Android DownloadProvider学习

DownloadProvider 简介

DownloadProvider 是Android提供的DownloadManager的增强版，亮点是支持断点下载，提供了“开始下载”，“暂停下载”，“重新下载”，“删除下载”接口。源码下载地址

DownloadProvider 详细分析

DownloadProvider开始下载的是由DownloadManager 的 enqueue方法启动的，启动一个新的下载任务的时序图

开始新的下载时候会调用DownloadManager的enqueue方法，然后再执行DownloadProvider的insert方法，将下载信
息写入数据库，包括下载链接地址等，然后再调用DownloadService的onCreate或者onStartCommand方法。
DownloadProvider类是非常重要的类，所有操作都跟此类有关，因为要保存下载状态。
在分析DownloadProvider的insert方法前，先看看insert方法的源码

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@Override public Uri insert(final Uri uri, final ContentValues values) {     checkInsertPermissions(values);     SQLiteDatabase db = mOpenHelper.getWritableDatabase();       // note we disallow inserting into ALL_DOWNLOADS     int match = sURIMatcher.match(uri);     if (match != MY_DOWNLOADS) {         Log.d(Constants.TAG, "calling insert on an unknown/invalid URI: "                 + uri);         throw new IllegalArgumentException("Unknown/Invalid URI " + uri);     }       ContentValues filteredValues = new ContentValues();       ......       Integer dest = values.getAsInteger(Downloads.COLUMN_DESTINATION);     if (dest != null) {               ......               }     Integer vis = values.getAsInteger(Downloads.COLUMN_VISIBILITY);     if (vis == null) {         if (dest == Downloads.DESTINATION_EXTERNAL) {             filteredValues.put(Downloads.COLUMN_VISIBILITY,                     Downloads.VISIBILITY_VISIBLE_NOTIFY_COMPLETED);         } else {             filteredValues.put(Downloads.COLUMN_VISIBILITY,                     Downloads.VISIBILITY_HIDDEN);         }     } else {         filteredValues.put(Downloads.COLUMN_VISIBILITY, vis);     }     ......       String pckg = values.getAsString(Downloads.COLUMN_NOTIFICATION_PACKAGE);     String clazz = values.getAsString(Downloads.COLUMN_NOTIFICATION_CLASS);     if (pckg != null && (clazz != null || isPublicApi)) {         ......     }     ......       //启动下载服务     Context context = getContext();     context.startService(new Intent(context, DownloadService.class));       //插入数据库     long rowID = db.insert(DB_TABLE, null, filteredValues);     if (rowID == -1) {         Log.d(Constants.TAG, "couldn't insert into downloads database");         return null;     }       insertRequestHeaders(db, rowID, values);     //启动下载服务     context.startService(new Intent(context, DownloadService.class));     notifyContentChanged(uri, match);     return ContentUris.withAppendedId(Downloads.CONTENT_URI, rowID); }

每次开始一个新的下载任务，都会插入数据库，然后启动启动下载服务类DownloadService，所以真正处理下载的是后台服务
DownloadService，DownloadService中有个下载线程类DownloadThread，DownloadService时序图

如果DownloadService没有启动将会执行onCreate()------>onStartCommand()方法，否则执行
onStartCommand()方法。然后执行updateFromProvider()方法启动UpdateThread线程，准备启动
DownloadThread线程。
分析UpdateThread的run方法前先看看run方法的源码：

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public void run() {     Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);       //如果数据里的存储的达到了1000以上时候，将会删除status>200即失败的记录     trimDatabase();     removeSpuriousFiles();       boolean keepService = false;     // for each update from the database, remember which download is     // supposed to get restarted soonest in the future     long wakeUp = Long.MAX_VALUE;

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//会一直在此循环，直到启动完所有下载任务     for (;;) {         synchronized (DownloadService.this) {             if (mUpdateThread != this) {                 throw new IllegalStateException(                         "multiple UpdateThreads in DownloadService");             }             if (!mPendingUpdate) {                 mUpdateThread = null;                 if (!keepService) {                     stopSelf();                 }                 if (wakeUp != Long.MAX_VALUE) {                     scheduleAlarm(wakeUp);                 }                 return;             }             mPendingUpdate = false;         }           long now = mSystemFacade.currentTimeMillis();         keepService = false;         wakeUp = Long.MAX_VALUE;         Set<long> idsNoLongerInDatabase = new HashSet<long>(                 mDownloads.keySet());           Cursor cursor = getContentResolver().query(                 Downloads.ALL_DOWNLOADS_CONTENT_URI, null, null, null,                 null);         if (cursor == null) {             continue;         }         try {             DownloadInfo.Reader reader = new DownloadInfo.Reader(                     getContentResolver(), cursor);             int idColumn = cursor.getColumnIndexOrThrow(Downloads._ID);               for (cursor.moveToFirst(); !cursor.isAfterLast(); cursor                     .moveToNext()) {                 long id = cursor.getLong(idColumn);                 idsNoLongerInDatabase.remove(id);                 DownloadInfo info = mDownloads.get(id);                 if (info != null) {                     updateDownload(reader, info, now);                 } else {                     info = insertDownload(reader, now);                 }                 if (info.hasCompletionNotification()) {                     keepService = true;                 }                 long next = info.nextAction(now);                 if (next == 0) {                     keepService = true;                 } else if (next > 0 && next < wakeUp) {                     wakeUp = next;                 }             }         } finally {             cursor.close();         }           for (Long id : idsNoLongerInDatabase) {             deleteDownload(id);         }           // is there a need to start the DownloadService? yes, if there         // are rows to be deleted.           for (DownloadInfo info : mDownloads.values()) {             if (info.mDeleted) {                 keepService = true;                 break;             }         }           mNotifier.updateNotification(mDownloads.values());           // look for all rows with deleted flag set and delete the rows         // from the database         // permanently         for (DownloadInfo info : mDownloads.values()) {             if (info.mDeleted) {                 Helpers.deleteFile(getContentResolver(), info.mId,                         info.mFileName, info.mMimeType);             }         }     } }</long></long>

UpdateThread线程负责从数据库中获取下载任务，该线程不会每次都新建新的对象，只有UpdateThread为空时候才会新建，在此线程的
run()方法中有两个for循环，外循环是从数据获取下载任务，确保插入数据库的任务都能被启动，直到启动完所有的下载任务才会退出。内循环是遍历从数
据库获取的没完成或者刚开始的下载任务，启动下载。

DownloadThrea线程是真正负责下载的线程，每次启动一个任务都会创建一个新的下载线程对象（对手机来说会耗很大的CPU,所有要加上同步锁或
者线程池来控制同时下载的数量），看看DownloadThread run方法的时序图：

从这个时序图可以看出，这里涉及的源码比较多，在这没有写，看的时候一定要对照的源码来看。其实在下载的时候会发生很多的异常，如网络异常，内存卡容量不足等，所以捕获异常很重要的，捕获后进行相关的处理，这也是体现出考虑全面的地方。