_netinfo.setText("loading...");
}
//doInBackground方法内部执行后台任务,不可在此方法内修改UI
@Override
protected String doInBackground(String... params) {
Log.i(TAG, "doInBackground(Params... params) called");
try {
HttpClient client = new DefaultHttpClient();
HttpGet get = new HttpGet(params[0]);
HttpResponse response = client.execute(get);
if (response.getStatusLine().getStatusCode() == HttpStatus.SC_OK) {
HttpEntity entity = response.getEntity();
InputStream is = entity.getContent();
long total = entity.getContentLength();
ByteArrayOutputStream baos = new ByteArrayOutputStream();
byte[] buf = new byte[1024];
int count = 0;
int length = -1;
while ((length = is.read(buf)) != -1) {
baos.write(buf, 0, length);
count += length;
//调用publishProgress公布进度,最后onProgressUpdate方法将被执行
publishProgress((int) ((count / (float) total) * 100));
//为了演示进度,休眠500毫秒
Thread.sleep(500);
}
return new String(baos.toByteArray(), "utf-8");
}
} catch (Exception e) {
Log.e(TAG, e.getMessage());
}
return null;
}
//onProgressUpdate方法用于更新进度信息
@Override
protected void onProgressUpdate(Integer... progresses) {
Log.i(TAG, "onProgressUpdate(Progress... progresses) called");
progressBar.setProgress(progresses[0]);
tv_netinfo.setText("loading..." + progresses[0] + "%");
}
//onPostExecute方法用于在执行完后台任务后更新UI,显示结果
@Override
protected void onPostExecute(String result) {
Log.i(TAG, "onPostExecute(Result result) called");
tv_netinfo.setText(result);
}
//onCancelled方法用于在取消执行中的任务时更改UI
@Override
protected void onCancelled() {
Log.i(TAG, "onCancelled() called");
tv_netinfo.setText("cancelled");
progressBar.setProgress(0);
}
}
class NetworkReceiver extends BroadcastReceiver{
@Override
public void onReceive(Context context, Intent intent) {
ConnectivityManager connectivityManager = (ConnectivityManager)
context.getSystemService(Context.CONNECTIVITY_SERVICE);
NetworkInfo networkInfo = connectivityManager.getActiveNetworkInfo();
refreshDisplay = (networkInfo != null&& networkInfo.isConnected())?true:false;
// 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, "wifi连接", 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, "断开连接", Toast.LENGTH_SHORT).show();
}
}
}
}
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