android 后台运行服务之创建后台服务篇

除非另行指定，大部分应用程序操作在前台称作为UI 线程的特殊线程中运行。这可能会造成问题，因为长时间运行的操作会影响你的用户界面的响应。这干扰用户交互操作，甚至可能导致系统错误。为了避免这种情况，Android框架提供了一些类，帮助您关闭负载操作放在一个独立运行于后台的线程。最有用的是intentservice。

接下来将描述如下三种操作。

1. 创建一个后台服务

这IntentServic类给单个后台线程运行某个操作提供了一个直接了断的结构， 允许它处理长时间运行的操作，不影响用户界面的响应，同时，一个IntentService 也不会被多用户界面生命周期事件的影响，因此它继续运行的情况下，将关闭AsyncTask。

一个IntentService 有一些限制：

• 不能和用户直接交互，为将结果更新UI， 必须发送结果给Activity。
• 工作请求是有序的，如果一个操作正在IntentService中运行,而且发送另一个请求给IntentService， 这个请求需要等待知道第一个请求操作完成.
• 运行在IntentService中的操作，不能够被中断.

然而，在大多数情况下，Intentservice 是简单的后台操作的首选方法。

接下来将给您讲述

如何创建您自己的IntentService子类以及创建请求回调方法onHandleIntent()，最后描述如何在Manifest 文件中定义IntentService。

