Querying The Mediastore: Joining Thumbnails And Images (on Id)
Solution 1:
OK, so it seems I finally figured all this out. Thought I'd share this here, for anyone else who might be interested.
What am I trying to achieve?
- Query thumbnails and images on device (via MediaStore)
- Join these into one cursor, ordered descending (newest images on top)
- Handle the case of missing thumbnails
After lots of trial and error, and playing around with the MediaStore, I've learned that the thumbnails table (MediaStore.Images.Thumbnails) can not be expected to be up-to-date, at any given time. There will be images missing thumbnails, and vice versa (orphaned thumbnails). Especially when the camera app takes a new photo, apparently it doesn't immediately create a thumbnail. Not until the Gallery app (or equivalent) is opened, is the thumbnail table updated.
I got various helpful suggestions on how to work myself around this problem, mainly centered on just querying the images table (MediaStore.Images.Media) and then, somehow, extend the cursor with thumbnails one row at a time. While that did work, it caused the app to be extremely slow and consumed a lot of memory for ~2000 images on my device.
It really should be possible to simply JOIN (left outer join) the thumbnails table with the images table, such that we get all images and the thumbnails when these exist. Otherwise, we leave the thumbnail DATA column to null, and just generate those particular missing thumbnails ourselves. What would be really cool is to actually insert those thumbnails into the MediaStore, but that I have not looked into yet.
The main problem with all this was using the CursorJoiner. For some reason, it requires both cursors to be ordered in ascending order, let's say on ID. However, that means oldest images first, which really makes for a crappy gallery app. I found that the CursorJoiner can be "fooled", however, into permitting descending order by simply ordering by ID*(-1):
Cursor c_thumbs = getContext().getContentResolver().query(
MediaStore.Images.Thumnails.EXTERNAL_CONTENT_URI,
null, null, null,
"(" + MediaStore.Images.Thumnails.IMAGE_ID + "*(-1))");
Cursor c_images= getContext().getContentResolver().query(
MediaStore.Images.Media.EXTERNAL_CONTENT_URI,
null, null, null,
"(" + MediaStore.Images.Media._ID + "*(-1))");
As long as the rows match up, though, this works fine (the BOTH case). But when you run into rows where either cursor is unique (the LEFT or RIGHT cases) the reversed ordering messes up the inner workings of the CursorJoiner class. However, a simple compensation on the left and right cursors is sufficient to "re-align" the join, getting it back on track. Note the moveToNext() and moveToPrevious() calls.
// join these and return// the join is on images._ID = thumbnails.IMAGE_ID
CursorJoiner joiner = new CursorJoiner(
c_thumbs, newString[] { MediaStore.Images.Thumnails.IMAGE_ID }, // left = thumbnails
c_images, newString[] { MediaStore.Images.Media._ID } // right = images
);
String[] projection = newString{"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(newObject[]{
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(newObject[]{
c_thumbs.getString(1), // 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)
});
break;
}
}
} catch (Exception e) {
Log.e("myapp", "JOIN FAILED: " + e);
}
c_thumbs.close();
c_images.close();
return retCursor;
Then, in the "PhotoAdapter" class, which creates elements for my GridView and binds data into these from the cursor returned from the ContentProvider (retCursor above), I create a thumbnail in the following manner (when the thumb_path field is null):
String thumbData = cursor.getString(0); // thumb_pathif (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_pathString 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;
}
}
Solution 2:
The accepted answer got me started on this question, but it contain a couple of small errors.
caseLEFT:
// handle casewhere a rowin cursorA isunique// images isunique (missing thumbnail)
caseRIGHT:
// handle casewhere a rowin cursorB isunique// thumbs isunique (missing image)
These are backwards. The documentation contradicts itself, and is likely where the mistake got made. From the source code of CursorJoiner:
caseLEFT:
// handle casewhere a rowin cursorA isuniqueThen in the enum for Result from the source code:
public enum Result {
/** The row currently pointed to by the left cursor is unique */RIGHT,
/** The row currently pointed to by the right cursor is unique */LEFT,
/** The rows pointed to by both cursors are the same */
BOTH
}
So I am guessing this is why you were force incrementing the cursors.
//compensate for CursorJoiner expecting cursors ordered ascending...
c_images.moveToNext();
c_thumbs.moveToPrevious();
The iterator in CursorJoiner automatically increments the cursors for you.
