Android源代码之Gallery专题研究(1)
前言
时光飞逝,从事Android系统开发已经两年了,总想写点什么来安慰自己。思考了非常久总是无法下笔,认为没什么好写的。如今最终决定写一些符合大多数人需求的东西,想必使用过Android手机的人们一定对“图库”(下面简称Gallery)这个应用非常熟悉。在Android市场里面有各种关于图库的应用,他们的最初原型事实上就是Android系统原生“图库”,仅仅是做了不同的差异化而已(UI差异化)。在研究Gallery源代码之前,我们须要对设计模式有一定的了解,依据自己对Gallery的了解,Gallery的设计就好比是一座设计精良的而且高效运转的机器(32个攒)。毫不夸张地说,在Android市场里,没有一款“图库”应用的设计设计可以和Gallery媲美。接下来的一段时间,就让我们共同来揭开Gallery的神奇面纱。
数据载入
在研究Gallery之前,我们还是来赞赏一下Gallery的总体效果,详细见图1-1所看到的:
图1-1
首先我们先来看一下Gallery的发展历史,在Android2.3之前Android系统的“图库”名为Gallery3D,在Android2.3之后系统将之前的Gallery3D更改为Gallery2,一直沿用到眼下最新版本号(4.4),Gallery2在UI和功能上面做了质的飞跃,是眼下Android源代码中很优秀的模块,对于Android应用开发人员来说是很好的开源项目,当中的设计新思想和设计模式都值得我们借鉴。
如今回到我们研究的主题-数据载入,我们先来看一下Gallery2在源代码中的路径(package/app/Gallery2/),在该路径下包括了“图库”使用的资源和源代码。我们在设计一款软件的时候首先考虑的是数据的存储和訪问,因此我们也依照这种设计思路来探究Gallery2的数据载入过程。讲到这儿略微提一下我分析源代码的方式,可能大家对Android源代码略微有一点了解的同学应该知道,Android源代码是很庞大的,因此选择分析程序的切入点大致能够分为两类:第一种是依照操程序操作步骤分析源代码——适用于界面跳转清晰的程序;另外一种是依据打印的Log信息分析程序的执行逻辑——适用于复杂的操作逻辑。
首先我们先来看一下BucketHelper.java类(/src/com/android/gallery3d/data/BucketHelper.java),该类主要是负责读取MediaProvider数据库中Image和Video数据,详细代码例如以下所看到的:
package com.android.gallery3d.data; import android.annotation.TargetApi; import android.content.ContentResolver; import android.database.Cursor; import android.net.Uri; import android.provider.MediaStore.Files; import android.provider.MediaStore.Files.FileColumns; import android.provider.MediaStore.Images; import android.provider.MediaStore.Images.ImageColumns; import android.provider.MediaStore.Video; import android.util.Log; import com.android.gallery3d.common.ApiHelper; import com.android.gallery3d.common.Utils; import com.android.gallery3d.util.ThreadPool.JobContext; import java.util.ArrayList; import java.util.Arrays; import java.util.Comparator; import java.util.HashMap; class BucketHelper { private static final String TAG = "BucketHelper"; private static final String EXTERNAL_MEDIA = "external"; // BUCKET_DISPLAY_NAME is a string like "Camera" which is the directory // name of where an image or video is in. BUCKET_ID is a hash of the path // name of that directory (see computeBucketValues() in MediaProvider for // details). MEDIA_TYPE is video, image, audio, etc. // BUCKET_DISPLAY_NAME字段为文件文件夹名称 BUCKET_ID字段为文件夹路径(path)的HASH值 // The "albums" are not explicitly recorded in the database, but each image // or video has the two columns (BUCKET_ID, MEDIA_TYPE). We define an // "album" to be the collection of images/videos which have the same value // for the two columns. // "专辑"的划分方式为:当文件具有同样的文件夹(BUCKET_ID)和多媒体类型(MEDIA_TYPE)即属于同一专辑 // The goal of the query (used in loadSubMediaSetsFromFilesTable()) is to // find all albums, that is, all unique values for (BUCKET_ID, MEDIA_TYPE). // In the meantime sort them by the timestamp of the latest image/video in // each of the album. // // The order of columns below is important: it must match to the index in // MediaStore. private static final String[] PROJECTION_BUCKET = { ImageColumns.BUCKET_ID, FileColumns.MEDIA_TYPE, ImageColumns.BUCKET_DISPLAY_NAME}; // The indices should match the above projections. private static final int INDEX_BUCKET_ID = 0; private static final int INDEX_MEDIA_TYPE = 1; private