MediaStore 中的 BUCKET_ID 是什么？

Question

我正在浏览一些查询 MediaStore.Images 内容提供者并使用名为 BUCKET_ID 的列的代码：

Uri uri = MediaStore.Images.Media.EXTERNAL_CONTENT_URI;
String projection[] = {
    "DISTINCT " + MediaStore.Images.Media.BUCKET_DISPLAY_NAME,
    MediaStore.Images.Media.BUCKET_ID,
    MediaStore.Images.Media.DATE_TAKEN,
    MediaStore.Images.Media.DATA
};
String BUCKET_GROUP_BY = "1) GROUP BY 1,(2";
String BUCKET_ORDER_BY = "MAX(datetaken) DESC";
Cursor imagecursor = getContentResolver().query(uri, projection, BUCKET_GROUP_BY, null, BUCKET_ORDER_BY);

一般来说，这些列代表什么？谁能用例子解释一下？（文档有一个关于这些列的单行说明，解释得很少）

Answer 1

文档对此非常不清楚，上面的答案是从文档中复制粘贴的。无论如何，我以评论的形式从Gallery 3D的来源中找到了答案：

    // 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.
    //
    // 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.

这是computeBucketValues()供参考：

/**
     * @param data The input path
     * @param values the content values, where the bucked id name and bucket display name are updated.
     *
     */
    private static void computeBucketValues(String data, ContentValues values) {
        File parentFile = new File(data).getParentFile();
        if (parentFile == null) {
            parentFile = new File("/");
        }

        // Lowercase the path for hashing. This avoids duplicate buckets if the
        // filepath case is changed externally.
        // Keep the original case for display.
        String path = parentFile.toString().toLowerCase();
        String name = parentFile.getName();

        // Note: the BUCKET_ID and BUCKET_DISPLAY_NAME attributes are spelled the
        // same for both images and video. However, for backwards-compatibility reasons
        // there is no common base class. We use the ImageColumns version here
        values.put(ImageColumns.BUCKET_ID, path.hashCode());
        values.put(ImageColumns.BUCKET_DISPLAY_NAME, name);
}