DownloadManager（强制版本更新和源码分析）

た入场券 2022-07-16 23:11 497阅读 0赞

ps:通常App更新是通过DownLoadManager来实现，更新方式分为强制升级（遇到重大bug时），自选是否版本升级。

#### DownLoadManager类: ####

--------------------

*  官方英文介绍：  
    
    
    >   
    > The download manager is a system service that handles long-running HTTP downloads.  
    > Clients may request that a URI be downloaded to a particular destination file.  
    > The download manager will conduct the download in the background, taking care of HTTP interactions and retrying downloads after failures or across connectivity changes and system reboots.  
    > Instances of this class should be obtained through getSystemService(String) by passing DOWNLOAD\_SERVICE.  
    > Apps that request downloads through this API should register a broadcast receiver for ACTION\_NOTIFICATION\_CLICKED to appropriately handle when the user clicks on a running download in a notification or from the downloads UI.  
    > Note that the application must have the INTERNET permission to use this class.
 *  #### 中文翻译介绍： ####
    
     *  含义：  
        DownLoadManager是一种长期执行http下载任务的系统服务（进程）。
     *  特性:  
        客户端可以通过URI下载相对应文件，DownloadManager是运行在后台，关注http交互和重新下载（在失败或者链接改变和系统重启）
     *  使用方式：  
        DownLoadManager对象=(DownloadManager)getSystemService(Context.DOWNLOAD\_SERVICE)；
     *  使用权限: `<uses-permission android:name="android.permission.INTERNET" />`
 *  #### 其常用的API： ####
    
    1.  enqueue(DownloadManager.Request request) : 添加request,开启下载，返回下载的id（唯一的）
    2.  remove(long… ids)：根据id取消下载
    3.  query(DownloadManager.Query query)： 查询指定id（即指定request）的下载情况，返回一个cursor
    4.  addCompletedDownload(String title, String description, boolean isMediaScannerScannable, String mimeType, String path, long length, boolean showNotification):  
        添加一个file文件到下载的数据库系统，因此当前程序能够管理下载情况。简单来说，根据指定条件开启下载文件
    5.  DownloadManager类的API :  
        [https://developer.android.com/reference/android/app/DownloadManager.html][https_developer.android.com_reference_android_app_DownloadManager.html]
 *  #### 相关类： ####
    
    1.  **DownloadManager.Request**：
        
         *  其常用api:
            
            1.  addRequestHeader(String header, String value):添加http的header
            2.  setAllowedNetworkTypes(int flags):设置允许的网络类型
            3.  setDestinationUri(Uri uri)：自定义下载目录
            4.  setNotificationVisibility(int visibility)：设置Notification的显示情况
            5.  setTitle(CharSequence title)：设置notification中显示的标题
    2.  **DownloadManager.Query**:
        
         *  setFilterById(long… ids)：添加下载的id,便于查询对应的下载情况
 *  #### **使用案例之自选更新与强制更新APP版本**: ####
    
    1.  思路分析：
        
         *  **App强制版本升级思路分析**：
         *  创建一个不可取消的dialog（按back键不能取消，按弹窗外不可取消）
         *  指定文件路径，通过DownloadManager开始下载
         *  查询下载情况，在dialog里中更新下载状态（handler回调主线程中，更新UI）
         *  下载完成后进行安装（静态注册广播接收器，实现安装功能）
         *  **自选更新思路**：借助系统的DownloadManager进行下载，Notifaction显示下载进度
    2.  在确定好更新版本需求后，设置Request:
        
