笔记56 | 管理网络的使用

2017-12-21 本文已影响7人项勇

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

isConnected()

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

isConnected()。

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

NetworkInfo

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_MANAGE_NETWORK_USAGE

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

PreferenceActivity

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

BroadcastReceiver

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();
            }
        }

    }
}