a。创建一个IntentService

为创建一个IntentService 组件，定义一个继承于IntentService的类， 并且在该类中实现onHandleIntent()方法。

例子如下：

/** * This service pulls RSS content from a web site URL contained in the incoming Intent (see * onHandleIntent()). As it runs, it broadcasts its status using LocalBroadcastManager; any * component that wants to see the status should implement a subclass of BroadcastReceiver and * register to receive broadcast Intents with category = CATEGORY_DEFAULT and action * Constants.BROADCAST_ACTION. * */public class RSSPullService extends IntentService {    // Used to write to the system log from this class.    public static final String LOG_TAG = "RSSPullService";    // Defines and instantiates an object for handling status updates.    private BroadcastNotifier mBroadcaster = new BroadcastNotifier(this);    /**     * An IntentService must always have a constructor that calls the super constructor. The     * string supplied to the super constructor is used to give a name to the IntentService's     * background thread.     */    public RSSPullService() {        super("RSSPullService");    }    /**     * In an IntentService, onHandleIntent is run on a background thread.  As it     * runs, it broadcasts its current status using the LocalBroadcastManager.     * @param workIntent The Intent that starts the IntentService. This Intent contains the     * URL of the web site from which the RSS parser gets data.     */    @Override    protected void onHandleIntent(Intent workIntent) {        // Gets a URL to read from the incoming Intent's "data" value        String localUrlString = workIntent.getDataString();        // Creates a projection to use in querying the modification date table in the provider.        final String[] dateProjection = new String[]        {            DataProviderContract.ROW_ID,            DataProviderContract.DATA_DATE_COLUMN        };        // A URL that's local to this method        URL localURL;        // A cursor that's local to this method.        Cursor cursor = null;        /*         * A block that tries to connect to the Picasa featured picture URL passed as the "data"         * value in the incoming Intent. The block throws exceptions (see the end of the block).         */        try {            // Convert the incoming data string to a URL.            localURL = new URL(localUrlString);            /*             * Tries to open a connection to the URL. If an IO error occurs, this throws an             * IOException             */            URLConnection localURLConnection = localURL.openConnection();            // If the connection is an HTTP connection, continue            if ((localURLConnection instanceof HttpURLConnection)) {                // Broadcasts an Intent indicating that processing has started.                mBroadcaster.broadcastIntentWithState(Constants.STATE_ACTION_STARTED);                // Casts the connection to a HTTP connection                HttpURLConnection localHttpURLConnection = (HttpURLConnection) localURLConnection;                // Sets the user agent for this request.                localHttpURLConnection.setRequestProperty("User-Agent", Constants.USER_AGENT);                /*                 * Queries the content provider to see if this URL was read previously, and when.                 * The content provider throws an exception if the URI is invalid.                 */                cursor = getContentResolver().query(                        DataProviderContract.DATE_TABLE_CONTENTURI,                        dateProjection,                        null,                        null,                        null);                // Flag to indicate that new metadata was retrieved                boolean newMetadataRetrieved;                /*                 * Tests to see if the table contains a modification date for the URL                 */                if (null != cursor && cursor.moveToFirst()) {                    // Find the URL's last modified date in the content provider                    long storedModifiedDate =                            cursor.getLong(cursor.getColumnIndex(                                    DataProviderContract.DATA_DATE_COLUMN)                            )                    ;                    /*                     * If the modified date isn't 0, sets another request property to ensure that                     * data is only downloaded if it has changed since the last recorded                     * modification date. Formats the date according to the RFC1123 format.                     */                    if (0 != storedModifiedDate) {                        localHttpURLConnection.setRequestProperty(                                "If-Modified-Since",                                org.apache.http.impl.cookie.DateUtils.formatDate(                                        new Date(storedModifiedDate),                                        org.apache.http.impl.cookie.DateUtils.PATTERN_RFC1123));                    }                    // Marks that new metadata does not need to be retrieved                    newMetadataRetrieved = false;                } else {                    /*                     * No modification date was found for the URL, so newmetadata has to be                     * retrieved.                     */                    newMetadataRetrieved = true;                }                // Reports that the service is about to connect to the RSS feed                mBroadcaster.broadcastIntentWithState(Constants.STATE_ACTION_CONNECTING);                // Gets a response code from the RSS server                int responseCode = localHttpURLConnection.getResponseCode();                switch (responseCode) {                    // If the response is OK                    case HttpStatus.SC_OK:                        // Gets the last modified data for the URL                        long lastModifiedDate = localHttpURLConnection.getLastModified();                        // Reports that the service is parsing                        mBroadcaster.broadcastIntentWithState(Constants.STATE_ACTION_PARSING);                        /*                         * Instantiates a pull parser and uses it to parse XML from the RSS feed.                         * The mBroadcaster argument send a broadcaster utility object to the                         * parser.                         */                        RSSPullParser localPicasaPullParser = new RSSPullParser();                        localPicasaPullParser.parseXml(                            localURLConnection.getInputStream(),                            mBroadcaster);                        // Reports that the service is now writing data to the content provider.                        mBroadcaster.broadcastIntentWithState(Constants.STATE_ACTION_WRITING);                        // Gets image data from the parser                        Vector<ContentValues> imageValues = localPicasaPullParser.getImages();                        // Stores the number of images                        int imageVectorSize = imageValues.size();                        // Creates one ContentValues for each image                        ContentValues[] imageValuesArray = new ContentValues[imageVectorSize];                        imageValuesArray = imageValues.toArray(imageValuesArray);                        /*                         * Stores the image data in the content provider. The content provider                         * throws an exception if the URI is invalid.                         */                        getContentResolver().bulkInsert(                                DataProviderContract.PICTUREURL_TABLE_CONTENTURI, imageValuesArray);                        // Creates another ContentValues for storing date information                        ContentValues dateValues = new ContentValues();                        // Adds the URL's last modified date to the ContentValues                        dateValues.put(DataProviderContract.DATA_DATE_COLUMN, lastModifiedDate);                        if (newMetadataRetrieved) {                            // No previous metadata existed, so insert the data                            getContentResolver().insert(                                DataProviderContract.DATE_TABLE_CONTENTURI,                                dateValues                            );                        } else {                            // Previous metadata existed, so update it.                            getContentResolver().update(                                    DataProviderContract.DATE_TABLE_CONTENTURI,                                    dateValues,                                    DataProviderContract.ROW_ID + "=" +                                            cursor.getString(cursor.getColumnIndex(                                                            DataProviderContract.ROW_ID)), null);                        }                        break;                }                // Reports that the feed retrieval is complete.                mBroadcaster.broadcastIntentWithState(Constants.STATE_ACTION_COMPLETE);            }        // Handles possible exceptions        } catch (MalformedURLException localMalformedURLException) {            localMalformedURLException.printStackTrace();        } catch (IOException localIOException) {            localIOException.printStackTrace();        } catch (XmlPullParserException localXmlPullParserException) {            localXmlPullParserException.printStackTrace();        } finally {            // If an exception occurred, close the cursor to prevent memory leaks.            if (null != cursor) {                cursor.close();            }        }    }}

注意： 这其他常规的Service组件的回调方法， 例如onStartCommand() 是自动被IntentServices调用，在一个IntentService中， 应当避免重载那些回调方法。

b.在Manifest 文件中定义IntentService

一个IntentService 也需要在应用manifest 文件中应该一个入口（entry），使用<service>元素提供这个入口service，<service>元素 是<application>元素的子项。具体定义如下：

   <application        android:icon="@drawable/icon"        android:label="@string/app_name">        ...        <!--            Because android:exported is set to "false",            the service is only available to this app.        -->        <service            android:name=".RSSPullService"            android:exported="false"/>        ...    <application/>

android:name 指定IntentService 类的名称，注意这<service>元素不包含intent filter，这Activity发送工作请求给intentService使用明确的Intent,因此没有filter的必要,这也意味这仅仅在同一个应用或其他具有相同user ID 的应用才能访问该服务。发送工作请求给后台服务将在下一篇“android 后台运行服务之发送工作请求给后台服务篇”讲述。