            /** * 设置Request参数 * @param url * @return */
            public DownloadManager.Request   CreateRequest(String url){
                DownloadManager.Request  request=new DownloadManager.Request(Uri.parse(url));
                switch (state){
                    case  Constants.OPTIONAL://自选更新
                        request.setTitle("秒加");// 文件下载时显示的标题
                        request.setNotificationVisibility(   
                          DownloadManager.Request.VISIBILITY_VISIBLE);// 下载完成后，notification会自动消失
                        break;
                    case Constants.FORCIBLEY://强制版本更新
                     request.setNotificationVisibility(   
                         DownloadManager.Request.VISIBILITY_HIDDEN);// 隐藏notification
                        break;
                }
                File file=MyUtils.getCacheFile(Constants.APPNAME);
                if(file!=null&&file.exists()){
                    file.delete();//删除掉存在的apk
                }
                request.setDestinationUri(Uri.fromFile(file));//指定apk缓存路径
                return  request;
            }
        
         *  在这里遇到个问题：
            
                // 若是使用VISIBILITY_HIDDEN，直接给报错：
                java.lang.SecurityException: Invalid value for visibility: 2
                at android.database.DatabaseUtils.readExceptionFromParcel(DatabaseUtils.java:137)
                at android.content.ContentProviderProxy.insert(ContentProviderNative.java:468)
                at android.content.ContentResolver.insert(ContentResolver.java:1190)
                at android.database.DatabaseUtils.readExceptionFromParcel(DatabaseUtils.java:185)
                at android.app.DownloadManager.enqueue(DownloadManager.java:1390)
                源码：public static final int VISIBILITY_HIDDEN = 2;
         *  通过查看源码，找到解决方式：
            
                /**
                 * Control whether a system notification is posted by the download manager while this
                 * download is running. If enabled, the download manager posts notifications about downloads
                 * through the system {@link android.app.NotificationManager}. By default, a notification is
                 * shown.
                 *
                 * If set to false, this requires the permission
                 * android.permission.DOWNLOAD_WITHOUT_NOTIFICATION.
                 *
                 * @param show whether the download manager should show a notification for this download.
                 * @return this object
                 * @deprecated use {@link #setNotificationVisibility(int)}
                 */
                @Deprecated
                public Request setShowRunningNotification(boolean show) {
                 return (show) ? setNotificationVisibility(VISIBILITY_VISIBLE) :
                            setNotificationVisibility(VISIBILITY_HIDDEN);
                }
                setShowRunningNotification(）源码可知： 添加权限，<uses-permission    
                android:name="android.permission.DOWNLOAD_WITHOUT_NOTIFICATION" />
    3.  通过DownLoadManager开始下载：
        
            /** * 通过DownloadManager开始下载 * @return */
            public long  startDownload(){
                 Context appContext=BaseApplication.getAppContext();
                 DownloadManager downloadManager=(DownloadManager)   
                       appContext.getSystemService(Context.DOWNLOAD_SERVICE);
                 long requestId= downloadManager.enqueue(CreateRequest(url));
                 if(state==Constants.FORCIBLEY){
                   //强制升级，显示自定义进度的弹窗
                         queryDownloadProgress(requestId,downloadManager);
                 }
                return  requestId;
            }
    4.  异步线程中通过DownloadManager.Query查询下载进度，Handler回调更新UI（强制版本升级需这步，自选可以跳过）
        
            /** * 查询下载进度 * @param requestId * * DownloadManager.COLUMN_TOTAL_SIZE_BYTES 下载文件的大小（总字节数） * DownloadManager.COLUMN_BYTES_DOWNLOADED_SO_FAR 当前下载文件的字节数 */
            public void queryDownloadProgress(long requestId,DownloadManager  downloadManager){
                DownloadManager.Query query=new DownloadManager.Query();
                query.setFilterById(requestId);
                try {
                    boolean isGoging=true;
                    while (isGoging) {
                         Cursor cursor = downloadManager.query(query);
                         if (cursor != null && cursor.moveToFirst()) {
                         int state = cursor.getInt( 
                               cursor.getColumnIndex(DownloadManager.COLUMN_STATUS));
                         switch (state) {
                            case DownloadManager.STATUS_SUCCESSFUL://下载成功
                                  isGoging=false;
                                handler.obtainMessage( 
                                downloadManager.STATUS_SUCCESSFUL).sendToTarget();//发送到主线程，更新ui
                                break;
                            case DownloadManager.STATUS_FAILED://下载失败
                                isGoging=false;
                                handler.obtainMessage(downloadManager.STATUS_FAILED).sendToTarget(); 
                                 //发送到主线程，更新ui
                                break;
            
