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  • 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:

        

     图1-2


    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。



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  • 原文地址:https://www.cnblogs.com/bhlsheji/p/4048469.html
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