static final int INDEX_BUCKET_NAME = 2; // We want to order the albums by reverse chronological order. We abuse the // "WHERE" parameter to insert a "GROUP BY" clause into the SQL statement. // The template for "WHERE" parameter is like: // SELECT ... FROM ... WHERE (%s) // and we make it look like: // SELECT ... FROM ... WHERE (1) GROUP BY 1,(2) // The "(1)" means true. The "1,(2)" means the first two columns specified // after SELECT. Note that because there is a ")" in the template, we use // "(2" to match it. private static final String BUCKET_GROUP_BY = "1) GROUP BY 1,(2"; private static final String BUCKET_ORDER_BY = "MAX(datetaken) DESC"; // Before HoneyComb there is no Files table. Thus, we need to query the // bucket info from the Images and Video tables and then merge them // together. // // A bucket can exist in both tables. In this case, we need to find the // latest timestamp from the two tables and sort ourselves. So we add the // MAX(date_taken) to the projection and remove the media_type since we // already know the media type from the table we query from. private static final String[] PROJECTION_BUCKET_IN_ONE_TABLE = { ImageColumns.BUCKET_ID, "MAX(datetaken)", ImageColumns.BUCKET_DISPLAY_NAME}; // We keep the INDEX_BUCKET_ID and INDEX_BUCKET_NAME the same as // PROJECTION_BUCKET so we can reuse the values defined before. private static final int INDEX_DATE_TAKEN = 1; // When query from the Images or Video tables, we only need to group by BUCKET_ID. private static final String BUCKET_GROUP_BY_IN_ONE_TABLE = "1) GROUP BY (1"; public static BucketEntry[] loadBucketEntries( JobContext jc, ContentResolver resolver, int type) { if (ApiHelper.HAS_MEDIA_PROVIDER_FILES_TABLE) {//当API1>= 11(即Android3.0版本号之后) return loadBucketEntriesFromFilesTable(jc, resolver, type);//获取MediaScanner数据库中多媒体文件(图片和视频)的文件夹路径和文件夹名称 } else {//Android3.0之前版本号 return loadBucketEntriesFromImagesAndVideoTable(jc, resolver, type); } } private static void updateBucketEntriesFromTable(JobContext jc, ContentResolver resolver, Uri tableUri, HashMap<Integer, BucketEntry> buckets) { Cursor cursor = resolver.query(tableUri, PROJECTION_BUCKET_IN_ONE_TABLE, BUCKET_GROUP_BY_IN_ONE_TABLE, null, null); if (cursor == null) { Log.w(TAG, "cannot open media database: " + tableUri); return; } try { while (cursor.moveToNext()) { int bucketId = cursor.getInt(INDEX_BUCKET_ID); int dateTaken = cursor.getInt(INDEX_DATE_TAKEN); BucketEntry entry = buckets.get(bucketId); if (entry == null) { entry = new BucketEntry(bucketId, cursor.getString(INDEX_BUCKET_NAME)); buckets.put(bucketId, entry); entry.dateTaken = dateTaken; } else { entry.dateTaken = Math.max(entry.dateTaken, dateTaken); } } } finally { Utils.closeSilently(cursor); } } private static BucketEntry[] loadBucketEntriesFromImagesAndVideoTable( JobContext jc, ContentResolver resolver, int type) { HashMap<Integer, BucketEntry> buckets = new HashMap<Integer, BucketEntry>(64); if ((type & MediaObject.MEDIA_TYPE_IMAGE) != 0) { updateBucketEntriesFromTable( jc, resolver, Images.Media.EXTERNAL_CONTENT_URI, buckets); } if ((type & MediaObject.MEDIA_TYPE_VIDEO) != 