                            case DownloadManager.STATUS_RUNNING://下载中
                                /** * 计算下载下载率； */
                                int totalSize = cursor.getInt( 
                                   cursor.getColumnIndex(DownloadManager.COLUMN_TOTAL_SIZE_BYTES));
                                int currentSize = cursor.getInt(  cursor.getColumnIndex(    
                                         DownloadManager.COLUMN_BYTES_DOWNLOADED_SO_FAR));
                                int progress = (int) (((float) currentSize) /  
                                                                      ((float) totalSize) * 100);
                                handler.obtainMessage(downloadManager.STATUS_RUNNING,  、
                                       progress).sendToTarget();//发送到主线程，更新ui
                                break;
            
                            case DownloadManager.STATUS_PAUSED://下载停止
                                handler.obtainMessage(DownloadManager.STATUS_PAUSED).sendToTarget();
                                break;
            
                            case DownloadManager.STATUS_PENDING://准备下载
                                handler.obtainMessage(DownloadManager.STATUS_PENDING).sendToTarget();
                                break;
                        }
                    }
                    if(cursor!=null){
                        cursor.close();
                    }
                }
            
              }catch (Exception e){
                e.printStackTrace();
              }
            } 
            /*handler更新ui*/
            private Handler handler = new Handler(new Handler.Callback() {
            @Override
            public boolean handleMessage(Message msg) {
                switch (msg.what) {
                    case DownloadManager.STATUS_SUCCESSFUL:
                        downloadDialog.setProgress(100);
                        cancleDialog();
            
                        break;
                    case DownloadManager.STATUS_RUNNING:
                        downloadDialog.setProgress((int) msg.obj);
                        break;
                    case DownloadManager.STATUS_FAILED:
                        cancleDialog();
                        break;
                    case DownloadManager.STATUS_PENDING:
                        showDialog();
                        break;
                }
                return false;
            }
            });
    5.  注册监听BroadcasetReceiver,监听下载完成后进行安装：
        
            <receiver android:name=".InstallAPk_BroadcastReceiver">
                <intent-filter>
                    <action android:name="android.intent.action.DOWNLOAD_COMPLETE" />
                </intent-filter>
            </receiver>
            
            public class InstallAPk_BroadcastReceiver extends BroadcastReceiver{ 
                 @Override
                 public void onReceive(Context context, Intent intent) {
                           install(context);
                  }
            
                 public void install(Context context){
                        File file = MyUtils.getCacheFile(Constants.APPNAME);
                        if (file == null || !file.exists()) {
                              return;
                         }
                         Intent installintent = new Intent();
                         installintent.setAction(Intent.ACTION_VIEW);
                    // 在Boradcast中启动活动需要添加Intent.FLAG_ACTIVITY_NEW_TASK
                        installintent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
                        installintent.setDataAndType(Uri.fromFile(file),
                               "application/vnd.android.package-archive");
                        context.startActivity(installintent);
               }
            }
    6.  效果图如下：
        
         *  强制版本更新效果图：  
            ![20160822174039181][]
         *  自选版本更新效果图：  
            ![这里写图片描述][20160822175725452]
    
    项目下载链接： [http://download.csdn.net/detail/hexingen/9609609][http_download.csdn.net_detail_hexingen_9609609]
 *  ### 源码分析： ###
    
    DownLoadManager中包含三个静态类和一些通过DownLoadProvider操作数据库的api，它采用了观察者模式，实现自动调配功能。
    
    1.  **DownloadManager.Request类的源码分析**：
        
            1.这类包含全部信息，唯一需求的参数URI
            2.最好自定义下载路径，防止被系统
            
            public static class Request { 
               //手机网络标志
                public static final int NETWORK_MOBILE = 1 << 0;
            
