Android(安卓)4.0 gallery2 生成video thumbnail的过程

通过print callstack得知gallery里面获得thumbnail 是通过LocalVideo.requestImage往下调用的：

com.android.gallery3d.data.LocalVideo.requestImage(LocalVideo.java:143)

com.android.gallery3d.ui.AlbumSlidingWindow$AlbumDisplayItem.startLoadBitmap(AlbumSlidingWindow.java:372)

com.android.gallery3d.ui.AbstractDisplayItem.requestImage(AbstractDisplayItem.java:81)

在LocalVideo.java里面直接调用return LocalVideoRequest对象：

所以简单的说thumbnail就是LocalVideoRequest得到的，细节就要往下看了。

@Override                                                                 public Job requestImage(int type) {    return new LocalVideoRequest(mApplication, getPath(), type, filePath);}

LocalVideoRequest直接调用ImageCacheRequest的实现

LocalVideoRequest里面override了onDecodeOriginal方法

public static class LocalVideoRequest extends ImageCacheRequest {    private String mLocalFilePath;    LocalVideoRequest(GalleryApp application, Path path, int type,            String localFilePath) {        super(application, path, type, LocalImage.getTargetSize(type));        mLocalFilePath = localFilePath;    }    @Override    public Bitmap onDecodeOriginal(JobContext jc, int type) {        Bitmap bitmap = BitmapUtils.createVideoThumbnail(mLocalFilePath);        if (bitmap == null || jc.isCancelled()) return null;                     return bitmap;                                                       }                                                                    }

ImageCacheRequest：

1. 首先创建ImageCacheService对象[在ImageCacheRequest::run()]

ImageCacheService cacheService = mApplication.getImageCacheService();

2. 通过CacheService.getImageData获取ImageData

ImageData data = cacheService.getImageData(mPath, mType);

3. 如果data不为null，对data进行decode，然后return，

如果data为空就调用上面说到的onDecodeOriginal创建新的thumbnail

再把经过处理的bitmap写进文件里面（cacheService.putImageData）

if (data != null) {                                                      BitmapFactory.Options options = new BitmapFactory.Options();         options.inPreferredConfig = Bitmap.Config.ARGB_8888;    Bitmap bitmap = DecodeUtils.requestDecode(jc, data.mData,                    data.mOffset, data.mData.length - data.mOffset, options);    if (bitmap == null && !jc.isCancelled()) {                               Log.w(TAG, "decode cached failed " + debugTag);                  }    return bitmap;} else {    Bitmap bitmap = onDecodeOriginal(jc, mType);    if (jc.isCancelled()) return null;    if (bitmap == null) {        Log.w(TAG, "decode orig failed " + debugTag);        return null;    }    if (mType == MediaItem.TYPE_MICROTHUMBNAIL) {        bitmap = BitmapUtils.resizeDownAndCropCenter(bitmap,                mTargetSize, true);    } else {        bitmap = BitmapUtils.resizeDownBySideLength(bitmap,                mTargetSize, true);    }    if (jc.isCancelled()) return null;    byte[] array = BitmapUtils.compressBitmap(bitmap);    if (jc.isCancelled()) return null;    cacheService.putImageData(mPath, mType, array);    return bitmap;}

ImageCacheService的实现：

有一个private对象: private BlobCache mCache;

通过在BlobCache.java增加log，BlobCache的路径为：

/mnt/sdcard/Android/data/com.android.gallery3d/cache/imgcache

// Creates the cache. Three files will be created:// path + ".idx", path + ".0", and path + ".1"// The ".0" file and the ".1" file each stores data for a region. Each of// them can grow to the size specified by maxBytes. The maxEntries parameter// specifies the maximum number of entries each region can have. If the// "reset" parameter is true, the cache will be cleared before use.public BlobCache(String path, int maxEntries, int maxBytes, boolean reset)        throws IOException {    this(path, maxEntries, maxBytes, reset, 0);}public BlobCache(String path, int maxEntries, int maxBytes, boolean reset,        int version) throws IOException {    mIndexFile = new RandomAccessFile(path + ".idx", "rw");    mDataFile0 = new RandomAccessFile(path + ".0", "rw");    mDataFile1 = new RandomAccessFile(path + ".1", "rw");    mVersion = version;

Gallery的cache目录:

/mnt/sdcard/Android/data/com.android.gallery3d/cache/

imgcache.0

imgcache.1

imgcache.idx

BlobCache的描述(BlobCache.java)：

// This is an on-disk cache which maps a 64-bits key to a byte array.//// It consists of three files: one index file and two data files. One of the// data files is "active", and the other is "inactive". New entries are// appended into the active region until it reaches the size limit. At that// point the active file and the inactive file are swapped, and the new active// file is truncated to empty (and the index for that file is also cleared).// The index is a hash table with linear probing. When the load factor reaches// 0.5, it does the same thing like when the size limit is reached.//// The index file format: (all numbers are stored in little-endian)// [0]  Magic number: 0xB3273030// [4]  MaxEntries: Max number of hash entries per region.// [8]  MaxBytes: Max number of data bytes per region (including header).// [12] ActiveRegion: The active growing region: 0 or 1.// [16] ActiveEntries: The number of hash entries used in the active region.// [20] ActiveBytes: The number of data bytes used in the active region.// [24] Version number.// [28] Checksum of [0..28).// [32] Hash entries for region 0. The size is X = (12 * MaxEntries bytes).// [32 + X] Hash entries for region 1. The size is also X.//// Each hash entry is 12 bytes: 8 bytes key and 4 bytes offset into the data// file. The offset is 0 when the slot is free. Note that 0 is a valid value// for key. The keys are used directly as index into a hash table, so they// should be suitably distributed.//// Each data file stores data for one region. The data file is concatenated// blobs followed by the magic number 0xBD248510.//// The blob format:// [0]  Key of this blob// [8]  Checksum of this blob// [12] Offset of this blob// [16] Length of this blob (not including header)// [20] Blob//// Below are the interface for BlobCache. The instance of this class does not// support concurrent use by multiple threads.//// public BlobCache(String path, int maxEntries, int maxBytes, boolean reset) throws IOException;// public void insert(long key, byte[] data) throws IOException;// public byte[] lookup(long key) throws IOException;// public void lookup(LookupRequest req) throws IOException;// public void close();// public void syncIndex();// public void syncAll();// public static void deleteFiles(String path);

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