This should be the working code (This code will also merge internal storage and external storage into a single cursor):
Cursor[] thumbs = newCursor[2];
thumbs[0] = mActivity.getContentResolver().query(
MediaStore.Images.Thumbnails.EXTERNAL_CONTENT_URI,
newString[]{
MediaStore.Images.Thumbnails._ID ,
MediaStore.Images.Thumbnails.IMAGE_ID,
MediaStore.Images.Thumbnails.DATA
},
null,
null,
MediaStore.Images.Thumbnails.IMAGE_ID + "*(-1)"
);
thumbs[1] = mActivity.getContentResolver().query(
MediaStore.Images.Thumbnails.INTERNAL_CONTENT_URI,
newString[]{
MediaStore.Images.Thumbnails._ID ,
MediaStore.Images.Thumbnails.IMAGE_ID,
MediaStore.Images.Thumbnails.DATA
},
null,
null,
MediaStore.Images.Thumbnails.IMAGE_ID + "*(-1)"
);
CursorthumbCursor=newMergeCursor(thumbs);
Cursor[] cursors = newCursor[2];
cursors[0] = mActivity.getContentResolver().query(
MediaStore.Images.Media.EXTERNAL_CONTENT_URI,
newString[]{
MediaStore.Images.Media._ID,
MediaStore.Images.Media.DATA,
MediaStore.Images.Media.ORIENTATION,
MediaStore.Images.Media.BUCKET_DISPLAY_NAME,
MediaStore.Images.Media.BUCKET_ID,
MediaStore.Images.Media.MIME_TYPE
},
null,
null,
MediaStore.Images.Media._ID + "*(-1)"
);
cursors[1] = mActivity.getContentResolver().query(
MediaStore.Images.Media.INTERNAL_CONTENT_URI,
newString[]{
MediaStore.Images.Media._ID,
MediaStore.Images.Media.DATA,
MediaStore.Images.Media.ORIENTATION,
MediaStore.Images.Media.BUCKET_DISPLAY_NAME,
MediaStore.Images.Media.BUCKET_ID,
MediaStore.Images.Media.MIME_TYPE
},
null,
null,
MediaStore.Images.Media._ID + "*(-1)"
);
CursorphotoCursor=newMergeCursor(cursors);
CursorJoinercursorJoiner=newCursorJoiner(
thumbCursor,
newString[]{
MediaStore.Images.Thumbnails.IMAGE_ID
},
photoCursor,
newString[]{
MediaStore.Images.Media._ID,
}
);
Cursor finalCursor= newMatrixCursor(newString[]{
MediaStore.Images.Media._ID,
MediaStore.Images.Media.DATA,
MediaStore.Images.Media.ORIENTATION,
MediaStore.Images.Media.BUCKET_DISPLAY_NAME,
MediaStore.Images.Media.BUCKET_ID,
MediaStore.Images.Media.MIME_TYPE,
"thumb_data"
});
for (CursorJoiner.Result joinerResult : cursorJoiner) {
switch (joinerResult) {
case RIGHT:
finalCursor.addRow(newObject[]{
photoCursor.getLong(photoCursor.getColumnIndex(MediaStore.Images.Media._ID)),
photoCursor.getString(photoCursor.getColumnIndex(MediaStore.Images.Media.DATA)),
photoCursor.getLong(photoCursor.getColumnIndex(MediaStore.Images.Media.ORIENTATION)),
photoCursor.getString(photoCursor.getColumnIndex(MediaStore.Images.Media.BUCKET_DISPLAY_NAME)),
photoCursor.getLong(photoCursor.getColumnIndex(MediaStore.Images.Media.BUCKET_ID)),
photoCursor.getString(photoCursor.getColumnIndex(MediaStore.Images.Media.MIME_TYPE)),
null
});
break;
case BOTH:
finalCursor.addRow(newObject[]{
photoCursor.getLong(photoCursor.getColumnIndex(MediaStore.Images.Media._ID)),
photoCursor.getString(photoCursor.getColumnIndex(MediaStore.Images.Media.DATA)),
photoCursor.getLong(photoCursor.getColumnIndex(MediaStore.Images.Media.ORIENTATION)),
photoCursor.getString(photoCursor.getColumnIndex(MediaStore.Images.Media.BUCKET_DISPLAY_NAME)),
photoCursor.getLong(photoCursor.getColumnIndex(MediaStore.Images.Media.BUCKET_ID)),
photoCursor.getString(photoCursor.getColumnIndex(MediaStore.Images.Media.MIME_TYPE)),
thumbCursor.getString(thumbCursor.getColumnIndex(MediaStore.Images.Thumbnails.DATA)),
});
break;
}
}
photoCursor.close();
thumbCursor.close();
Solution 3:
Here is my test case, which demonstrates the lack of support in CursorJoiner for descending ordered cursors. This, however, is documented specifically in the CursorJoiner source code, so I'm not trying to critize but merely show how this can be circumvented (or hacked).
The test case shows how the assumption of ascending ordering makes the need for "flipping", or reversing, all of the choices made by the CursorJoiner (comparator result, incrementation of cursors, etc). What I'd really like to try next is to modify the CursorJoiner class directly, to try to add support for DESC ordering.