               //wifi网络标志
                public static final int NETWORK_WIFI = 1 << 1;
            
                //bluetootch标志
                public static final int NETWORK_BLUETOOTH = 1 << 2;
            
                private Uri mUri;
                private Uri mDestinationUri;
                private List<Pair<String, String>> mRequestHeaders = new ArrayList<Pair<String, String>>();
                private CharSequence mTitle;
                private CharSequence mDescription;
                private String mMimeType;
                private int mAllowedNetworkTypes = ~0; 
                private boolean mRoamingAllowed = true;
                private boolean mMeteredAllowed = true;
                private boolean mIsVisibleInDownloadsUi = true;
                private boolean mScannable = false;
                private boolean mUseSystemCache = false;
            
               //可以被媒体扫描的文件，对应存在数据库中列
                private static final int SCANNABLE_VALUE_YES = 0;
            
                //私有文件（不被媒体扫描到），对应在数据库中列
                private static final int SCANNABLE_VALUE_NO = 2;
            
                //显示在下载的通知，下载完后消失
                public static final int VISIBILITY_VISIBLE = 0;
            
               //下载时显示通知，完成通知
                public static final int VISIBILITY_VISIBLE_NOTIFY_COMPLETED = 1;
            
                //不显示下载通知
                public static final int VISIBILITY_HIDDEN = 2;
            
                //下载完后显示通知
                public static final int VISIBILITY_VISIBLE_NOTIFY_ONLY_COMPLETION = 3;
            
                 //默认显示哪种通知
                private int mNotificationVisibility = VISIBILITY_VISIBLE;
            
                //添加uri
                public Request(Uri uri) {
                    if (uri == null) {
                        throw new NullPointerException();
                    }
                    String scheme = uri.getScheme();
                    if (scheme == null || (!scheme.equals("http") && !scheme.equals("https"))) {
                        throw new IllegalArgumentException("Can only download HTTP/HTTPS URIs: " + uri);
                    }
                    mUri = uri;
                }
            
                Request(String uriString) {
                    mUri = Uri.parse(uriString);
                }
            
                //自定下载文件路径
                public Request setDestinationUri(Uri uri) {
                    mDestinationUri = uri;
                    return this;
                }
            
                //设置系统下载缓存到自定义目录中，适合系统权限的应用
                public Request setDestinationToSystemCache() {
                    mUseSystemCache = true;
                    return this;
                }
            
               //自定义不被媒体扫描的sdcard文件路径
                public Request setDestinationInExternalFilesDir(Context context, String dirType,
                        String subPath) {
                    final File file = context.getExternalFilesDir(dirType);
                    if (file == null) {
                        throw new IllegalStateException("Failed to get external storage files directory");
                    } else if (file.exists()) {
                        if (!file.isDirectory()) {
                            throw new IllegalStateException(file.getAbsolutePath() +
                                    " already exists and is not a directory");
                        }
                    } else {
                        if (!file.mkdirs()) {
                            throw new IllegalStateException("Unable to create directory: "+
                                    file.getAbsolutePath());
                        }
                    }
                    setDestinationFromBase(file, subPath);
                    return this;
                }
            
                //设置自定义的公共sdcard文件路径,可以被媒体扫描到
                public Request setDestinationInExternalPublicDir(String dirType, String subPath) {
                    File file = Environment.getExternalStoragePublicDirectory(dirType);
                    if (file == null) {
                        throw new IllegalStateException("Failed to get external storage public directory");
                    } else if (file.exists()) {
                        if (!file.isDirectory()) {
                            throw new IllegalStateException(file.getAbsolutePath() +
                                    " already exists and is not a directory");
                        }
                    } else {
                        if (!file.mkdirs()) {
                            throw new IllegalStateException("Unable to create directory: "+
                                    file.getAbsolutePath());
                        }
                    }
                    setDestinationFromBase(file, subPath);
                    return this;
                }
            
                private void setDestinationFromBase(File base, String subPath) {
                    if (subPath == null) {
                        throw new NullPointerException("subPath cannot be null");
                    }
                    mDestinationUri = Uri.withAppendedPath(Uri.fromFile(base), subPath);
                }
            