0) { updateBucketEntriesFromTable( jc, resolver, Video.Media.EXTERNAL_CONTENT_URI, buckets); } BucketEntry[] entries = buckets.values().toArray(new BucketEntry[buckets.size()]); Arrays.sort(entries, new Comparator<BucketEntry>() { @Override public int compare(BucketEntry a, BucketEntry b) { // sorted by dateTaken in descending order return b.dateTaken - a.dateTaken; } }); return entries; } private static BucketEntry[] loadBucketEntriesFromFilesTable( JobContext jc, ContentResolver resolver, int type) { Uri uri = getFilesContentUri(); Cursor cursor = resolver.query(uri, PROJECTION_BUCKET, BUCKET_GROUP_BY, null, BUCKET_ORDER_BY); if (cursor == null) { Log.w(TAG, "cannot open local database: " + uri); return new BucketEntry[0]; } ArrayList<BucketEntry> buffer = new ArrayList<BucketEntry>(); int typeBits = 0; if ((type & MediaObject.MEDIA_TYPE_IMAGE) != 0) { typeBits |= (1 << FileColumns.MEDIA_TYPE_IMAGE); } if ((type & MediaObject.MEDIA_TYPE_VIDEO) != 0) { typeBits |= (1 << FileColumns.MEDIA_TYPE_VIDEO); } try { while (cursor.moveToNext()) { if ((typeBits & (1 << cursor.getInt(INDEX_MEDIA_TYPE))) != 0) { BucketEntry entry = new BucketEntry( cursor.getInt(INDEX_BUCKET_ID), cursor.getString(INDEX_BUCKET_NAME));//构造元数据BucketEntry if (!buffer.contains(entry)) { buffer.add(entry);//加入数据信息 } } if (jc.isCancelled()) return null; } } finally { Utils.closeSilently(cursor); } return buffer.toArray(new BucketEntry[buffer.size()]); } private static String getBucketNameInTable( ContentResolver resolver, Uri tableUri, int bucketId) { String selectionArgs[] = new String[] {String.valueOf(bucketId)}; Uri uri = tableUri.buildUpon() .appendQueryParameter("limit", "1") .build(); Cursor cursor = resolver.query(uri, PROJECTION_BUCKET_IN_ONE_TABLE, "bucket_id = ?", selectionArgs, null); try { if (cursor != null && cursor.moveToNext()) { return cursor.getString(INDEX_BUCKET_NAME); } } finally { Utils.closeSilently(cursor); } return null; } @TargetApi(ApiHelper.VERSION_CODES.HONEYCOMB) private static Uri getFilesContentUri() { return Files.getContentUri(EXTERNAL_MEDIA); } public static String getBucketName(ContentResolver resolver, int bucketId) { if (ApiHelper.HAS_MEDIA_PROVIDER_FILES_TABLE) { String result = getBucketNameInTable(resolver, getFilesContentUri(), bucketId); return result == null ? "" : result; } else { String result = getBucketNameInTable( resolver, Images.Media.EXTERNAL_CONTENT_URI, bucketId); if (result != null) return result; result = getBucketNameInTable( resolver, Video.Media.EXTERNAL_CONTENT_URI, bucketId); return result == null ? "" : result; } } public static class BucketEntry { public String bucketName; public int bucketId; public int dateTaken; public BucketEntry(int id, String name) { bucketId = id; bucketName = Utils.ensureNotNull(name); } @Override public int hashCode() { return bucketId; } @Override public boolean equals(Object object) { if (!(object instanceof BucketEntry)) return false; BucketEntry entry = (BucketEntry) object; return bucketId == entry.bucketId; } } }
接下来我们再来看看BucketHelper类的调用关系的时序图,详细如1-2所看到的:
到眼下为止我们大致了解了Gallery数据载入的一个大体流程,接下来的文章将分析Album数据的读取以及数据封装。
郑重声明:本站内容如果来自互联网及其他传播媒体,其版权均属原媒体及文章作者所有。转载目的在于传递更多信息及用于网络分享,并不代表本站赞同其观点和对其真实性负责,也不构成任何其他建议。