Android進階練習-執行網路操作

執行網路操作
     這一部分闡述了如何來進行最基本的網路連接任務，管理網路連接（包含網路狀態的改變），還有讓使用者來管理一個應用網路的用法，還有描述了如何來解析和運用XML資料     經過學習了這些課程後，你能夠基本的在一個應用中有效從網路上下載和解析資料，並且使用最少的網路資源     通過本章你將會學到     串連到網路          怎麼樣去串連一個網路，選擇合適的HTTP的用戶端，在UI祝線程外執行一個網路操作          管理網路的使用          怎麼樣去檢查網路連接狀態，建立一個前台介面去管理網路的使用，怎麼去應對網路狀態改變     解析
XML 資料          怎麼樣去解析和使用XML資料網路操作最佳實務

取得device的網路連接狀態

         // Checks the network connection and sets the wifiConnected and mobileConnected    // variables accordingly.    private void updateConnectedFlags() {              //網路管理服務        ConnectivityManager connMgr =                (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE );        NetworkInfo activeInfo = connMgr.getActiveNetworkInfo();        if (activeInfo != null && activeInfo.isConnected()) {              //wifi 串連狀態            wifiConnected = activeInfo.getType() == ConnectivityManager.TYPE_WIFI ;            //mobile串連狀態            mobileConnected = activeInfo.getType() == ConnectivityManager.TYPE_MOBILE ;        } else {            wifiConnected = false ;            mobileConnected = false ;        }    }

在UI主線程外運用AsyncTask類非同步執行網路連接操作

    // Implementation of AsyncTask used to download XML feed from stackoverflow.com.    private class DownloadXmlTask extends AsyncTask<String, Void, String> {        @Override        protected String doInBackground( String... urls) {            try {                return loadXmlFromNetwork(urls[0]);            } catch (IOException e) {                return getResources().getString(R.string.connection_error);            } catch (XmlPullParserException e) {                return getResources().getString(R.string.xml_error);            }        }        @Override        protected void onPostExecute(String result) {            setContentView(R.layout. activity_main);            // Displays the HTML string in the UI via a WebView            WebView myWebView = (WebView) findViewById(R.id. webview);            myWebView.loadData(result, "text/html", null );        }    }

執行網路連接

    // Given a string representation of a URL, sets up a connection and gets    // an input stream.    private InputStream downloadUrl(String urlString) throws IOException {        URL url = new URL(urlString);        HttpURLConnection conn = (HttpURLConnection) url. openConnection();        //設定讀取網路流的逾時時間        conn.setReadTimeout(10000 /* milliseconds */);        //設定網路連接逾時時間        conn.setConnectTimeout(15000 /* milliseconds */);        //設定為GET請求        conn. setRequestMethod("GET");        //設定允許讀取網路輸入資料流        conn.setDoInput( true);        // Starts the query        conn. connect();        InputStream stream = conn. getInputStream();        return stream;    }

在Android作業系統中，每次網路狀態有變化時都會發送一個廣播，我們只要註冊了該廣播的接收器，就可以得知網路狀態的變化了

         // Register BroadcastReceiver to track connection changes.        // 動態註冊廣播接收器去接收網路狀態改變的廣播        IntentFilter filter = new IntentFilter(ConnectivityManager.CONNECTIVITY_ACTION );        receiver = new NetworkReceiver();        this.registerReceiver( receiver, filter);

    /**     *     * This BroadcastReceiver intercepts the android.net.ConnectivityManager.CONNECTIVITY_ACTION,     * which indicates a connection change. It checks whether the type is TYPE_WIFI.     * If it is, it checks whether Wi- Fi is connected and sets the wifiConnected flag in the     * main activity accordingly.     * 網路狀態改變時，進行一些處理     */    public class NetworkReceiver extends BroadcastReceiver {        @Override        public void onReceive(Context context, Intent intent) {            ConnectivityManager connMgr =                    (ConnectivityManager) context.getSystemService(Context.CONNECTIVITY_SERVICE );            NetworkInfo networkInfo = connMgr.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 SDK 內建的解析器PULL解析xml時最佳實務點

//設定不使用namespace特性，節省資源，加快解析速度            parser.setFeature(XmlPullParser. FEATURE_PROCESS_NAMESPACES, false );

         //測試一下是否是標籤的起始，支援任何 namespace，標籤的名為feed.有效減少一些無效的解析操作        parser.require(XmlPullParser. START_TAG, ns , "feed" );

    // Skips tags the parser isn't interested in. Uses depth to handle nested tags. i.e.,    // if the next tag after a START_TAG isn't a matching END_TAG, it keeps going until it    // finds the matching END_TAG (as indicated by the value of "depth" being 0).    // 跳過一些對我們無用的標籤不進行解析，不需要每次都去判斷當前標籤是否是我們需要的標籤，加快解析速度    private void skip(XmlPullParser parser) throws XmlPullParserException, IOException {        if (parser.getEventType() != XmlPullParser.START_TAG) {            throw new IllegalStateException();        }        int depth = 1;        while (depth != 0) {            switch (parser.next()) {            case XmlPullParser.END_TAG:                    depth--;                    break;            case XmlPullParser.START_TAG:                    depth++;                    break;            }        }    }

總結

在Android中去執行一個網路任務，一般要在UI主線程之外的線程中去執行以免阻塞UI主線程，監聽到網路狀態的變化，進而做一些操作，是很有必要的!有助於我們的應用提供更好的使用者體驗。xml資料的解析也需要更好的進行最佳化，以更快的速度把資料展現在使用者面前，這些都是很好的實踐。

Sample：http://www.android-doc.com/training/basics/network-ops/index.html