                //若是文件是可以被媒体扫描，在enqueue()前调用
                public void allowScanningByMediaScanner() {
                    mScannable = true;
                }
            
                //添加header
                public Request addRequestHeader(String header, String value) {
                    if (header == null) {
                        throw new NullPointerException("header cannot be null");
                    }
                    if (header.contains(":")) {
                        throw new IllegalArgumentException("header may not contain ':'");
                    }
                    if (value == null) {
                        value = "";
                    }
                    mRequestHeaders.add(Pair.create(header, value));
                    return this;
                }
            
               //设置Notification的显示标题
                public Request setTitle(CharSequence title) {
                    mTitle = title;
                    return this;
                }
            
                //设置显示在notification中内容
                public Request setDescription(CharSequence description) {
                    mDescription = description;
                    return this;
                }
            
                //设置下载的文件类型
                public Request setMimeType(String mimeType) {
                    mMimeType = mimeType;
                    return this;
                }
                //setNotificationVisibility（）所取代
                @Deprecated
                public Request setShowRunningNotification(boolean show) {
                    return (show) ? setNotificationVisibility(VISIBILITY_VISIBLE) :
                            setNotificationVisibility(VISIBILITY_HIDDEN);
                }
            
                //设置Notification得使用
                public Request setNotificationVisibility(int visibility) {
                    mNotificationVisibility = visibility;
                    return this;
                }
            
                 //设置允许下载的网络类型
                public Request setAllowedNetworkTypes(int flags) {
                    mAllowedNetworkTypes = flags;
                    return this;
                }
            
               //设置是否可以漫游状态网络连接,默认true
                public Request setAllowedOverRoaming(boolean allowed) {
                    mRoamingAllowed = allowed;
                    return this;
                }
            
            
            //设置是否可以计量网络连接，默然true
                public Request setAllowedOverMetered(boolean allow) {
                    mMeteredAllowed = allow;
                    return this;
                }
            
                 //设置下载Uri是否可见
                public Request setVisibleInDownloadsUi(boolean isVisible) {
                    mIsVisibleInDownloadsUi = isVisible;
                    return this;
                }
            
                //获取到Request中信息，在系统下载对应的数据库insert
                ContentValues toContentValues(String packageName) {
                    ContentValues values = new ContentValues();
                    assert mUri != null;
                    values.put(Downloads.Impl.COLUMN_URI, mUri.toString());
                    values.put(Downloads.Impl.COLUMN_IS_PUBLIC_API, true);
                    values.put(Downloads.Impl.COLUMN_NOTIFICATION_PACKAGE, packageName);
            
                    if (mDestinationUri != null) {
                        values.put(Downloads.Impl.COLUMN_DESTINATION,  
                        Downloads.Impl.DESTINATION_FILE_URI);
                        values.put(Downloads.Impl.COLUMN_FILE_NAME_HINT, mDestinationUri.toString());
                    } else {
                        values.put(Downloads.Impl.COLUMN_DESTINATION,
                                   (this.mUseSystemCache) ?
                                           Downloads.Impl.DESTINATION_SYSTEMCACHE_PARTITION :
            
