我想要实现的目标是什么？

>在设备上查询缩略图和图像(通过MediaStore)
>将这些加入一个光标,按顺序降序(最新的图像在上面)
>处理丢失缩略图的情况

经过大量的试验和错误,以及使用MediaStore,我了解到在任何给定时间都不能指望缩略图表(MediaStore.Images.Thumbnails)是最新的.将丢失缩略图的图像,反之亦然(孤立的缩略图).特别是当相机应用拍摄新照片时,显然它不会立即创建缩略图.直到Gallery应用程序(或等效的)打开,缩略图表才会更新.

关于如何解决这个问题,我得到了各种有用的建议,主要集中在查询图像表(MediaStore.Images.Media),然后,不知何故,一次一行地用缩略图扩展光标.虽然这确实有效,但它导致应用程序非常慢并且在我的设备上消耗了大量内存〜2000个图像.

真的应该可以简单地使用图像表JOIN(左外连接)缩略图表,这样我们就可以得到所有图像和缩略图.否则,我们将缩略图DATA列保留为null,并自己生成那些特定的缺失缩略图.真正酷的是将这些缩略图实际插入MediaStore,但我还没有考虑过.

所有这一切的主要问题是使用CursorJoiner.由于某种原因,它要求两个游标按升序排序,让我们说ID.然而,这意味着首先是最古老的图像,这真的是一个糟糕的画廊应用程序.我发现CursorJoiner可以被“欺骗”,但是,通过简单地按ID *( – 1)排序允许降序：

Cursor c_thumbs = getContext().getContentResolver().query(
                    MediaStore.Images.Thumnails.EXTERNAL_CONTENT_URI,null,"(" + MediaStore.Images.Thumnails.IMAGE_ID + "*(-1))");

Cursor c_images= getContext().getContentResolver().query(
                    MediaStore.Images.Media.EXTERNAL_CONTENT_URI,"(" + MediaStore.Images.Media._ID + "*(-1))");

但是,只要行匹配,就可以正常工作(两种情况).但是当你遇到光标是唯一的行(LEFT或RIGHT情况)时,反向排序会混淆CursorJoiner类的内部工作.但是,左右光标的简单补偿足以“重新对齐”连接,使其恢复正常.注意moveToNext()和moveToPrevIoUs()调用.

// join these and return
// the join is on images._ID = thumbnails.IMAGE_ID
CursorJoiner joiner = new CursorJoiner(
        c_thumbs,new String[] { MediaStore.Images.Thumnails.IMAGE_ID },// left = thumbnails
        c_images,new String[] { MediaStore.Images.Media._ID }   // right = images
);

String[] projection = new String{"thumb_path","ID","title","desc","datetaken","filename","image_path"};

MatrixCursor retCursor = new MatrixCursor(projection);

try {
    for (CursorJoiner.Result joinerResult : joiner) {

        switch (joinerResult) {
            case LEFT:
                // handle case where a row in cursorA is unique
                // images is unique (missing thumbnail)

                // we want to show ALL images,even (new) ones without thumbnail!
                // data = null will cause a temporary thumbnail to be generated in PhotoAdapter.bindView()

                retCursor.addRow(new Object[]{
                        null,// data
                        c_images.getLong(1),// image id
                        c_images.getString(2),// title
                        c_images.getString(3),// desc
                        c_images.getLong(4),// date
                        c_images.getString(5),// filename
                        c_images.getString(6)
                });

                // compensate for CursorJoiner expecting cursors ordered ascending...
                c_images.moveToNext();
                c_thumbs.moveToPrevIoUs();
                break;

            case RIGHT:
                // handle case where a row in cursorB is unique
                // thumbs is unique (missing image)

                // compensate for CursorJoiner expecting cursors ordered ascending...
                c_thumbs.moveToNext();
                c_images.moveToPrevIoUs();
                break;

            case BOTH:

                // handle case where a row with the same key is in both cursors
                retCursor.addRow(new Object[]{
                        c_thumbs.getString(1),// filename
                        c_images.getString(6)
                });

                break;
        }
    }
} catch (Exception e) {
    Log.e("myapp","JOIN Failed: " + e);
}

c_thumbs.close();
c_images.close();

return retCursor;

然后,在“PhotoAdapter”类中,它为我的GridView创建元素,并将数据从ContentProvider返回的光标(上面的retCursor)中绑定到这些元素中,我按以下方式创建缩略图(当thumb_path字段为null时)：

String thumbData = cursor.getString(0);  // thumb_path
if (thumbData != null) {
    Bitmap thumbBitmap;
    try {
        thumbBitmap = BitmapFactory.decodeFile(thumbData);
        viewHolder.iconView.setImageBitmap(thumbBitmap);
    } catch (Exception e) {
        Log.e("myapp","PhotoAdapter.bindView() can't find thumbnail (file) on disk (thumbdata = " + thumbData + ")");
        return;
    }

} else {

    String imgPath = cursor.getString(6);   // image_path
    String imgId = cursor.getString(1);  // ID 
    Log.v("myapp","PhotoAdapter.bindView() thumb path for image ID " + imgId + " is null. Trying to generate,with path = " + imgPath);

    try {
        Bitmap thumbBitmap = ThumbnailUtils.extractThumbnail(BitmapFactory.decodeFile(imgPath),512,384);
        viewHolder.iconView.setImageBitmap(thumbBitmap);
    }  catch (Exception e) {
        Log.e("myapp","PhotoAdapter.bindView() can't generate thumbnail for image path: " + imgPath);
        return;
    }
}