Please note that it seems the part about ordering by ID*(-1) maybe is not strictly necessary for this to work. In the following example, I did not negate the ID columns (plain DESC ordering, not "pseudo-ASC" with negative sequences), and it still works.
Test case
String[] colA = newString[] { "_id", "data", "B_id" };
String[] colB = newString[] { "_id", "data" };
MatrixCursor cursorA = new MatrixCursor(colA);
MatrixCursor cursorB = new MatrixCursor(colB);
// add 4 items to cursor A, linked to cursor B// the data is ordered DESCENDING// all cases, LEFT/RIGHT/BOTH, are included
cursorA.addRow(newObject[] { 5, "Item A", 1004 }); // BOTH
cursorA.addRow(newObject[] { 4, "Item B", 1003 }); // LEFT
cursorA.addRow(newObject[] { 3, "Item C", 1002 }); // BOTH
cursorA.addRow(newObject[] { 2, "Item D", 1001 }); // LEFT
cursorA.addRow(newObject[] { 1, "Item E", 1000 }); // BOTH
cursorA.addRow(newObject[] { 0, "Item F", 500 }); // LEFT// similarily for cursorB (DESC)
cursorB.addRow(newObject[] { 1004, "X" }); // BOTH
cursorB.addRow(newObject[] { 1002, "Y" }); // BOTH
cursorB.addRow(newObject[] { 999, "Z" }); // RIGHT
cursorB.addRow(newObject[] { 998, "S" }); // RIGHT
cursorB.addRow(newObject[] { 900, "A" }); // RIGHT
cursorB.addRow(newObject[] { 1000, "G" }); // BOTH// join these on ID
CursorJoiner cjoiner = new CursorJoiner(
cursorA, newString[] { "B_id" }, // left = A
cursorB, newString[] { "_id" } // right = B
);
// enable workaroundboolean desc = true;
int count = 0;
for (CursorJoiner.Result joinerResult : cjoiner) {
Log.v("TEST", "Processing (left)=" + (cursorA.isAfterLast() ? "<empty>" : cursorA.getLong(2))
+ " / (right)=" + (cursorB.isAfterLast() ? "<empty>" : cursorB.getLong(0)));
// flip the CursorJoiner.Result (unless Result.BOTH, or either cursor is exhausted)if (desc && joinerResult != CursorJoiner.Result.BOTH
&& !cursorB.isAfterLast() && !cursorA.isAfterLast())
joinerResult = (joinerResult == CursorJoiner.Result.LEFT ? CursorJoiner.Result.RIGHT : CursorJoiner.Result.LEFT);
switch (joinerResult) {
case LEFT:
// handle case where a row in cursorA is unique
Log.v("TEST", count + ") join LEFT. cursorA is unique");
if (desc) {
// compensate cursor incrementsif (!cursorB.isAfterLast()) cursorB.moveToPrevious();
if (!cursorA.isLast()) cursorA.moveToNext();
}
break;
case RIGHT:
Log.v("TEST", count + ") join RIGHT. cursorB is unique");
// handle case where a row in cursorB is uniqueif (desc) {
if (!cursorB.isLast()) cursorB.moveToNext();
if (!cursorA.isAfterLast()) cursorA.moveToPrevious();
}
break;
case BOTH:
Log.v("TEST", count + ") join BOTH: " + cursorA.getInt(0) + "," + cursorA.getString(1) + "," + cursorA.getInt(2) + "/" + cursorB.getInt(0) + "," + cursorB.getString(1));
// handle case where a row with the same key is in both cursorsbreak;
}
count++;
}
Log.v("TEST", "Join done!");
and the output:
V/TEST: Processing (left)=5/ (right)=1004
V/TEST: 0) joinBOTH: 4,Item A,1004/1004,X
V/TEST: Processing (left)=4/ (right)=1002
V/TEST: 1) join LEFT. cursorA isunique
V/TEST: Processing (left)=3/ (right)=1002
V/TEST: 2) joinBOTH: 2,Item C,1002/1002,Y
V/TEST: Processing (left)=2/ (right)=999
V/TEST: 3) join RIGHT. cursorB isunique
V/TEST: Processing (left)=2/ (right)=998
V/TEST: 4) join RIGHT. cursorB isunique
V/TEST: Processing (left)=2/ (right)=900
V/TEST: 5) join RIGHT. cursorB isunique
V/TEST: Processing (left)=2/ (right)=1000
V/TEST: 6) join LEFT. cursorA isunique
V/TEST: Processing (left)=1/ (right)=1000
V/TEST: 7) joinBOTH: 0,Item D,1000/1000,F
V/TEST: Processing (left)=0/ (right)=---
V/TEST: 8) join LEFT. cursorA isunique
V/TEST: Join done!
Post a Comment for "Querying The Mediastore: Joining Thumbnails And Images (on Id)"