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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@Overridepublic 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。
