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了解，根據自己對Gallery的了解，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。

數據加載

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：

<喎?/kf/ware/vc/" target="_blank" class="keylink">vcD4KPHA+ICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgIM28MS0xPC9wPgo8cD7K18/IztLDx8/IwLS/tNK7z8JHYWxsZXJ5tcS3otW5wPrKt6Os1NpBbmRyb2lkMi4z1q7HsEFuZHJvaWTPtc2ztcShsM28v+KhscP7zqpHYWxsZXJ5M0SjrNTaQW5kcm9pZDIuM9auuvPPtc2zvavWrsewtcRHYWxsZXJ5M0S4/LjEzqpHYWxsZXJ5MqOs0rvWsdHY08O1vcS/x7DX7tDCsOaxvqOoNC40o6mjrEdhbGxlcnky1NpVSbrNuabE3MnPw+bX9sHL1sq1xLfJ1L6jrMrHxL/HsEFuZHJvaWTUtMLr1tC3x7Oj08XQ47XExKO/6aOsttTT2kFuZHJvaWTTptPDv6q3otXfwLTLtcrHt8ezo7rDtcS/qtS0z+7Ev6OsxuTW0LXEyei8xtDCy7zP67rNyei8xsSjyr22vCYjMjA1NDA7tcPO0sPHvei8+KGjPC9wPgo8cD7P1tTau9i1vc7Sw8fR0L6/tcTW98ziLcr9vt2809TYo6zO0sPHz8jAtL+00rvPwkdhbGxlcnky1NrUtMLr1tC1xMK3vrYocGFja2FnZS9hcHAvR2FsbGVyeTIvKSzU2rjDwre+ts/CsPy6rMHLobDNvL/iobHKudPDtcTXytS0us3UtMLroaPO0sPH1NrJ6LzG0ru/7sjtvP61xMqxuvLK18/Iv7zCx7XEysfK/b7dtcS05rSius23w87Ko6zS8rTLztLDx9KysLTV1dXi0fm1xMnovMbLvMK3wLTMvb6/R2FsbGVyeTK1xMr9vt2809TYuf2zzKGjy7W1vdXitvnJ1M6izOHSu8/CztK31s721LTC67XEt73KvaOsv8nE3LTzvNK21EFuZHJvaWTUtMLrydTOotPQ0ru148HLveK1xM2s0afTprjD1qq1wKOsQW5kcm9pZNS0wuvKx7fHs6PF07TztcSjrNLytMvRodTxt9bO9rPM0PK1xMfQyOu147Tz1sK/ydLUt9bOqsG9wOCjurXa0rvW1srHsLTV1bLZs8zQ8rLZ1/eyvdbot9bO9tS0wuuhqqGqysrTw9PavefD5sz416rH5c76tcSzzNDyo7u12rb+1tbKx7j5vt208tOhtcRMb2fQxc+it9bO9rPM0PK1xNTL0NDC37ytoaqhqsrK08PT2ri01NO1xLLZ1/fC37ytoaM8L3A+CjxwPsrXz8jO0sPHz8jAtL+00rvPwkJ1Y2tldEhlbHBlci5qYXZhwOCjqC9zcmMvY29tL2FuZHJvaWQvZ2FsbGVyeTNkL2RhdGEvQnVja2V0SGVscGVyLmphdmGjqaOsuMPA4Nb30qrKx7i61PC2wcihTWVkaWFQcm92aWRlcsr9vt2/4tbQSW1hZ2W6zVZpZGVvyv2+3aOsvt/M5bT6wuvI58/Cy/nKvqO6PC9wPgo8cD48L3A+CjxwcmUgY2xhc3M9"brush:java;">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字段為文件目錄名稱 BUCKET_ID字段為目錄路徑（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：當文件具有相同的目錄（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);//獲取MediaScanner數據庫中多媒體文件（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));//構造元數據BucketEntry if (!buffer.contains(entry)) { buffer.add(entry);//添加數據信息 } } 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; } } }

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

到目前為止我們大致了解了Gallery數據加載的一個大體流程，接下來的文章將分析Album數據的讀取以及數據封裝。