                                        Downloads.Impl.DESTINATION_CACHE_PARTITION_PURGEABLE);
                    }
                    // is the file supposed to be media-scannable?
                    values.put(Downloads.Impl.COLUMN_MEDIA_SCANNED, (mScannable) ? SCANNABLE_VALUE_YES   :SCANNABLE_VALUE_NO);
            
                    if (!mRequestHeaders.isEmpty()) {
                        encodeHttpHeaders(values);
                    }
            
                    putIfNonNull(values, Downloads.Impl.COLUMN_TITLE, mTitle);
                    putIfNonNull(values, Downloads.Impl.COLUMN_DESCRIPTION, mDescription);
                    putIfNonNull(values, Downloads.Impl.COLUMN_MIME_TYPE, mMimeType);
            
                    values.put(Downloads.Impl.COLUMN_VISIBILITY, mNotificationVisibility);
                    values.put(Downloads.Impl.COLUMN_ALLOWED_NETWORK_TYPES 
                        , mAllowedNetworkTypes);
                    values.put(Downloads.Impl.COLUMN_ALLOW_ROAMING, mRoamingAllowed);
                    values.put(Downloads.Impl.COLUMN_ALLOW_METERED, mMeteredAllowed);
                    values.put(Downloads.Impl.COLUMN_IS_VISIBLE_IN_DOWNLOADS_UI 
                        , mIsVisibleInDownloadsUi);
            
                    return values;
                }
            
                private void encodeHttpHeaders(ContentValues values) {
                    int index = 0;
                    for (Pair<String, String> header : mRequestHeaders) {
                        String headerString = header.first + ": " + header.second;
                        values.put(Downloads.Impl.RequestHeaders.INSERT_KEY_PREFIX + index 
                        , headerString);
                        index++;
                    }
                }
            
                private void putIfNonNull(ContentValues contentValues, String key, Object value) {
                    if (value != null) {
                        contentValues.put(key, value.toString());
                    }
                }
            }
    2.  **DownloadManager.Query类的源码**:
        
            本类：用于筛选指定条件的下载
            public static class Query {
            //上升排序
            public static final int ORDER_ASCENDING = 1;
            
            //降序
            public static final int ORDER_DESCENDING = 2;
            
            private long[] mIds = null;
            private Integer mStatusFlags = null;
            private String mOrderByColumn = Downloads.Impl.COLUMN_LAST_MODIFICATION;
            private int mOrderDirection = ORDER_DESCENDING;
            private boolean mOnlyIncludeVisibleInDownloadsUi = false;
            
            //给定下载id
            public Query setFilterById(long... ids) {
                mIds = ids;
                return this;
            }
            
            //返回匹配到的下载状态的Query
            public Query setFilterByStatus(int flags) {
                mStatusFlags = flags;
                return this;
            }
            //查询系统的下载UI是否可见，默认false
            public Query setOnlyIncludeVisibleInDownloadsUi(boolean value) {
                mOnlyIncludeVisibleInDownloadsUi = value;
                return this;
            }
            
            
            /*改变查询结果的排序 String column只支持COLUMN_LAST_MODIFIED_TIMESTAMP或者COLUMN_TOTAL_SIZE_BYTES int direction只支持ORDER_ASCENDING，ORDER_DESCENDING */
            public Query orderBy(String column, int direction) {
                if (direction != ORDER_ASCENDING && direction != ORDER_DESCENDING) {
                    throw new IllegalArgumentException("Invalid direction: " + direction);
                }
            
                if (column.equals(COLUMN_LAST_MODIFIED_TIMESTAMP)) {
                    mOrderByColumn = Downloads.Impl.COLUMN_LAST_MODIFICATION;
                } else if (column.equals(COLUMN_TOTAL_SIZE_BYTES)) {
                    mOrderByColumn = Downloads.Impl.COLUMN_TOTAL_BYTES;
                } else {
                    throw new IllegalArgumentException("Cannot order by " + column);
                }
                mOrderDirection = direction;
                return this;
            }
            
