Android源碼之Gallery專題研究（1）

前言

時光飛逝，從事Android系統開發已經兩年了，總想寫點什麼來安慰自己。思考了很久總是無法下筆，覺得沒什麼好寫的。現在終於決定寫一些符合大多數人需求的東西，想必使用過Android手機的人們一定對“圖庫”（以下簡稱Gallery）這個應用非常熟悉。在Android市場裡面有各種關於圖庫的應用，他們的最初原型其實就是Android系統原生“圖庫”，只是做了不同的差異化而已（UI差異化）。在研究Gallery源碼之前，我們需要對設計模式有一定的瞭解，根據自己對Gallery的瞭解，Gallery的設計就好比是一座設計精良的並且高效運轉的機器（32個攢）。毫不誇張地說，在Android市場裡，沒有一款“圖庫”應用的設計設計能夠和Gallery媲美。接下來的一段時間，就讓我們共同來揭開Gallery的神秘面紗。

資料載入

在研究Gallery之前，我們還是來欣賞一下Gallery的整體效果，具體見圖1-1所示：

圖1-1

首先我們先來看一下Gallery的發展曆史，在Android2.3之前Android系統的“圖庫”名為Gallery3D，在Android2.3之後系統將之前的Gallery3D更改為Gallery2，一直沿用到目前最新版本（4.4），Gallery2在UI和功能上面做了質的飛躍，是目前Android源碼中非常優秀的模組，對於Android應用開發人員來說是非常好的開源項目，其中的設計新思想和設計模式都值得我們借鑒。

現在回到我們研究的主題-資料載入，我們先來看一下Gallery2在源碼中的路徑(package/app/Gallery2/),在該路徑下包含了“圖庫”使用的資源和源碼。我們在設計一款軟體的時候首先考慮的是資料的儲存和訪問，因此我們也按照這樣的設計思路來探究Gallery2的資料載入過程。說到這兒稍微提一下我分析源碼的方式，可能大家對Android源碼稍微有一點瞭解的同學應該知道，Android源碼是非常龐大的，因此選擇剖析器的切入點大致可以分為兩類：第一種是按照操程式操作步驟分析源碼——適用於介面跳轉清晰的程式；第二種是根據列印的Log資訊剖析器的運行邏輯——適用於複雜的操作邏輯。

首先我們先來看一下BucketHelper.java類（/src/com/android/gallery3d/data/BucketHelper.java），該類主要是負責讀取MediaProvider資料庫中Image和Video資料，具體代碼如下所示：

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欄位為檔案目錄名稱 BUCKET_ID欄位為目錄路徑（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.    // "專輯"的劃分方式為：當檔案具有相同的目錄（BUCKET_ID）和多媒體類型（MEDIA_TYPE）即屬於同一專輯    // 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) {//當API1>= 11(即Android3.0版本之後)            return loadBucketEntriesFromFilesTable(jc, resolver, type);//擷取MediaScanner資料庫中多媒體檔案（圖片和視頻）的目錄路徑和目錄名稱        } else {//Android3.0之前版本            return loadBucketEntriesFromImagesAndVideoTable(jc, resolver, type);        }    }    private static void updateBucketEntriesFromTable(JobContext jc,            ContentResolver resolver, Uri tableUri, HashMap 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 buckets = new HashMap(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() {            @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 buffer = new ArrayList();        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));//構造中繼資料BucketEntry                    if (!buffer.contains(entry)) {                        buffer.add(entry);//添加資料資訊                    }                }                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;        }    }}

接下來我們再來看看BucketHelper類的調用關係的時序圖，具體如1-2所示：

圖1-2

到目前為止我們大致瞭解了Gallery資料載入的一個大體流程，接下來的文章將分析Album資料的讀取以及資料封裝。