Android Training - 执行网络操作(Lesson 1 - HTTP连接网络)

这一章会介绍一些基本的网络操作，监视网络链接（包括网络改变），让用户来控制app对网络的选择使用。还会介绍如何解析与使用XML数据。

通过学习这章节的课程，你已经会学习一些基础知识，如何在最小化网络阻塞的情况下，创建一个高效的app，用来下载数据与解析数据。

你还可以参考下面文章进阶学习:

第1课:Connecting to the Network

这一课会演示如何实现一个简单的连接到网络的程序。它提供了一些你应该follow的最好示例，用来创建最简单的网络连接程序。请注意，想要执行网络操作首先需要在程序的manifest文件中添加下面的permissions:

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

Choose an HTTP Client(选择一个HTTP Client)

大多数连接网络的Android app会使用HTTP来发送与接受数据。Android提供了两种HTTP clients: HttpURLConnection 与Apache HttpClient。他们二者均支持HTTPS ，都以流方式进行上传与下载，都有可配置timeout, IPv6 与连接池（connection pooling）. 推荐从Gingerbread版本开始使用 HttpURLConnection 。关于这部分的更多详情，请参考 Android’s HTTP Clients。

Check the Network Connection(检测网络连接)

在你的app尝试进行网络连接之前，需要检测当前是否有可用的网络。请注意，设备可能会不在网络覆盖范围内，或者用户可能关闭Wi-Fi与移动网络连接。关于这方面更多的资料，请参考：Managing Network Usage.(下一课内容)

public void myClickHandler(View view) {
    ...
    ConnectivityManager connMgr = (ConnectivityManager)
        getSystemService(Context.CONNECTIVITY_SERVICE);
    NetworkInfo networkInfo = connMgr.getActiveNetworkInfo();
    if (networkInfo != null && networkInfo.isConnected()) {
        // fetch data
    } else {
        // display error
    }
    ...
}

Perform Network Operations on a Separate Thread(在另外一个Thread执行网络操作)

网络操作会遇到不可预期的延迟。显然为了避免一个不好的用户体验，总是在UI Thread之外去执行网络操作。AsyncTask 类提供了一种简单的方式来处理这个问题。关于更多的详情，请参考： Multithreading For Performance.

在下面的代码示例中，myClickHandler() 方法会触发一个新的DownloadWebpageTask().execute(stringUrl). 它继承自AsyncTask，实现了下面两个方法:

doInBackground()) 执行 downloadUrl()方法。Web URL作为参数，方法downloadUrl() 获取并处理网页返回的数据，执行完毕后，传递结果到onPostExecute()。参数类型为String.
onPostExecute()) 获取到返回数据并显示到UI上。

public class HttpExampleActivity extends Activity {
    private static final String DEBUG_TAG = "HttpExample";
    private EditText urlText;
    private TextView textView;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        urlText = (EditText) findViewById(R.id.myUrl);
        textView = (TextView) findViewById(R.id.myText);
    }

    // When user clicks button, calls AsyncTask.
    // Before attempting to fetch the URL, makes sure that there is a network connection.
    public void myClickHandler(View view) {
        // Gets the URL from the UI's text field.
        String stringUrl = urlText.getText().toString();
        ConnectivityManager connMgr = (ConnectivityManager)
            getSystemService(Context.CONNECTIVITY_SERVICE);
        NetworkInfo networkInfo = connMgr.getActiveNetworkInfo();
        if (networkInfo != null && networkInfo.isConnected()) {
            new DownloadWebpageText().execute(stringUrl);
        } else {
            textView.setText("No network connection available.");
        }
    }

     // Uses AsyncTask to create a task away from the main UI thread. This task takes a 
     // URL string and uses it to create an HttpUrlConnection. Once the connection
     // has been established, the AsyncTask downloads the contents of the webpage as
     // an InputStream. Finally, the InputStream is converted into a string, which is
     // displayed in the UI by the AsyncTask's onPostExecute method.
     private class DownloadWebpageText extends AsyncTask {
        @Override
        protected String doInBackground(String... urls) {