            //通过 ContentResolver.query()，返回cursor
            
            Cursor runQuery(ContentResolver resolver, String[] projection, Uri baseUri) {
                Uri uri = baseUri;
                List<String> selectionParts = new ArrayList<String>();
                String[] selectionArgs = null;
            
                if (mIds != null) {
                    selectionParts.add(getWhereClauseForIds(mIds));
                    selectionArgs = getWhereArgsForIds(mIds);
                }
            
                if (mStatusFlags != null) {
                    List<String> parts = new ArrayList<String>();
                    if ((mStatusFlags & STATUS_PENDING) != 0) {
                        parts.add(statusClause("=", Downloads.Impl.STATUS_PENDING));
                    }
                    if ((mStatusFlags & STATUS_RUNNING) != 0) {
                        parts.add(statusClause("=", Downloads.Impl.STATUS_RUNNING));
                    }
                    if ((mStatusFlags & STATUS_PAUSED) != 0) {
                        parts.add(statusClause("=", Downloads.Impl.STATUS_PAUSED_BY_APP));
                        parts.add(statusClause("=", Downloads.Impl.STATUS_WAITING_TO_RETRY));
                        parts.add(statusClause("=", Downloads.Impl.STATUS_WAITING_FOR_NETWORK));
                        parts.add(statusClause("=", Downloads.Impl.STATUS_QUEUED_FOR_WIFI));
                    }
                    if ((mStatusFlags & STATUS_SUCCESSFUL) != 0) {
                        parts.add(statusClause("=", Downloads.Impl.STATUS_SUCCESS));
                    }
                    if ((mStatusFlags & STATUS_FAILED) != 0) {
                        parts.add("(" + statusClause(">=", 400)
                                  + " AND " + statusClause("<", 600) + ")");
                    }
                    selectionParts.add(joinStrings(" OR ", parts));
                }
            
                if (mOnlyIncludeVisibleInDownloadsUi) {
                    selectionParts.add(Downloads.Impl.COLUMN_IS_VISIBLE_IN_DOWNLOADS_UI + " != '0'");
                }
            
                // only return rows which are not marked 'deleted = 1'
                selectionParts.add(Downloads.Impl.COLUMN_DELETED + " != '1'");
            
                String selection = joinStrings(" AND ", selectionParts);
                String orderDirection = (mOrderDirection == ORDER_ASCENDING ? "ASC" : "DESC");
                String orderBy = mOrderByColumn + " " + orderDirection;
            
                return resolver.query(uri, projection, selection, selectionArgs, orderBy);
            }
            
            private String joinStrings(String joiner, Iterable<String> parts) {
                StringBuilder builder = new StringBuilder();
                boolean first = true;
                for (String part : parts) {
                    if (!first) {
                        builder.append(joiner);
                    }
                    builder.append(part);
                    first = false;
                }
                return builder.toString();
            }
            
            private String statusClause(String operator, int value) {
                return Downloads.Impl.COLUMN_STATUS + operator + "'" + value + "'";
             }
            }
    3.  **DownloadManager类的源码**:
        
         *  enqueue（）对应源码：插入一条Requst信息，观察者模式下自动开始下载
            
                private ContentResolver mResolver;
                public long enqueue(Request request) {
                     ContentValues values = request.toContentValues(mPackageName);
                     Uri downloadUri = mResolver.insert(Downloads.Impl.CONTENT_URI, values);
                     long id = Long.parseLong(downloadUri.getLastPathSegment());
                     return id;
                }
         *  remove(）： 更新id对应requst中删除标记
            
                public int remove(long... ids) {
                   return markRowDeleted(ids);
                }
                public int markRowDeleted(long... ids) {
                    if (ids == null || ids.length == 0) {
                      // called with nothing to remove!
                      throw new IllegalArgumentException("input param 'ids' can't be null");
                     }
                    ContentValues values = new ContentValues();
                    values.put(Downloads.Impl.COLUMN_DELETED, 1);
                     // if only one id is passed in, then include it in the uri itself.
                    // this will eliminate a full database scan in the download service.
                    if (ids.length == 1) {
                       return mResolver.update(ContentUris.withAppendedId(mBaseUri, ids[0]),  
                       values,null, null);
                    } 
                   return mResolver.update(mBaseUri, values, getWhereClauseForIds(ids),
                    getWhereArgsForIds(ids));
                }
         *  query（）：查询到一些关于下载的信息
            