            // params comes from the execute() call: params[0] is the url.
            try {
                return downloadUrl(urls[0]);
            } catch (IOException e) {
                return "Unable to retrieve web page. URL may be invalid.";
            }
        }
        // onPostExecute displays the results of the AsyncTask.
        @Override
        protected void onPostExecute(String result) {
            textView.setText(result);
       }
    }
    ...
}

关于上面那段代码的示例详解，请参考下面:

When users click the button that invokes myClickHandler(), the app passes the specified URL to the AsyncTask subclass DownloadWebpageTask.
The AsyncTask method doInBackground() calls the downloadUrl() method.
The downloadUrl() method takes a URL string as a parameter and uses it to create a URL object.
The URL object is used to establish an HttpURLConnection.
Once the connection has been established, the HttpURLConnection object fetches the web page content as an InputStream.
The InputStream is passed to the readIt() method, which converts the stream to a string.
Finally, the AsyncTask’s onPostExecute() method displays the string in the main activity’s UI.

Connect and Download Data(连接并下载数据)

在执行网络交互的线程里面，你可以使用 HttpURLConnection 来执行一个 GET 类型的操作并下载数据。在你调用 connect()之后，你可以通过调用getInputStream()来得到一个包含数据的InputStream 对象。

在下面的代码示例中， doInBackground() 方法会调用downloadUrl(). 这个 downloadUrl() 方法使用给予的URL，通过 HttpURLConnection 连接到网络。一旦建立连接，app使用 getInputStream() 来获取数据。

// Given a URL, establishes an HttpUrlConnection and retrieves
// the web page content as a InputStream, which it returns as
// a string.
private String downloadUrl(String myurl) throws IOException {
    InputStream is = null;
    // Only display the first 500 characters of the retrieved
    // web page content.
    int len = 500;

    try {
        URL url = new URL(myurl);
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setReadTimeout(10000 /* milliseconds */);
        conn.setConnectTimeout(15000 /* milliseconds */);
        conn.setRequestMethod("GET");
        conn.setDoInput(true);
        // Starts the query
        conn.connect();
        int response = conn.getResponseCode();
        Log.d(DEBUG_TAG, "The response is: " + response);
        is = conn.getInputStream();

        // Convert the InputStream into a string
        String contentAsString = readIt(is, len);
        return contentAsString;

    // Makes sure that the InputStream is closed after the app is
    // finished using it.
    } finally {
        if (is != null) {
            is.close();
        }
    }
}

请注意，getResponseCode() 会返回连接状态码( status code). 这是一种获知额外网络连接信息的有效方式。status code 是 200 则意味着连接成功.

Convert the InputStream to a String(把InputStream的数据转换为String)

InputStream 是一种可读的byte数据源。如果你获得了一个 InputStream, 通常会进行decode或者转换为制定的数据类型。例如，如果你是在下载一张image数据，你可能需要像下面一下进行decode：

InputStream is = null;
...
Bitmap bitmap = BitmapFactory.decodeStream(is);
ImageView imageView = (ImageView) findViewById(R.id.image_view);
imageView.setImageBitmap(bitmap);

在上面演示的示例中， InputStream 包含的是web页面的文本内容。下面会演示如何把 InputStream 转换为string，以便显示在UI上。

// Reads an InputStream and converts it to a String.
public String readIt(InputStream stream, int len) throws IOException, UnsupportedEncodingException {
    Reader reader = null;
    reader = new InputStreamReader(stream, "UTF-8");
    char[] buffer = new char[len];
    reader.read(buffer);
    return new String(buffer);
}

学习自：http://developer.android.com/training/basics/network-ops/connecting.html，请多指教，谢谢！
转载请注明出自http://kesenhoo.github.com，谢谢配合！

胡凯