                public Cursor query(Query query) {
                   Cursor underlyingCursor = query.runQuery(mResolver,  
                          UNDERLYING_COLUMNS, mBaseUri);
                   if (underlyingCursor == null) {
                         return null;
                   }
                  return new CursorTranslator(underlyingCursor, mBaseUri);
                } 
                //CursorTranslator这个类封装DownloadProvide返回的cursor,通过一些列信息，从而计算出需返回的数据
         *  addCompletedDownload(）：和enqueue（）方法类似，唯一不的是这里有一个默认的Request

private static final String NON_DOWNLOADMANAGER_DOWNLOAD =
                "non-dwnldmngr-download-dont-retry2download";
    
          public long addCompletedDownload(String title, String description,
                boolean isMediaScannerScannable, String mimeType, String path, long length,
                boolean showNotification) {
            return addCompletedDownload(title, description, isMediaScannerScannable, mimeType, path,
                    length, showNotification, false);
          }
    
        public long addCompletedDownload(String title, String description,
                boolean isMediaScannerScannable, String mimeType, String path, long length,
                boolean showNotification, boolean allowWrite) {
         //检查参数是否为空
            validateArgumentIsNonEmpty("title", title);
            validateArgumentIsNonEmpty("description", description);
            validateArgumentIsNonEmpty("path", path);
            validateArgumentIsNonEmpty("mimeType", mimeType);
            if (length < 0) {
                throw new IllegalArgumentException(" invalid value for param: totalBytes");
            }
    
            //在downloads.db中，给一个默认的路径,返回下载的id
            Request request = new Request(NON_DOWNLOADMANAGER_DOWNLOAD)
                    .setTitle(title)
                    .setDescription(description)
                    .setMimeType(mimeType);
            ContentValues values = request.toContentValues(null);
            values.put(Downloads.Impl.COLUMN_DESTINATION,
                    Downloads.Impl.DESTINATION_NON_DOWNLOADMANAGER_DOWNLOAD);
            values.put(Downloads.Impl._DATA, path);
            values.put(Downloads.Impl.COLUMN_STATUS, Downloads.Impl.STATUS_SUCCESS);
            values.put(Downloads.Impl.COLUMN_TOTAL_BYTES, length);
            values.put(Downloads.Impl.COLUMN_MEDIA_SCANNED,
                    (isMediaScannerScannable) ? Request.SCANNABLE_VALUE_YES :
                            Request.SCANNABLE_VALUE_NO);
            values.put(Downloads.Impl.COLUMN_VISIBILITY, (showNotification) ?
                    Request.VISIBILITY_VISIBLE_NOTIFY_ONLY_COMPLETION : Request.VISIBILITY_HIDDEN);
            values.put(Downloads.Impl.COLUMN_ALLOW_WRITE, allowWrite ? 1 : 0);
            Uri downloadUri = mResolver.insert(Downloads.Impl.CONTENT_URI, values);
            if (downloadUri == null) {
                return -1;
            }
            return Long.parseLong(downloadUri.getLastPathSegment());
        }

**通过查看DownloadManager的源码发现：添加下载的任务，移除下载的任务，都是对数据库中数据的增，改操作。然后结合观察者模式实现自动调配，下载任务。**

[https_developer.android.com_reference_android_app_DownloadManager.html]: https://developer.android.com/reference/android/app/DownloadManager.html
[20160822174039181]: /images/20220717/d601ee921e5440f2bca1b821d2fac9ce.png
[20160822175725452]: /images/20220717/8b2c332a16fa4123a33e4ec56355a4f2.png
[http_download.csdn.net_detail_hexingen_9609609]: http://download.csdn.net/detail/hexingen/9609609